Students should finish high school with a good understanding of where humans came from, and of our place in the universe, up to the present.

For the most part, what is taught in schools lags behind what we know/understand.

Of course the construction of detailed curricula, the methods use to impart the information, the construction of lessons, the use of visual aids and media to assist understanding, etc., etc., is a matter for content experts and education experts in the various fields. .

Also, the coverage in the modules outlined below is quite extensive, and there is certainly too much information overall to cover in the years of elementary-, middle- and high school, especially after factoring all the other material that needs to be covered. Consequently, some choices will need to be made about which content to include in each module, while still presenting the overall picture.

The content in the various pop-up boxes reflects my own non-specialist understanding of these topics, acquired mostly from Wikipedia, and in any proper school curriculum will certainly need to be re-written by experts in the various fields.

After expanding any of the accordion tabs, click on any of the topic links to get a popup window with a brief description of the topic, and a link to the Wikipedia page with a more detailed description of the topic.

Remark: At this time, this page (with its many popups and hover boxes) is not designed for viewing on mobile devices, although the popups and hover boxes appear to work on android phones (and possibly other types of phone).

The reason for the accordions, popup, and tooltip boxes is to allow for rapid navigation between topics.
If you find a topic that you wish to read more about, it is strongly recommended that you follow the link (which may be found at the bottom of the popup- and tooltip boxes) to the corresponding page on Wikipedia, where the article will be more legible and where there will be more live links.

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The aim here should be to leave students with an understanding of the earth's place in space and time, how it arose in terms of the processes that led to stellar and planetary formation, how the earth and the solar system relate to the rest of the universe, both in terms of the vastness of space and the history of the universe since the Big Bang.

Topics that should be covered include (click on a heading to open a popup window):

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The Big Bang

The Big Bang Theory is presently the most widely accepted theory of the origin of the Universe.

It holds that the Universe expanded outwards from a singularity (infinitely small region of space) around 13.8 billion years ago.

All the stars, planets, galaxies, nebulae, comets, etc., etc., have arisen through physical changes in the Universe since the time of the Big Bang.

The theory does not make any claims about the origin of the Universe, if it has an origin, and indeed one theory, The Big Bounce, posits that the present expansion of the Universe is just one expanion phase in an infinite sequence of expansion and contraction that both preceded the present cycle and will also follow it.

For more details, see The Big Bang. close

The expanding Universe

Around 1930 the astronomer Edwin Hubble discovered that galaxies were moving further apart from each other.

It is now believed that the reason for this is that space itself is expanding, and thus, working backward in time, that at one time the Universe and everything we presently observe was located in an infinitemially small region.

Current estimates are that the Big Bang, the beginning of the expansion of the Universe, took place around 13 billion years ago.

There are many unanswered questions about the expansion of the Universe:
- Why did the Universe start to expand in the first place?
- Will the Universe keep expanding forever, or will it stop expanding at some point in the future, and start contracting again?
- Why was the expansion not uniform in all directions, in the sense that there is presently large variation in the spatial distribution of matter in the Universe? For more details, see Expansion of the universe. close

Estimates of the age the universe

The current age of the universe, the time elapsed since The Big Bang, is estimated to be close to 13.8 billion years.

This estimate has been arrived at in a number of ways.

Measurements of the cosmic background radiation lead to estimates of the cooling time since The Big Bang.

Measurement of the expansion rate of the universe can be used to extrapolate backwards to estimate when expansion started (how long ago The Big Bang happened).

For more details, see Age of the Universe. close

Estimates of the number of stars in the universe

Estimates of the number of stars in the observable universe vary, but most recent estimates are around 10²³.

This number is hard to visualize, but by current estimates there as many stars in the universe as grains of medium-sized sand in a pile of such sand that would cover the continent of North America (Canada, the United States, Mexico and Central America) to a depth of over 260 Km, or that would cover the entire earth to a depth of about 13 Km.

The relation of our solar system to all the other stars the universe, may be partially grasped by considering it as a single grain of sand in this pile of sand that would cover North America to a depth of over 260 kilometers.

Note the use of the term observable universe - some parts of the universe are so far away that light has not reached Earth yet, and some parts of the universe are so far away that light will never reach Earth.

For more information, see Composition of the Universe. close

Estimates of the size of the universe

The observable universe has a diameter of roughly 91 billion light years, where a light year is the distance travelled by light in one year, or about 6 billion (6 × 10¹²) miles.

This is hard to visualize so we use the analogy of comparing stars to grains of sand used elsewhere (ignoring for the moment the huge variation in the size of stars).

The number of stars in the universe is roughly the same as the number of grains of fine sand in a sphere of sand around 2300 kilometers in diameter (or in a sphere of sand with diameter roughly one fifth the diameter of the earth).

If the universe and everything in it were shrunk by a factor of around 6 × 10¹² (so that a star the size of our sun were shrunk to the size of a grain of fine sand), then the average distance between stars would have shrunk to around 71 kilometers, and the universe would have shrunk to occupy a spherical region of space with diameter extending around 100 times the diameter of our solar system.

Remark: This comparison model is assuming that as well as shrinking the universe and stars, that also the grains have been uniformly distributed throughout this spherical region of space with diameter extending around 100 times the diameter of our solar system. It is after this uniform redistribution that the average distance between the grains of sand representing the stars becomes `~ 71 km.

For more information, see Size and regions of the Universe. close

Large-scale structure of the universe

The 10²²-10²⁴ stars in the universe are not uniformally distributed in space.

From Wikipedia:
"Stars are organized into galaxies, which in turn form galaxy groups, galaxy clusters, superclusters, sheets, walls and filaments, which are separated by immense voids, creating a vast foam-like structure sometimes called the "cosmic web"."

Our galaxy, the Milky Way , contains 100-400 billion stars, and is contained in the Local Group of galaxies, which are a part of the Virgo Supercluster, which is in turn part of the Laniakea Supercluster (see the image above).

The number of galaxies in the observable universe is estimated to be between 200 billion (2×10¹¹) and 2 trillion (2×10¹²) or more.

Our solar system iscontained in a cluster of superclusters called the Laniakea Supercluster, which contains our own Milky Way and roughly 100,000 other nearby galaxies, stretched out over 520 million light-years.

There are even larger galactic superstructures, for example the Hercules–Corona Borealis Great Wall, containing many billions of galaxies.

For more information, see The observable universe - large scale structure. close

Formation of stars and galaxies

Little is known about the early history of the universe, but by about 377,000 years after the Big Bang the universe had cooled sufficiently so that the first atoms (mostly hydrogen and helium with some traces of lithium) could form.

Sometime around 400 to 700 million years after the Big Bang, the first stars and galaxies form (under the action of gravity). These first stars were relatively short lived, exploding as supernovae after just millions of years. Most of the heavier elements such as those that make up all the material in the planets were first formed at this time.

Galactic clusters and superclusters start to develop, and by about 1 billion years after the Big Bang, the universe looked pretty much like it does today (although it continues to expand, and new stars and galaxies continue to form, and older stars and galaxies burn out).

Our galaxy, the Milky Way, contains some stars that formed shortly after the Big Bang, while others (in the thin disk) formed between 8 and 10 billion years ago.

Our solar system with the sun and planets began to form around 9.2 million years after the Big Bang (about 4.6 billion years ago).

For more information, see Star Formation. close

The Milky Way

The Milky Way galaxy began to form shortly after the Big Bang (around 13.8 billion years ago). Many of the stars in the thin disk formed between 8 and 10 billion years ago. Our sun and the planets in the solar system formed about 4.6 billion years ago. Star formation is ongoing, and stars less than 100 million years old have been observed.

The Milky Way and the galaxy Andromeda form a binary galactic system in a cluster of around 50 galaxies, called the Local Group. The Local Group is part of the Virgo Supercluster, containing about 100 galaxies, and one of an estimated 10 million galactic super clusters in the observable universe.

It is a spiral galaxy with diameter estimated to be between 100,000 light years and 180,000 light years. It contains between 200 billion and 400 billion stars, and probably at least 100 billion planets. This is roughly as many stars as grains of 1 millimeter diameter sand in a box 20 feet on a side, but spread out so that the distance between our sun and the nearest neighbouring star would correspond to a distance of around 30 km between the corresponding grains of sand (and the corresponding "sand galaxy" having a diameter between 700,000 km and 1,300,000 km).

About 95% of the Milky Way is made up of dark matter, which encloses the visible disc of the galaxy in a roughly spherical halo containing around 6 x 10¹¹ to 3 x 10¹² solar masses. Luminous matter makes up about 9 x 10¹⁰ solar masses. In addition there is interstellar dust and gas (mostly hydrogen and helium) with total mass amounting to between 10% and 15% of luminous matter.

Outside the galactic center, the Milky Way consists of 4 spiral arms. Our solar system is in the Orion Arm, about 26,000 light years from the galactic center, and rotates about the galactic center about every 240 million years.

For more information, see The Milky Way. close

Formation of The Solar System

The Solar System formed about 4.6 billion years ago, when a large molecular cloud, perhaps several light years across, collapsed under gravity, with the material in the cloud clumping to give rise to several stars, one of them being our sun with its associated system of planets.

This molecular cloud was composed mostly of hydrogen, a lesser amount of helium, and smaller amounts of heavier elements which had formed in the bodies of previous generations of stars.

As the material in the part of the cloud that eventually became the solar system contracted under gravity, the center began to heat up, and the mass of material began to spin faster. This rotation in turn caused the mass of material to flatten into a disc with diameter rougly 200 times the radius of the earth's orbit around the sun.

The material at the center of the disc became a hot dense protostar, which eventually became our sun, while the dust and gas further out coalesced (under gravity) to form ever larger clumps of matter, which eventually combined to form the presently existing planets, protoplanets, and other minor bodies.

Because of the high temperatures close to the sun, the planets that formed closer to the sun (Mercury, Venus, Earth and Mars) were rocky planets, becuse of the relative scarcity of metallic elements, these planets were smaller than the planets further out (Jupiter, Saturn, Uranus and Neptune), which are formed primarily from Hydrogen and helium in gaseous, liquid and solid forms together with other materials (although possibly with a rocky core).

The remaining material in the solar system, that which did not coalesce to form the sun and planets, gathered into various belts and clouds circling the sun,some of which lie at distances far outside the orbits of the planets.

For more information, see Formation of the Solar System. close

The Sun, Planets and Other Bodies in The Solar System

The Solar System consists of the sun and the bodies, such as the planets, which orbit it. It is in the Orion Arm of the Milky Way galaxy, about 26,000 light years from the galactic center, and rotates about the galactic center about every 240 million years. It formed from a molecular cloud of gas and dust about 4.6 billion years ago.

If the sun, which has a diameter of around 864,000 miles, were scaled down to the size of a basketball (about 9.55 inches), then on the same scale the Earth would have a diameter of around 0.0874 inches, and would orbit the basketball sun at an average distance of around 86 feet. The most distant planet, Neptune, would have a diameter of about 1/3 of an inch on this scale, and would orbit at an average distance of over 2500 feet. Also on this scale, smaller bodies orbiting this basketball sun may exist as far out as 1000 miles.

Informally, the Solar System is divided into three regions.
The inner Solar System contains the Sun, the four smaller rocky planets (Mercury, Venus, Earth and Mars) and the asteroid belt.
The outer Solar System contains the four giant planets (Jupiter, Saturn, Uranus and Neptune).
The region beyond Neptune, which contains the Kuiper belt and other objects, is now regarded as a separate region of the Solar System.

The principal bodies in our Solar System, in order of distance from the sun, are (hover over a name to get an information "hover box"):

The Sun The Sun is a star, and occupies the central position in the Solar system.
Its diameter is about 1.39 million kilometers, around 109 times that of Earth.
Its mass is about 2x10³⁰ kg, over 300,000 times that of Earth, which accounts for nearly 99.9% of the total mass of the Solar System.
Nuclear fusion at the core of the sun is the source of its light and heat, and has raised the temperature of the core to around 27 million degrees Fahrenheit, and its surface to around 10,000 degrees Fahrenheit.
The sun will remain essentially as it is today for another 5 billion years, when nuclear fusion of most of its hydrogen will have been completed, it will expand into a red giant star.
Its chemical composition is about 71.1% Hydrogen, 27.4% Helium, and the remainder heavier elements.
The sun is also the source of the "solar wind", a stream of charged particles that spreads out from the sun throughout the Solar System, and is responsible for Mercury and Venus losing much of their atmosphere.
For more information, see The Sun.

Mercury Mercury is the planet in the Solar System that is closest to the sun.
It is a rocky planet, like earth, and appears to have a solid silicate crust and mantle between 500 and 600 km thick. Mercury's core is believed to occupy about 55% of its volume, and to be partly molten.
Mercury's gravity is about 0.38 that of Earth
Its average radius is about 2497 km, about 0.38 that of the earth, and its volume is around 1/18th that of the earth.
It orbits around the sun roughly every 87.97 days.
Mercury's orbit is the most eccentric of all the planets in the solar system, with its closest distance to the sun (about 46 million km) being only about 66% of its farthest distance from the sun (about 69.8 million km).
It used to be thought that Mercury kept the same face towards the sun, but it is now known that it does rotate about an axis that is nearly perpendicular to its orbit, with a Mercurian day lasting two Mercurian years.
The surface of Mercury looks quite similar to the surface of the Moon, with many craters and plains. There are remains of ancient volcanoes, from perhaps as long ago as 3.5 billion years.
There are no oceans, but the polar regions contain large amounts of ice.
Mercury has no signicant atmosphere, but does have a thin exosphere containing hydrogen, helium, oxygen, sodium, calcium, potassium and other trace elements.
It does have a planetary magnetic field whose strength is about 1.1% that of the Earth's.
Temperatures range from -280 ℉ to 800 ℉ across the equatorial regions. Temperatures near the poles are consistently below -136 ℉. Temperatures on the dark side average about -262 ℉.
For more information, see Mercury.

Venus Venus is the second planet from the sun, orbiting the sun between the orbits of Mercury and Earth.
It orbits around the sun roughly every every 224.7 Earth days, at an average distance of around 108 million km.
Venus rotates on its axis roughly every 243 Earth days, and a day on Venus lasts 118 Earth days.
Venus is roughly the same size as earth, with mean radius about 0.95 that of the earth, and mass about 81.5% that of the earth.
It has no satellites.
The atmosphere is denser than that of the earth, being around 96.5% carbon dioxide, 3.5% nitrogen, and traces of other gases, mostly sulphur dioxide. The atmospheric pressure is about 90 times that on Earth.
About 80% of the surface consists of smooth volcanic plains, with most of the remainder made of two high continents, one in the northern hemisphere and the other in the southern.
There are relatively few impact craters, indicating that the surface formed within the last 300-600 million years. There are several times as many ancient volcanoes on Venus as there are on Earth. There is some evidence of ongoing volcanic activity.
Internally, Venus is believed to be similar to Earth, with a core, mantle and crust, although it has no internally generated magnetic field.
The atmosphere of carbon dioxide has caused a strong greenhouse effect, leading to surface temperatures of at least 860℉. Temperatures on Venus are much more uniform, from the equator to the poles, than they are on Earth.
Thick clouds in the upper atmosphere obscure the surface. These clouds may also be capable of producing ligntning. The atmosphere also has an ozone layer.
The extreme temperature- and pressure conditions make water-based life very unlikely to exist on Venus.
For more information, see Venus.

Earth Earth is the third planet from the sun, and the one on which we live. It is the only planet known (at this time) to be inhabited by lifeforms.
It orbits the sun at an average distance of around 93 million miles, completing one revolution around the sun in around 365.26 days, at an average speed of around 18.5 miles per second. The solar day (period of rotation relative to the sun) is approximately 24 hours (and varies in length up to 2 milliseconds longer than the average length of a day in the 19th century).
The earth is a slightly flattened sphere, with an equatorial circumference of around 24,900 miles. Its mass is around 5.97×10²¹ tons. It is the densest planet in the Solar System.
The Earth's gravitational interaction with its satellite, the Moon, causess the ocean tides, stabilizes its orientation on its axis, and gradually slows down its rotation.
The Earth's axis of rotation is tilted by about 23.44° to the plane of its orbit around the sun, the resulting variation in the amount of sunlight reaching any particular point on the Earth's surface throughout the year causing the seasonal changes in climate. The surface temperature on Earth varies from around -128.5℉ to around 134.3℉.
The Earth has a solid iron inner core, a liquid outer core that generates the magnetic field, and a mantle that drives the motion of the tectonic plates that comprise the crust.
The Earth's crust is composed of several large tectonic plates that move slowly over the mantle over millions of years.
Around 71% of the Earth's surface is covered by oceans, with the remaining 29% being continents and Islands. The present continents formed as a result of these tectonic plates forming into super-continents, and breaking apart again over hundreds of millions of years. The majority of the polar regions have an ice covering, which has expanded and contracted repeatedly over the past 2.6 million years, to cover more extensive areas of the Earth's surface.
The Earth's atmosphere presently consists of around 78% nitrogen, 21% oxygen, and traces of other gases, including water vapour and carbon dioxide. The composition of the Earth's atmosphere has changed considerably over the billions of years since the Earth formed.
As well as having a biosphere of lifeforms, the planet Earth is also unique in being inhabited by a technologically-advanced relatively intelligent species.
For more information, see Earth.

Mars Mars is the fourth planet from the sun, and orbits at an average distance of 141.6 million miles. It takes roughly 1.88 Earth years for Mars to complete a orbit around the Sun. The orbit of mars has been getting slightly more eccentric due to the gravitational attraction of the other planets.
The Martian solar day lasts about 24 hours and 40 minutes.
The equatorial radius of Mars is about 2110 miles, about half that of Earth. Its mass is around 6.32×10²⁰ tons, about 11% that of the earth, and its gravity is about 38% that of the Earth.
Mars has a core (mostly iron and nickel, with around 16% sulphur), a silicate mantle, and a crust (the average thickness of the crust on Mars is about 31 miles, compared with about 25 miles for Earth).
Mars is a rocky planet. The predominant reddish colour is caused by a widespread layer of iron oxide (rust).
It appears to have large quantities of ice located at the poles, and also large quantities of underground water elsewhere.
It has the largest volcano and one of the largest canyons in the solar system. It has craters like the moon, but also valleys and deserts like the Earth. There is evidence of volcanic activity having occurred as recently as 20 million years ago.
Physical conditions presently prevent the presence of liquid water on the surface of Mars, although landform patterns indicate that water did exist on the surface in the past.
Mars has a thin atmosphere, which is about 96% carbon dioxide, along with argon and nitrogen, as well as traces of oxygen and water.
Mars has two small moons, called Phobos and Deimos.
The axis of rotation of Mars is tilted by about 25.19° to its orbital plane. This has resulted in Mars having seasons similar to Earth. The surface temperature on Mars varies from -225℉ at the polar caps in winter to around 95℉ at the equator in summer. Mars experiences gigantic dust storms, which can cover the entire planet, with wind speeds up to 100mph.
For more information, see Mars.

The Asteroid Belt The asteroid belt is a disc of several million rocky bodies of various sizes that exist between the orbits of Mars and Jupiter.
The total mass of material in the asteroid belt is estimated to be between 2.76×10¹⁸ and 3.15×10¹⁸ tons, about 4% of the mass of the moon. The four largest objects account for half the total mass of the belt, with the largest, Ceres, which has a diameter of nearly 950 km, accounting for nearly 1/3 of the total mass. Over 200 of the asteroids are larger than 100 km across, and there are between 700,000 and 1.7 million asteroids with a diameter of 1 km or more. Other bodies range in size down to grains of dust.
The asteroid belt formed from accretion of particles in the primordial solar nebula. The influence of Jupiter gave these asteroids too much orbital energy for the asteroids to accrete into a planet, resulting in 99.9% of the original mass of the asteroid belt being lost in the first 100 million years of the Solar System's history.
Within the asteroid belt, there is some clustering into various families.
For more information, see The Asteroid Belt.

Jupiter Jupiter is the largest planet in the solar system, with its total mass being around 1.868×10²⁴ tons, or around 318 times the mass of the Earth. The (equatorial) radius is around 44,400 miles, about 11.2 times that of the Earth, and the surface area is about 122 times that of the Earth.
Jupiter orbits around the sun every 11.86 Earth years, at a mean orbital radius of 483.8 million miles.
Jupiter is mostly a liquid planet. It is believed to have a central core of rock and ice, surrounded by a much deeper layer of liquid metallic hydrogen with some helium, and an outer layer of molecular hydrogen.
Jupiter's atmosphere is over 3000 miles in altitude. The atmosphere has an upper cloud layer composed of ammonia crystals and possibly ammonium hydrosulfide. This cloud layer exists in bands, which interact with each other to cause storms. There is believed to be a thin layer of water clouds below the ammonia cloud layer, and these water clouds are believed to generate powerful thunderstorms. Some of the storms on Jupiter have extended over areas larger than the surface of the earth, and have lasted for centuries.
Jupiter has at least 69 moons, four of these being large, varying in size from slightly smaller than the Earth's moon to slightly larger than Mercury. Io has over 400 active volcanoes. Europa, Ganymede and Callisto possibly have internal oceans existing beneath their surfaces.
Jupiter's magnetic field is 14 times stronger than that of Earth.
Jupiter rotates on its axis in slightly less than 10 hours.
Jupiter has a faint planetary ring system composed of three main rings, which are primarily made of dust.
For more information, see Jupiter.

Saturn Saturn is the sixth planet from the sun, and the second largest, after Jupiter.
Its mass is about 5.7707×10²³ tons, which is around 95 times the mass of the Earth. Its equatorial radius is 37,449 miles, which is about 9.45 times that of the Earth, and its surface is about 83.7 times that of the Earth.
Saturn is believed to have a core conssiting of iron, nickel and rock. Surrounding this there is a deep layer of metallic hydrogen, which in turn is enclosed in a layer of liquid hydrogen and liquid helium, which in turn is enclosed in a gaseous outer layer. Saturn is mostly composed of hydrogen and helium, and overall is less dense than water.
It orbits the sun once every 29.46 Earth years, at an average distance of around 886 million miles, about 9.5 times the average distance of the Earth from the sun. Saturn rotates on its axis roughly every 10.7 hours.
Saturn's atmosphere is about 96.3% hydrogen by volume, and about 3.25% helium by volume, with trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane. Clouds in the upper atmosphere are composed of ammonium crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide or water. Wind speeds on Saturn can reach 1,100 mph.
It has a magnetic field that is slightly weaker than that of the Earth.
Saturn is surrounded by a system of rings, which are mostly composed of ice, but also contain some rocky material and dust.
Saturn has 62 known moons, as well as possibly hundreds of moonlets, with diameter between 50 and 500 metres, existing in the rings. The largest moon, Titan, is 50% larger than the Earth's moon, and larger than the planet Mercury. Titan has its own atmosphere (mostly nitrogen), and surface features such as dunes, rivers, lakes, seas (probably liquid methane and ethane) and mountains. There is some speculation that Titan may be capable of supporting primitive lifeforms.
For more information, see Saturn.

Uranus Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System.
Its mass is about 8.5439×10²² tons, which is around 14.5 times the mass of the Earth. Its equatorial radius is 15,882 miles, which is about 4 times that of the Earth, and its surface is about 15.9 times that of the Earth.
Uranus has a small rocky, silicate/iron-nickel core (about 3.8% of the total mass); an icy mantle consisting of water, ammonia and methane (about 92% of the total mass); and an outer gaseous layer (hydrogen and helium) making up the rest.
Uranus has an axial tilt of 97.7 °. This results in each pole getting around 42 years of continuous sunlight, followed by 42 years of darkness. Temperatures in the lower atmosphere on Uranus vary from -371℉ to around 116℉. The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature around 800 to 850 K.
Uranus orbits the sun once every 84.02 Earth years, at an average distance of around 1874 million miles, about 19.2 times the average distance of the Earth from the sun. Uranus rotates on its axis roughly every 17 hours and 14 minutes.
Uranus has 27 known natural satellites. The largest satellite, Titania, has a radius of only 788.9 km (490.2 mi), or less than half that of the Moon.
Uranus is surrounded by a system of 13 rings, composed of dark material ranging in size from dust to a fraction of a metre.
For more information, see Uranus.

Neptune Neptune is the eighth and farthest known planet from the Sun in the Solar System.
In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet.
Neptune is about 17.15 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune.
The surface area of Neptune is about 14.98 times that of the Earth.
Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 AU (4.5 billion km).
Neptune rotates on its axis once every 16 hours and 6 minutes.
Like Uranus, Neptune has a small rocky, silicate/iron-nickel core (about 7% of the total mass); an icy mantle consisting of water, ammonia and methane (between 58% and 87% of the total mass); and an outer gaseous layer (hydrogen and helium) making up the rest.
Neptune has the strongest sustained wind speeds in the solar system, with recorded wind speeds as high as 1300 mph.
Anticyclonic storms covering an area comparable to the surface area of the Earth, and lasting several years, have been observed.
Temperatures in the upper atmosphere on can get as low as -361℉.
The temperature at the center of the planet is around 9300℉.
Neptune has 14 known natural satellites.
The largest satellite, Triton, has a diameter of 2,710 kilometres (1,680 mi), making it the seventh-largest moon in the Solar System.
Neptune has a faint and fragmented ring system.
For more information, see Neptune.

The Kuiper Belt From Wikipedia:
The Kuiper belt, occasionally called the Edgeworth–Kuiper belt, is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun.[2] It is similar to the asteroid belt, but is far larger—20 times as wide and 20 to 200 times as massive.
Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed.
While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water.
The Kuiper belt is home to three officially recognized dwarf planets: Pluto, Haumea and Makemake.
Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.
For more information, see The Kuiper Belt.

Pluto From Wikipedia:
Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune.
It was the first Kuiper belt object to be discovered.

Pluto is the largest and second-most-massive known dwarf planet in the Solar System, and the ninth-largest and tenth-most-massive known object directly orbiting the Sun.
It is the largest known trans-Neptunian object by volume but is less massive than Eris.
other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume.
It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU (4.4–7.4 billion km) from the Sun.
This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding.
Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.5 AU). ...

... Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra.
Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body. ...

... Pluto's rotation period, its day, is equal to 6.39 Earth days.[84] Like Uranus, Pluto rotates on its "side" in its orbital plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at its solstices, one-fourth of its surface is in continuous daylight, whereas another fourth is in continuous darkness. ...

... The plains on Pluto's surface are composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.
The mountains are made of water ice. ....
... Scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of water ice. ...

For more information, see Pluto.

The Scattered Disc From Wikipedia:
The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, and are a subset of the broader family of trans-Neptunian objects.
The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5×109 km; 2.8×109 mi).
These extreme orbits are thought to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.

Although the closest scattered-disc objects approach the Sun at about 30–35 AU, their orbits can extend well beyond 100 AU.
This makes scattered objects among the coldest and most distant objects in the Solar System.
The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects traditionally called the Kuiper belt, but its outer limits reach much further away from the Sun and further above and below the ecliptic than the Kuiper belt proper.
For more information, see The Scattered Disc.

Comets more later. From Wikipedia:
A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing.
This produces a visible atmosphere or coma, and sometimes also a tail.
These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet.
Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles.
The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit.
If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky.
Comets have been observed and recorded since ancient times by many cultures.
For more information, see Comets.

The Oort Cloud From Wikipedia:
The Oort cloud named after the Dutch astronomer Jan Oort, sometimes called the Öpik–Oort cloud, is a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 50,000 and 200,000 AU (0.8 and 3.2 ly).
It is divided into two regions: a disc-shaped inner Oort cloud (or Hills cloud) and a spherical outer Oort cloud.
Both regions lie beyond the heliosphere and in interstellar space.
The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud.
For more information, see The Oort Cloud.

For more information, see The Solar System. close

The various types of star and the life and death of stars

From Wikipedia:
Stellar evolution is the process by which a star changes over the course of time.
Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe.
...
All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds.
Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

Star Formation From Wikipedia:
Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars.
As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products.
It is closely related to planet formation, another branch of astronomy.
Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function.

For more information, see Early Galaxy Formation.

Molecular Cloud From Wikipedia:
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H2).
This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.

For more information, see Molecular Cloud.

Protostar From Wikipedia:
A protostar is a very young star that is still gathering mass from its parent molecular cloud.
The protostellar phase is the earliest one in the process of stellar evolution.
For a one solar-mass star it lasts about 1,000,000 years.
The phase begins when a molecular cloud first collapses under the force of self-gravity.
It ends when the protostar blows back the infalling gas and is revealed as an optically visible pre-main-sequence star, which later contracts to become a main sequence star.

For more information, see Protostar.

Pre-main-sequence Star From Wikipedia:
A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence.
Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope of interstellar dust and gas.
After the protostar blows away this envelope, it is optically visible, and appears on the stellar birthline in the Hertzsprung-Russell diagram.
At this point, the star has acquired nearly all of its mass but has not yet started hydrogen burning (i.e. nuclear fusion of hydrogen).
The star then contracts, its internal temperature rising until it begins hydrogen burning on the zero age main sequence.
This period of contraction is the pre-main sequence stage.

For more information, see Pre-main-sequence Star.

Brown Dwarf From Wikipedia:
Brown dwarfs are substellar objects that occupy the mass range between the heaviest gas giant planets and the lightest stars, having masses between approximately 13 to 75–80 times that of Jupiter (M_J), or approximately 2.5×10²⁸ kg to about 1.5×10²⁹ kg.
Below this range are the sub-brown dwarfs, and above it are the lightest red dwarfs (M9?V).
Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.

Unlike the stars in the main sequence, brown dwarfs are not massive enough to sustain nuclear fusion of ordinary hydrogen (¹H) to helium in their cores.

For more information, see Brown Dwarf.

Sub-brown Dwarf From Wikipedia:
A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a mass below the limiting mass for thermonuclear fusion of deuterium (about 13 M_J).
Some researchers call them free-floating planets whereas others call them planetary-mass brown dwarfs.

For more information, see Sub-brown Dwarf.

Main Sequence From Wikipedia:
For a more-massive protostar (than a brown dwarf), the core temperature will eventually reach 10 million kelvin, initiating the proton–proton chain reaction and allowing hydrogen to fuse, first to deuterium and then to helium.
In stars of slightly over 1 solar mass (2.0×10³⁰ kg), the carbon–nitrogen–oxygen fusion reaction (CNO cycle) contributes a large portion of the energy generation.
The onset of nuclear fusion leads relatively quickly to a hydrostatic equilibrium in which energy released by the core maintains a high gas pressure, balancing the weight of the star's matter and preventing further gravitational collapse.
The star thus evolves rapidly to a stable state, beginning the main-sequence phase of its evolution.

A new star will sit at a specific point on the main sequence of the Hertzsprung–Russell diagram, with the main-sequence spectral type depending upon the mass of the star.
Small, relatively cold, low-mass red dwarfs fuse hydrogen slowly and will remain on the main sequence for hundreds of billions of years or longer, whereas massive, hot O-type stars will leave the main sequence after just a few million years.
A mid-sized yellow dwarf star, like the Sun, will remain on the main sequence for about 10 billion years.
The Sun is thought to be in the middle of its main sequence lifespan.

For more information, see Main Sequence.

Red Dwarf From Wikipedia:
A red dwarf (or M dwarf) is a small and relatively cool star on the main sequence, of M spectral type.
Red dwarfs range in mass from a low of 0.075 to about 0.50 solar mass and have a surface temperature of less than 4,000 K.
Sometimes K-type main-sequence stars, with masses between 0.50-0.8 solar mass, are also included.

Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot be easily observed.
From Earth, not one is visible to the naked eye.
Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are fifty of the sixty nearest stars.
According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way. ...

... Recent astrophysical models suggest that red dwarfs of 0.1 solar masses may stay on the main sequence for some six to twelve trillion years, gradually increasing in both temperature and luminosity, and take several hundred billion more to collapse, slowly, into a white dwarf.
Such stars will not become red giants as they are fully convective and will not develop a degenerate helium core with a shell burning hydrogen.
Instead, hydrogen fusion will proceed until almost the whole star is helium.

For more information, see Red Dwarf.

Red Giant From Wikipedia:
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ) in a late phase of stellar evolution.
The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K (4,700 ℃; 8,500 ℉) or lower.
The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.

The most common red giants are stars on the red-giant branch (RGB) that are still fusing hydrogen into helium in a shell surrounding an inert helium core.
Other red giants are the red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process; and the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen burning shell just beyond that.

For more information, see Red Giant.

White Dwarf From Wikipedia:
A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter.
A white dwarf is very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth.
A white dwarf's faint luminosity comes from the emission of stored thermal energy; no fusion takes place in a white dwarf wherein mass is converted to energy.
The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star.
There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun.
The unusual faintness of white dwarfs was first recognized in 1910.
The name white dwarf was coined by Willem Luyten in 1922.

White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star, which would include the Sun and over 97% of the other stars in the Milky Way.

For more information, see White Dwarf.

Black Dwarf From Wikipedia:
A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits significant heat or light.
Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe (13.8 billion years), no black dwarfs are expected to exist in the universe now, and the temperature of the coolest white dwarfs is one observational limit on the age of the universe.

For more information, see Black Dwarf.

Supergiant Star From Wikipedia:
Supergiants are among the most massive and most luminous stars.
Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about -3 and -8.
The temperature range of supergiant stars spans from about 3,500 K to over 20,000 K.

For more information, see Supergiant Star.

Supernova From Wikipedia:
A supernova (abbreviations: SN and SNe) is a transient astronomical event that occurs during the last stellar evolutionary stages of a star's life, either a massive star or a white dwarf, whose destruction is marked by one final, titanic explosion.
This causes the sudden appearance of a "new" bright star, before slowly fading from sight over several weeks or months or years.

Supernovae are more energetic than novae.
In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star.
Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous.
The word supernova was coined by Walter Baade and Fritz Zwicky in 1931.

For more information, see Supernova.

Neutron Star From Wikipedia:
A neutron star is the collapsed core of a large star which before collapse had a total of between 10 and 29 solar masses.
Neutron stars are the smallest and densest stars, not counting hypothetical quark stars and strange stars.
Typically, neutron stars have a radius on the order of 10 kilometres (6.2 mi) and a mass between 1.4 and 2.16 solar masses.
They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past the white dwarf star density to that of atomic nuclei.
Once formed, they no longer actively generate heat, and cool over time; however, they may still evolve further through collision or accretion.
Most of the basic models for these objects imply that neutron stars are composed almost entirely of neutrons (subatomic particles with no net electrical charge and with slightly larger mass than protons); the electrons and protons present in normal matter combine to produce neutrons at the conditions in a neutron star.
Neutron stars are supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle, just as white dwarfs are supported against collapse by electron degeneracy pressure.
If the remnant star has a mass greater than about 3 solar masses, it continues collapsing to form a black hole.

For more information, see Neutron Star.

Black Hole From Wikipedia:
A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing—not even particles and electromagnetic radiation such as light—can escape from inside it.
The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole.
The boundary of the region from which no escape is possible is called the event horizon.
Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed.
In many ways a black hole acts like an ideal black body, as it reflects no light.
Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass.
This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe. ...

... Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle.
After a black hole has formed, it can continue to grow by absorbing mass from its surroundings.
By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form.
There is general consensus that supermassive black holes exist in the centers of most galaxies.

For more information, see Black Hole.

Barium Star From Wikipedia:
Barium stars are spectral class G to K giants, whose spectra indicate an overabundance of s-process elements by the presence of singly ionized barium, Ba II, at wavelength 455.4 nm.
Barium stars also show enhanced spectral features of carbon, the bands of the molecules CH, CN and C<sub>2<\sub>.
The class was originally recognized and defined by William P. Bidelman and Philip Keenan.

Observational studies of their radial velocity suggested that all barium stars are binary stars.
Observations in the ultraviolet using International Ultraviolet Explorer detected white dwarfs in some barium star systems.

Barium stars are believed to be the result of mass transfer in a binary star system.
The mass transfer occurred when the now-observed giant star was on the main sequence.
Its companion, the donor star, was a carbon star on the asymptotic giant branch (AGB), and had produced carbon and s-process elements in its interior.
These nuclear fusion products were mixed by convection to its surface.
Some of that matter "polluted" the surface layers of the main-sequence star as the donor star lost mass at the end of its AGB evolution, and it subsequently evolved to become a white dwarf.
These systems are being observed at an indeterminate amount of time after the mass transfer event, when the donor star has long been a white dwarf, and the "polluted" recipient star has evolved to become a red giant.

For more information, see Barium Star.</sub>

Carbon Star From Wikipedia:
A carbon star is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance.
There are also some dwarf and supergiant carbon stars, with the more common giant stars sometimes being called classical carbon stars to distinguish them.

In most stars (such as the Sun), the atmosphere is richer in oxygen than carbon.
Ordinary stars not exhibiting the characteristics of carbon stars but cool enough to form carbon monoxide are therefore called oxygen-rich stars.

Carbon stars have quite distinctive spectral characteristics, and they were first recognized by their spectra by Angelo Secchi in the 1860s, a pioneering time in astronomical spectroscopy.

For more information, see Carbon Star.

Mercury-manganese Star From Wikipedia:
A mercury-manganese star is a type of chemically peculiar star with a prominent spectral line at 398.4 nm, due to absorption from ionized mercury.
These stars are of spectral type B8, B9, or A0, corresponding to surface temperatures between about 10,000 and 15,000 K, with two distinctive characteristics:

An atmospheric excess of elements like phosphorus, manganese, gallium, strontium, yttrium, zirconium, platinum and mercury.
A lack of a strong dipole magnetic field.

Their rotation is relatively slow, and as a consequence their atmosphere is relatively calm.
It is thought, but has not been proven, that some types of atoms sink under the force of gravity, while others are lifted towards the exterior of the star by radiation pressure, making a heterogeneous atmosphere.

For more information, see Mercury-manganese Star.

S-type Star From Wikipedia:
An S-type star (or just S star) is a cool giant with approximately equal quantities of carbon and oxygen in its atmosphere.
The class was originally defined in 1922 by Paul Merrill for stars with unusual absorption lines and molecular bands now known to be due to s-process elements. The bands of zirconium monoxide (ZrO) are a defining feature of the S stars.

The carbon stars have more carbon than oxygen in their atmospheres.
In most stars, such as class M giants, the atmosphere is richer in oxygen than carbon and they are referred to as oxygen-rich stars.
S-type stars are intermediate between carbon stars and normal giants.
They can be grouped into two classes: intrinsic S stars, which owe their spectra to convection of fusion products and s-process elements to the surface; and extrinsic S stars, which are formed through mass transfer in a binary system.

The intrinsic S-type stars are on the most luminous portion of the asymptotic giant branch, a stage of their lives lasting less than a million years.
Many are long period variable stars.
The extrinsic S stars are less luminous and longer-lived, often smaller-amplitude semiregular or irregular variables.
S stars are relatively rare, with intrinsic S stars forming less than 10% of asymptotic giant branch stars of comparable luminosity, while extrinsic S stars form an even smaller proportion of all red giants.

For more information, see S-type Star.

Technetium Star From Wikipedia:
A technetium star, or more properly a Tc-rich star, is a star whose stellar spectrum contains absorption lines of the light radioactive metal technetium.
The most stable isotope of technetium is 98Tc with a half-life of 4.2 million years, which is too short a time to allow the metal to be material from before the star's formation.
Therefore, the detection in 1952 of technetium in stellar spectra provided unambiguous proof of nucleosynthesis in stars, one of the more extreme cases being R Geminorum.

Stars containing technetium belong to the class of asymptotic giant branch stars (AGB)—stars that are like red giants, but with a slightly higher luminosity, and which burn hydrogen in an inner shell.
Members of this class of stars switch to helium shell burning with an interval of some 100,000 years, in "dredge-ups".
Technetium stars belong to the classes M, MS, S, SC and C-N.
They are most often variable stars of the long period variable types.

For more information, see Technetium Star.

Mira Variable From Wikipedia:
Mira variables ("Mira", Latin, adj. - feminine form of adjective "wonderful"), named for the prototype star Mira, are a class of pulsating variable stars characterized by very red colours, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitude at visual wavelengths.
They are red giants in the very late stages of stellar evolution, on the asymptotic giant branch, that will expel their outer envelopes as planetary nebulae and become white dwarfs within a few million years.

Mira variables are stars massive enough that they have undergone helium fusion in their cores but are less than two solar masses, stars that have already lost about half their initial mass.
However, they can be thousands of times more luminous than the Sun due to their very large distended envelopes.
They are pulsating due to the entire star expanding and contracting. This produces a change in temperature along with radius, both of which factors cause the variation in luminosity.
The pulsation depends on the mass and radius of the star and there is a well-defined relationship between period and luminosity (and colour).
The very large visual amplitudes are not due to large luminosity changes, but due to a shifting of energy output between infra-red and visual wavelengths as the stars change temperature during their pulsations.

For more information, see Mira Variable.

Variable Star From Wikipedia:
A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.

This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.
Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.

Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.

For more information, see Variable Star.

Formation of the Sun From Wikipedia:
The Sun formed about 4.6 billion years ago from the collapse of part of a giant molecular cloud that consisted mostly of hydrogen and helium and that probably gave birth to many other stars.
This age is estimated using computer models of stellar evolution and through nucleocosmochronology.
The result is consistent with the radiometric date of the oldest Solar System material, at 4.567 billion years ago.
Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes, such as iron-60, that form only in exploding, short-lived stars.
This indicates that one or more supernovae must have occurred near the location where the Sun formed.
A shock wave from a nearby supernova would have triggered the formation of the Sun by compressing the matter within the molecular cloud and causing certain regions to collapse under their own gravity.
As one fragment of the cloud collapsed it also began to rotate because of conservation of angular momentum and heat up with the increasing pressure.
Much of the mass became concentrated in the center, whereas the rest flattened out into a disk that would become the planets and other Solar System bodies.
Gravity and pressure within the core of the cloud generated a lot of heat as it accreted more matter from the surrounding disk, eventually triggering nuclear fusion.
Thus, the Sun was born.

For more information, see Formation of the Sun.

Future of the Sun From Wikipedia:
Once the Sun stops fusing helium in its core and ejects its layers in a planetary nebula in about 8 billion years, it will become a white dwarf and, over trillions of years time, eventually will no longer emit any light.
After that, the Sun will not be visible to the equivalent of the naked human eye, removing it from optical view even if the gravitational effects are evident.
The estimated time for the Sun to cool enough to become a black dwarf is about 10¹⁵ (1 quadrillion) years, though it could take much longer than this, if weakly interacting massive particles (WIMPs) exist, as described above.

For more information, see Future of the Sun.

Stellar Classification From Wikipedia:
In astronomy, stellar classification is the classification of stars based on their spectral characteristics.
Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines.
Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element.
The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences.
The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.

Most stars are currently classified under the Morgan-Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type).
Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. A8, A9, F0, and F1 form a sequence from hotter to cooler).
The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.

For more information, see Stellar Classification.

For more information, see Stellar Evolution. close

The various types of galaxy, and the origins, life and death of galaxies

From Wikipedia:
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.
The word galaxy is derived from the Greek galaxias, literally "milky", a reference to the Milky Way.
Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.

Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular.
Many galaxies are thought to have supermassive black holes at their active centers.
The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun.
As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang.

Active Galaxy From Wikipedia:
A portion of the observable galaxies are classified as active galaxies if the galaxy contains an active galactic nucleus (AGN).
A significant portion of the total energy output from the galaxy is emitted by the active galactic nucleus, instead of the stars, dust and interstellar medium of the galaxy.

The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassive black hole (SMBH) at the core region of the galaxy.
The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc.
In about 10% of these galaxies, a diametrically opposed pair of energetic jets ejects particles from the galaxy core at velocities close to the speed of light.
The mechanism for producing these jets is not well understood.

For more information, see Active Galaxy.

Dark Galaxy From Wikipedia:
A dark galaxy is a hypothesized galaxy with no, or very few, stars.
They received their name because they have no visible stars, but may be detectable if they contain significant amounts of gas.
Astronomers have long theorized the existence of dark galaxies, but there are no confirmed examples to date.
Dark galaxies are distinct from intergalactic gas clouds caused by galactic tidal interactions, since these gas clouds do not contain dark matter, so they do not technically qualify as galaxies.
Distinguishing between intergalactic gas clouds and galaxies is difficult; most candidate dark galaxies turn out to be tidal gas clouds.

For more information, see Dark Galaxy.

Dark Matter Halo From Wikipedia:
A dark matter halo is a hypothetical component of a galaxy that envelops the galactic disc and extends well beyond the edge of the visible galaxy.
The halo's mass dominates the total mass.
Thought to consist of dark matter, halos have not been observed directly.
Their existence is inferred through their effects on the motions of stars and gas in galaxies.
Dark matter halos play a key role in current models of galaxy formation and evolution.
The dark matter halo is not fully explained by the presence of massive compact halo objects (MACHOs).

For more information, see Dark Matter Halo .

Disc Galaxy From Wikipedia:
A disc galaxy is a galaxy characterized by a disc, a flattened circular volume of stars.
These galaxies may or may not include a central non-disc-like region (a galactic bulge).

For more information, see Disc Galaxy .

Early Galaxy Formation From Wikipedia:
The detailed process by which early galaxies formed is an open question in astrophysics.
Theories can be divided into two categories: top-down and bottom-up.
In top-down correlations (such as the Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years.
In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.

Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared within them.
These were composed almost entirely of hydrogen and helium, and may have been massive.
If so, these huge stars would have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium.
This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.

For more information, see Early Galaxy Formation.

Elliptical Galaxy From Wikipedia:
An elliptical galaxy is a type of galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless image.
Unlike flat spiral galaxies with organization and structure, elliptical galaxies are more three-dimensional, without much structure, and their stars are in somewhat random orbits around the center.
They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. ...

... Elliptical galaxies range in size from tens of millions to over one hundred trillion stars. ...

... Every massive elliptical galaxy contains a supermassive black hole at its center.
Observations of 46 elliptical galaxies, 20 classical bulges, and 22 pseudobulges show that each contain a black hole at the center.

For more information, see Elliptical Galaxy.

Galactic Bulge From Wikipedia:
In astronomy, a bulge is a tightly packed group of stars within a larger formation.
The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid).
Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy.
It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.

For more information, see Galactic Bulge .

Galactic Corona From Wikipedia:
The terms galactic corona and gaseous corona have been used in the first decade of the 21st century to describe a hot, ionised, gaseous component in the Galactic halo of the Milky Way.
A similar body of very hot and tenuous gas in the halo of any spiral galaxy may also be described by these terms.

This coronal gas may be sustained by the galactic fountain, in which superbubbles of ionised gas from supernova remnants expand vertically through galactic chimneys into the halo.
As the gas cools, it is pulled back into the galactic disc of the galaxy by gravitational forces.

Galactic coronas have been and are currently being studied extensively, in the hope of gaining a further understanding of galaxy formation.
Although, considering how galaxies differ in shaping and sizing, no particular theory has been able to adequately illustrate how the galaxies in the Universe originally formed.

For more information, see Galactic Corona .

Galactic Disc From Wikipedia:
The Galactic Disc is a component of disc galaxies, such as spiral galaxies and lenticular galaxies.
Galactic discs consist of a stellar component ( composed of most of the galaxy's stars) and a gaseous component (mostly composed of cool gas and dust).
The stellar population of galactic discs tend to exhibit very little random motion with most of its stars undergoing nearly circular orbits about the galactic center.
Discs can be fairly thin because the disc material's motion lies predominantly on the plane of the disc (very little vertical motion).
The Milky Way's disc, for example is approximately 1 kpc thick but thickness can vary for discs in other galaxies.

For more information, see Galactic Disc.

Galactic Halo From Wikipedia:
A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component.
Several distinct components of galaxies comprise the halo:

- the stellar halo
- the galactic corona (hot gas, i.e. a plasma)
- the dark matter halo

The distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the spherical shape of the halo contrasts with the flat disc.
In an elliptical galaxy, there is no sharp transition between the other components of the galaxy and the halo.

For more information, see Galactic Halo.

Globular Cluster From Wikipedia:
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite.
Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers.
The name of this category of star cluster is derived from the Latin globulus—a small sphere.
A globular cluster is sometimes known more simply as a globular.

Globular clusters are found in the halo of a galaxy and contain considerably more stars and are much older than the less dense open clusters, which are found in the disk of a galaxy.
Globular clusters are fairly common; there are about 150 to 158 currently known globular clusters in the Milky Way, with perhaps 10 to 20 more still undiscovered.
Larger galaxies can have more: Andromeda Galaxy, for instance, may have as many as 500.
Some giant elliptical galaxies (particularly those at the centers of galaxy clusters), such as M87, have as many as 13,000 globular clusters.

For more information, see Globular Cluster.

Irregular Galaxy From Wikipedia:
An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy.
Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.

Collectively they are thought to make up about a quarter of all galaxies.
Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force.
Irregular galaxies may contain abundant amounts of gas and dust.
This is not necessarily true for dwarf irregulars.

Irregular galaxies are commonly small, about one tenth the mass of the Milky Way galaxy.
Due to their small sizes, they are prone to environmental effects like crashing with large galaxies and intergalactic clouds.

For more information, see Irregular Galaxy.

Lenticular Galaxy From Wikipedia:
A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes.
They contain large-scale discs but they do not have large-scale spiral arms.
Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation.
They may, however, retain significant dust in their disks.
As a result, they consist mainly of aging stars (like elliptical galaxies).

For more information, see Lenticular Galaxy.

Low-surface-brightness Galaxy From Wikipedia:
A low-surface-brightness galaxy, or LSB galaxy, is a diffuse galaxy with a surface brightness that, when viewed from Earth, is at least one magnitude lower than the ambient night sky.

Most LSBs are dwarf galaxies, and most of their baryonic matter is in the form of neutral gaseous hydrogen, rather than stars.
They appear to have over 95% of their mass as non-baryonic dark matter.
There appears to be no supernova activity in these galaxies.

For more information, see Low-surface-brightness Galaxy.

Luminous Infrared Galaxy From Wikipedia:
Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of brightness, above 10¹¹ solar luminosities.
They are also referred to as submillimeter galaxies (SMGs) through their normal method of detection.
LIRGs are more abundant than starburst galaxies, Seyfert galaxies and quasi-stellar objects at comparable luminosity.
Infrared galaxies emit more energy in the infrared than at all other wavelengths combined.
A LIRG's luminosity is 100 billion times that of our sun.

For more information, see Luminous Infrared Galaxy.

Nebula From Wikipedia:
A nebula (Latin for "cloud" or "fog"; pl. nebulae, nebulæ, or nebulas) is an interstellar cloud of dust, hydrogen, helium and other ionized gases.
Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the Milky Way.
The Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others.

Most nebulae are of vast size, some are hundreds of light years in diameter.
A nebula that is barely visible to the human eye from Earth would appear larger, but no brighter, from close by.
The Orion Nebula, the brightest nebula in the sky that occupies a region twice the diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers.
Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth – a nebular cloud the size of the Earth would have a total mass of only a few kilograms.
Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffuse they can only be detected with long exposures and special filters.
Some nebulae are variably illuminated by T Tauri variable stars.
Nebulae are often star-forming regions, such as in the "Pillars of Creation" in the Eagle Nebula.
In these regions the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter, and eventually will become dense enough to form stars.
The remaining material is then believed to form planets and other planetary system objects.

For more information, see Nebula.

Number of Galaxies From Wikipedia:
There are about one hundred billion galaxies in the universe (see list of nearest galaxies for a complete list), on the order of 100,000 in our Local Supercluster and an estimated number of about one to two trillion in all of the observable universe.

For more information, see Number of Galaxies.

Peculiar Galaxy From Wikipedia:
A peculiar galaxy is a galaxy of unusual size, shape, or composition.
Between five and ten percent of known galaxies are categorized as peculiar.
Astronomers have identified two types of peculiar galaxies: interacting galaxies and active galactic nuclei (AGN).

When two galaxies come close to each other, their mutual gravitational forces can cause them to acquire highly irregular shapes.
The terms 'peculiar galaxy' and 'interacting galaxy' have now become synonymous because the majority of peculiar galaxies attribute their forms to such gravitational forces.

For more information, see Peculiar Galaxy.

Protogalaxy From Wikipedia:
In physical cosmology, a protogalaxy, which could also be called a "primeval galaxy", is a cloud of gas which is forming into a galaxy.
It is believed that the rate of star formation during this period of galactic evolution will determine whether a galaxy is a spiral or elliptical galaxy; a slower star formation tends to produce a spiral galaxy.
The smaller clumps of gas in a protogalaxy form into stars.

For more information, see Protogalaxy.

Quasar From Wikipedia:
A quasar (also known as a QSO or quasi-stellar object) is an extremely luminous active galactic nucleus (AGN).
Most large galaxies contain a supermassive central black hole with mass ranging from millions to billions of Solar masses.
In quasars and other types of AGN, the black hole is surrounded by a gaseous accretion disk.
As gas in the accretion disk falls toward the black hole, energy is released in the form of electromagnetic radiation.
This radiation can be observed across the electromagnetic spectrum at radio, infrared, visible, ultraviolet, and X-ray, and gamma wavelengths.
The power radiated by quasars is enormous: the most powerful quasars have luminosities exceeding 10⁴¹ watts, thousands of times greater than an ordinary large galaxy such as the Milky Way.

For more information, see Quasar.

Radio Galaxy From Wikipedia:
Radio galaxies and their relatives, radio-loud quasars and blazars, are types of active galaxy that are very luminous at radio wavelengths, with luminosities up to 10³⁹ W between 10 MHz and 100 GHz.
The radio emission is due to the synchrotron process.
The observed structure in radio emission is determined by the interaction between twin jets and the external medium, modified by the effects of relativistic beaming.
The host galaxies are almost exclusively large elliptical galaxies.
Radio-loud active galaxies can be detected at large distances, making them valuable tools for observational cosmology.
Recently, much work has been done on the effects of these objects on the intergalactic medium, particularly in galaxy groups and clusters.

For more information, see Radio Galaxy.

Ring Galaxy From Wikipedia:
A ring galaxy is a galaxy with a circle-like appearance.
Hoag's Object, discovered by Art Hoag in 1950, is an example of a ring galaxy.
The ring contains many massive, relatively young blue stars, which are extremely bright.
The central region contains relatively little luminous matter.
Some astronomers believe that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy.
Because most of a galaxy consists of empty space, this "collision" rarely results in any actual collisions between stars.
However, the gravitational disruptions caused by such an event could cause a wave of star formation to move through the larger galaxy.
Other astronomers think that rings are formed around some galaxies when external accretion takes place.
Star formation would then take place in the accreted material because of the shocks and compressions of the accreted material.

For more information, see Ring Galaxy.

Seyfert Galaxy From Wikipedia:
Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars.
They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses whose spectra reveal strong, high-ionisation emission lines, but unlike quasars, their host galaxies are clearly detectable.

Seyfert galaxies account for about 10% of all galaxies and are some of the most intensely studied objects in as they are thought to be powered by the same phenomena that occur in quasars, although they are closer and less luminous than quasars.
These galaxies have supermassive black holes at their centers which are surrounded by accretion discs of in-falling material.
The accretion discs are believed to be the source of the observed ultraviolet radiation.
Ultraviolet emission and absorption lines provide the best diagnostics for the composition of the surrounding material.

For more information, see Seyfert Galaxy.

Spindle Galaxy From Wikipedia:
A Prolate rotator galaxy is an unusual class of galaxy that is cigar shaped and rotates around its long axis.
They are also called Spindle galaxies.
A prolate rotator galaxy is an elliptical galaxy in prolate rotation.
That is they possess a significant amount of rotation around the major axis.
To make a prolate rotator galaxy, two large spiral disk galaxies need to collide at right angles.
One forms the central bar , the other the disk.
The bar then dominates the system.
At 2017 20 such galaxies were known, while another group of researcher found 59 likely objects.

They are not to be confused with lenticular galaxies which are spiral galaxies seen edge on and have sometimes been given the name 'spindle' because of their long thin appearance when viewed from this direction , eg NGC 5866.

For more information, see Spindle Galaxy.

Spiral Galaxies From Wikipedia:
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence.
Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge.
These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc.
The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. ...

Spiral galaxies may consist of several distinct components:

- A flat, rotating disc of stars and interstellar matter of which spiral arms are prominent components
- A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
- A bar-shaped distribution of stars
- A near-spherical halo of stars, including many in globular clusters
- A supermassive black hole at the very center of the central bulge
- A near-spherical dark matter halo

For more information, see Spiral Galaxies.

Starburst Galaxy From Wikipedia:
A starburst galaxy is a galaxy undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies.
For example, the star formation rate of the Milky Way galaxy is approximately 3 solar masses/yr, however, starburst galaxies can experience star formation rates that are more than a factor of 100 times greater.
In a starburst galaxy, the rate of star formation is so large that the galaxy will consume all of its gas reservoir, from which the stars are forming, on a timescale much shorter than the age of the galaxy.
As such, the starburst nature of a galaxy is a phase, and one that typically occupies a brief period of a galaxy's evolution.
The majority of starburst galaxies are in the midst of a merger or close encounter with another galaxy.

For more information, see Starburst Galaxy.

Ultra Diffuse Galaxy From Wikipedia:
An ultra diffuse galaxy (UDG) is an extremely low luminosity galaxy, the first example of which was discovered in the nearby Virgo Cluster by Allan Sandage and Bruno Binggeli in 1984.
Such a galaxy may have the same size and mass as the Milky Way but a visible star count of only 1%.
Their lack of luminosity is due to the lack of star-forming gas in the galaxy.
This results in old stellar populations.

Some ultra diffuse galaxies found in the Coma Cluster, about 330 million light years from Earth, have diameters of 60 kly (18 kpc) (more than half the size of our galaxy) with 1% of the stars of the Milky Way Galaxy.
The distribution of ultra diffuse galaxies in the Coma Cluster is the same as luminous galaxies; this suggests that the cluster environment strips the gas from the galaxies, while allowing them to populate the cluster the same as more luminous galaxies.
The similar distribution in the higher tidal force zones suggests a larger dark matter fraction to hold the galaxies together under the higher stress.

For more information, see Ultra Diffuse Galaxy.

For more information, see Galaxy. close

Other Matter and Energy in the Universe

From Wikipedia:
Dramatic advances in observational cosmology since the 1990s, including the cosmic microwave background, distant supernovae and galaxy redshift surveys, have led to the development of a standard model of cosmology.
This model requires the universe to contain large amounts of dark matter and dark energy whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations.

Dark Matter From Wikipedia:
Dark matter is a hypothetical form of matter that is thought to account for approximately 80% of the matter in the universe, and about a quarter of its total energy density.
The majority of dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet undiscovered subatomic particles.
Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained unless more matter is present than can be seen. For this reason, most experts think dark matter to be ubiquitous in the universe and to have had a strong influence on its structure and evolution.
The name dark matter refers to the fact that it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible (or 'dark') to the entire electromagnetic spectrum, making it extremely difficult to detect using usual astronomical equipment.

For more information, see Dark Matter.

Dark Energy From Wikipedia:
In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.
Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate.

Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68.3% of the total energy in the present-day observable universe.
The mass–energy of dark matter and ordinary (baryonic) matter contribute 26.8% and 4.9%, respectively, and other components such as neutrinos and photons contribute a very small amount.
The density of dark energy (~ 7 × 10^-30 g/cm3) is very low, much less than the density of ordinary matter or dark matter within galaxies.
However, it dominates the mass–energy of the universe because it is uniform across space.

For more information, see Dark Energy.

For more information, see Physical Cosmology. close

The future history of the Universe

From Wikipedia:
The ultimate fate of the universe is a topic in physical cosmology, whose theoretical restrictions allow possible scenarios for the evolution and ultimate fate of the universe to be described and evaluated.
Based on available observational evidence, deciding the fate and evolution of the universe have now become valid cosmological questions, being beyond the mostly untestable constraints of mythological or theological beliefs.
Many possible dark futures have been predicted by rival scientific hypotheses, including that the universe might have existed for a finite and infinite duration, or towards explaining the manner and circumstances of its beginning.

Big Bounce From Wikipedia:
The Big Bounce is a hypothetical cosmological model for the origin of the known universe.
It was originally suggested as a phase of the cyclic model or oscillatory universe interpretation of the Big Bang, where the first cosmological event was the result of the collapse of a previous universe.
It receded from serious consideration in the early 1980s after inflation theory emerged as a solution to the horizon problem, which had arisen from advances in observations revealing the large-scale structure of the universe.
In the early 2000s, inflation was found by some theorists to be problematic and unfalsifiable in that its various parameters could be adjusted to fit any observations, so that the properties of the observable universe are a matter of chance.
An alternative picture including a Big Bounce was conceived as a predictive and falsifiable possible solution to the horizon problem, and has been under active investigation since 2017.

For more information, see Big Bounce.

Big Crunch From Wikipedia:
The Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately causing the cosmic scale factor to reach zero or causing a reformation of the universe starting with another Big Bang.

Recent experimental evidence suggests that the expansion of the universe is not being slowed down by gravity but rather accelerating.
However, since the nature of the dark energy that is postulated to drive the acceleration is unknown, a Big Crunch is still possible, although not observationally supported as of today.

For more information, see Big Crunch.

Big Rip From Wikipedia:
In physical cosmology, the Big Rip is a hypothetical cosmological model concerning the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, and even spacetime itself, is progressively torn apart by the expansion of the universe at a certain time in the future.
According to the hypothesis, first published in 2003, the scale factor of the universe and with it all distances in the universe will become infinite at a finite time in the future.
The possibility of sudden singularities and crunch or rip singularities at late times occur only for hypothetical matter with implausible physical properties.

For more information, see Big Rip.

Cosmic Uncertainty From Wikipedia:
Each possibility described so far is based on a very simple form for the dark energy equation of state.
But as the name is meant to imply, very little is currently known about the physics of dark energy.
If the theory of inflation is true, the universe went through an episode dominated by a different form of dark energy in the first moments of the Big Bang; but inflation ended, indicating an equation of state far more complex than those assumed so far for present-day dark energy.
It is possible that the dark energy equation of state could change again resulting in an event that would have consequences which are extremely difficult to predict or parametrize.
As the nature of dark energy and dark matter remain enigmatic, even hypothetical, the possibilities surrounding their coming role in the universe are currently unknown.

For more information, see Cosmic Uncertainty.

Cyclic Model From Wikipedia:
A cyclic model (or oscillating model) is any of several cosmological models in which the universe follows infinite, or indefinite, self-sustaining cycles.
For example, the oscillating universe theory briefly considered by Albert Einstein in 1930 theorized a universe following an eternal series of oscillations, each beginning with a big bang and ending with a big crunch; in the interim, the universe would expand for a period of time before the gravitational attraction of matter causes it to collapse back in and undergo a bounce.

For more information, see Cyclic Model,
Eternal Return.

False Vacuum From Wikipedia:
In order to best understand the false vacuum collapse theory, one must first understand the Higgs field which permeates the universe.
Much like an electromagnetic field, it varies in strength based upon its potential.
A true vacuum exists so long as the universe exists in its lowest energy state, in which case the false vacuum theory is irrelevant.
However, if the vacuum is not in its lowest energy state (a false vacuum), it could tunnel into a lower energy state.
This is called vacuum decay.
This has the potential to fundamentally alter our universe; in more audacious scenarios even the various physical constants could have different values, severely affecting the foundations of matter, energy, and spacetime.
It is also possible that all structures will be destroyed instantaneously, without any forewarning.
Studies of a particle similar to the Higgs boson support the theory of a false vacuum collapse billions of years from now.

For more information, see False Vacuum.

Heat Death of the Universe From Wikipedia:
The heat death of the universe is a plausible ultimate fate of the universe in which the universe has diminished to a state of no thermodynamic free energy and therefore can no longer sustain processes that increase entropy.
Heat death does not imply any particular absolute temperature; it only requires that temperature differences or other processes may no longer be exploited to perform work.
In the language of physics, this is when the universe reaches thermodynamic equilibrium (maximum entropy).

If the topology of the universe is open or flat, or if dark energy is a positive cosmological constant (both of which are consistent with current data), the universe will continue expanding forever and a heat death is expected to occur, with the universe cooling to approach equilibrium at a very low temperature after a very long time period.

For more information, see Heat Death of the Universe,
Future of an Expanding Universe.

For more information, see
Ultimate Fate of the Universe,
Future of an Expanding Universe,
Timeline of the Far Future. close

For more information, see
Chronology of the universe .

The aim here is to explain how the present physical earth (everything apart from life forms) arose from the time when the material in the earth coalesced from a dispersed cloud of gas and dust in space into the primeval earth around 4.6 billion years ago (as outlined elsewhere, in another module).
This area of information should include explanations of the origin and development of all the major features of the earth, the transitions they went through over time, and how these processes causing these changes continue to shape the earth.

Topics covered should include:

0 close

Formation of the Earth 4.6 billion years ago

The Earth, along with the other planets, formed by accretion of gas and dust in the solar nebula shortly after the formation of the sun.
Early in its history, a period of intense volcanic activity, during which the earth was likely covered in oceans of molten rock, led to the differentiation of core, crust and mantle, and the outgassing of the water oceans and the early atmosphere.

Formation of the Planets From Wikipedia:
The various planets are thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun's formation.
The currently accepted method by which the planets formed is accretion, in which the planets began as dust grains in orbit around the central protostar.
Through direct contact, these grains formed into clumps up to 200 metres in diameter, which in turn collided to form larger bodies (planetesimals) of ~10 kilometres (km) in size.
These gradually increased through further collisions, growing at the rate of centimetres per year over the course of the next few million years.

The inner Solar System, the region of the Solar System inside 4 AU, was too warm for volatile molecules like water and methane to condense, so the planetesimals that formed there could only form from compounds with high melting points, such as metals (like iron, nickel, and aluminium) and rocky silicates.
These rocky bodies would become the terrestrial planets (Mercury, Venus, Earth, and Mars).
These compounds are quite rare in the Universe, comprising only 0.6% of the mass of the nebula, so the terrestrial planets could not grow very large.
The terrestrial embryos grew to about 0.05 Earth masses and ceased accumulating matter about 100,000 years after the formation of the Sun; subsequent collisions and mergers between these planet-sized bodies allowed terrestrial planets to grow to their present sizes. ...

... At the end of the planetary formation epoch the inner Solar System was populated by 50–100 Moon- to Mars-sized planetary embryos.
Further growth was possible only because these bodies collided and merged, which took less than 100 million years.
These objects would have gravitationally interacted with one another, tugging at each other's orbits until they collided, growing larger until the four terrestrial planets we know today took shape.
One such giant collision is thought to have formed the Moon, while another removed the outer envelope of the young Mercury.

For more information, see Formation of the Planets .

Magma Oceans From Wikipedia:
Magma oceans exist during periods of Earth's or any planet's accretion when the planet is completely or partly molten.
In the early solar system, energy to melt objects came largely from the decay of radioactive aluminum-26.
As planets grew larger, the energy was supplied from large or giant impacts.
During its formation, the Earth likely suffered a series of magma oceans resulting from giant impacts, the final one being the Moon-forming impact.

Magma oceans are integral parts of planetary formation as they facilitate formation of a core through metal segregation and an atmosphere and hydrosphere through degassing.
Magma oceans may survive for millions to tens of millions of years, interspersed by relatively clement conditions.

Magma oceans are widely accepted to have existed on Earth, and the best chemical evidence for them is the abundance of certain siderophile elements in the mantle that record magma ocean depths of approximately 1000 km during accretion.
A magma ocean also occurred on the Moon during and following its formation.

For more information, see Magma Oceans .

For more information, see Nebular Hypothesis. close

Formation and History of the Atmosphere

The Earth has had three quite different atmospheres during its 4.6 billion year history.

Earth's Three Atmospheres From Wikipedia:
Earth is often described as having had three atmospheres.
The first atmosphere, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted of these elements compared to cosmic abundances.
After the impact which created the moon, the molten Earth released volatile gases; and later more gases were released by volcanoes, completing a second atmosphere rich in greenhouse gases but poor in oxygen.
Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga.
Note: Ga = 10⁹ years.
For more information, see Earth's Three Atmospheres .

Earliest Atmosphere From Wikipedia:
The first atmosphere would have consisted of gases in the solar nebula, primarily hydrogen.
In addition, there would probably have been simple hydrides such as those now found in gas giants like Jupiter and Saturn, notably water vapor, methane, and ammonia.
As the solar nebula dissipated, the gases would have escaped, partly driven off by the solar wind.

For more information, see Earliest Atmosphere .

Second Atmosphere From Wikipedia:
The next atmosphere, consisting largely of nitrogen, carbon dioxide, and inert gases, was produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids.
A major part of carbon dioxide emissions were soon dissolved in water and built up carbonate sediments.

Water-related sediments have been found dating from as early as 3.8 billion years ago.
About 3.4 billion years ago, nitrogen was the major part of the then stable "second atmosphere".
An influence of life has to be taken into account rather soon in the history of the atmosphere because hints of early life forms have been dated to as early as 3.5 billion years ago.
The fact that it is not perfectly in line with the 30% lower solar radiance (compared to today) of the early Sun has been described as the "faint young Sun paradox".

For more information, see Second Atmosphere .

Third Atmosphere From Wikipedia:
The constant rearrangement of continents by plate tectonics influences the long-term evolution of the atmosphere by transferring carbon dioxide to and from large continental carbonate stores.
Free oxygen did not exist in the atmosphere until about 2.4 billion years ago, during the Great Oxygenation Event, and its appearance is indicated by the end of the banded iron formations.
Until then, any oxygen produced by photosynthesis was consumed by oxidation of reduced materials, notably iron.
Molecules of free oxygen did not start to accumulate in the atmosphere until the rate of production of oxygen began to exceed the availability of reducing materials.
That point was a shift from a reducing atmosphere to an oxidizing atmosphere.
O₂ showed major variations until reaching a steady state of more than 15% by the end of the Precambrian.
The following time span was the Phanerozoic eon, during which oxygen-breathing metazoan life forms began to appear.

The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of 35% during the Carboniferous period, significantly higher than today's 21%.
Two main processes govern changes in the atmosphere: plants use carbon dioxide from the atmosphere, releasing oxygen and the breakdown of pyrite and volcanic eruptions release sulfur into the atmosphere, which oxidizes and hence reduces the amount of oxygen in the atmosphere.
However, volcanic eruptions also release carbon dioxide, which plants can convert to oxygen.
The exact cause of the variation of the amount of oxygen in the atmosphere is not known.
Periods with much oxygen in the atmosphere are associated with rapid development of animals.
Today's atmosphere contains 21% oxygen, which is high enough for rapid development of animals.

For more information, see Third Atmosphere .

For more information, see Evolution of Earth's Atmosphere. close

Formation and History of the Oceans

Many of the rocks from which the original Earth formed contained water in the form of mineral hydrates.
Also, many of the asteroids, comets and other bodies which struck Earth subsequently, were also rich and water.
During planetary differentiation , in the first few hundreds of millions of years shortly after the material that formed the Earth first came together around 4.6 billion years ago, much of the material in the earth was heated into a magma ocean.
Heavier materials sank towards the core, and lighter materials moved towards the surface.
This lighter material would have included not only the lighter rocky material which eventually cooled to form the crust, but also the gases which formed the second atmosphere, and the water which collected in the "hollows" in the crust to form the oceans.
The location and boundaries of the oceans have changed over the billions of years of Earth's history, as the continents have moved slowly due to plate tectonics .

From Wikipedia:
The origin of water on Earth, or the reason that there is clearly more liquid water on Earth than on the other rocky planets of the Solar System, is not completely understood.
There exist numerous more or less mutually compatible hypotheses as to how water may have accumulated on Earth's surface over the past 4.5 billion years in sufficient quantity to form oceans. ...

... Comets, trans-Neptunian objects or water-rich meteoroids (protoplanets) from the outer reaches of the asteroid belt colliding with Earth may have brought water to the world's oceans. ...

Gradual "dehydration melting"—leakage of water stored in hydrate minerals of Earth's rocks—could have formed a portion of its water.

Extraplanetary Sources From Wikipedia:
Comets, trans-Neptunian objects or water-rich meteoroids (protoplanets) from the outer reaches of the asteroid belt colliding with Earth may have brought water to the world's oceans.
Asteroids may have been primarily responsible based on several studies, including measurements of the ratio of the hydrogen isotopes deuterium and protium, since similar percentage impurities as in carbon-rich chondrites were found in oceanic water, whereas previous measurement of the isotopes' concentrations in comets and trans-Neptunian objects correspond only slightly to water on Earth. ...
... Large enough Planetesimals were heated by the decay of aluminium isotope.
This could cause water to rise to the surface. ...
... Recent measurements of the chemical composition of Moon rocks suggest that Earth was born with its water already present.

For more information, see Extraplanetary sources .

Internal Sources From Wikipedia:
Gradual "dehydration melting"—leakage of water stored in hydrate minerals of Earth's rocks—could have formed a portion of its water.
Water may also have come from volcanism: water vapor in the atmosphere that originated in volcanic eruptions may have condensed to form rain, slowly filling Earth's oceanic basins.

For more information, see Internal Sources .

First Oceans There are differing theories about the history of the earth in the first few hundred million years after its formation 4.6 billion years ago.
In particular, there are different theories about how long the period of volcanic activity that eventually led to the formation of the crust lasted.
This in turn has led to different estimates for when water collected in low level regions to form the first oceans.
From Wikipedia:
... Nevertheless, detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, suggesting that the Earth already had oceans or seas at that time. ...

For more information, see Internal Sources .

List of Ancient Oceans From Wikipedia:
This is a list of former oceans that disappeared due to tectonic movements and other geographical and climatic changes.
In alphabetic order:

- Bridge River Ocean, the ocean between the ancient Insular Islands (i.e. Stikinia) and North America
- Cache Creek Ocean, a Paleozoic ocean between the Wrangellia Superterrane and Yukon-Tanana Terrane
- Iapetus Ocean, the southern hemisphere ocean between Baltica and Avalonia
- Khanty Ocean, the Precambrian to Silurian ocean between Baltica and the Siberian continent
- Mezcalera Ocean, the ocean between the Guerrero terrane and Laurentia
- Mirovia, the ocean that surrounded the Rodinia supercontinent
...

For more information, see List of Ancient Oceans .

For more information, see Origin of Water on Earth. close

Formation of the Moon

The Moon is believed to have formed about 4.51 billion years ago.
The prevailing theory is that it formed after a Mars-sized object struck the earth.

Formation of the Moon From Wikipedia:
The Moon formed 4.51 billion years ago, some 60 million years after the origin of the Solar System.
Several forming mechanisms have been proposed, including the fission of the Moon from Earth's crust through centrifugal force (which would require too great an initial spin of Earth), the gravitational capture of a pre-formed Moon (which would require an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon), and the co-formation of Earth and the Moon together in the primordial accretion disk (which does not explain the depletion of metals in the Moon).
These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.

The prevailing hypothesis is that the Earth–Moon system formed after an impact of a Mars-sized body (named Theia) with the proto-Earth (giant impact).
The impact blasted material into Earth's orbit and then the material accreted and formed the Moon.

For more information, see Formation of the Moon .

Giant-impact Hypothesis From Wikipedia:
The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago, in the Hadean eon; about 20 to 100 million years after the solar system coalesced.
The colliding body is sometimes called Theia, from the name of the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.
Analysis of lunar rocks, published in a 2016 report, suggests that the impact may have been a direct hit, causing a thorough mixing of both parent bodies.

For more information, see Giant-impact Hypothesis .

For more information, see Origin of the Moon. close

Formation of the Earth's Crust, Mantle and Core

After the initial accretion of gas and dust to form the early Earth, there followed a period of intense volcanism and many collisions with other bodies in the early solar system.
It was during this time that much of the material in the Earth went through a molten phase.
This led to heavier materials sinking towards the Earth's center to form the core and mantle, and lighter material that eventually solidified to form the Earth's crust rising to the surface.

Early Volcanism From Wikipedia:
Earth was initially molten due to extreme volcanism and frequent collisions with other bodies.
Eventually, the outer layer of the planet cooled to form a solid crust when water began accumulating in the atmosphere.

For more information, see Geological History in the Early Hadeon Eon .

Formation of the Core From Wikipedia:
In early stages of Earth's formation about 4.6 billion years ago, melting would have caused denser substances to sink toward the center in a process called planetary differentiation (see also the iron catastrophe), while less-dense materials would have migrated to the crust.
The core is thus believed to largely be composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials).
Some have argued that the inner core may be in the form of a single iron crystal.

For more information, see Formation of the Core .

Formation of the Mantle From Wikipedia:
The interior of Earth, similar to the other terrestrial planets, is divided into layers of different composition.
The mantle is a layer between the crust and the outer core.
Earth's mantle is a silicate rocky shell with an average thickness of 2,886 kilometres (1,793 mi).
The mantle makes up about 84% of Earth's volume.
It is predominantly solid but in geological time it behaves as a viscous fluid.
The mantle encloses the hot core rich in iron and nickel, which makes up about 15% of Earth's volume.
Past episodes of melting and volcanism at the shallower levels of the mantle have produced a thin crust of crystallized melt products near the surface.

For more information, see Formation of the Mantle .

Formation of the Crust The Earth's crust is quite thin, compared to its radius, and the crust on the surface of the Earth may be compared, relatively speaking, to a postage stamp on the surface of a soccer ball.

From Wikipedia:
Earth formed approximately 4.6 billion years ago from a disk of dust and gas orbiting the newly formed Sun.
It formed via accretion, where planetesimals and other smaller rocky bodies collided and stuck, gradually growing into a planet.
This process generated an enormous amount of heat, which caused early Earth to melt completely.
As planetary accretion slowed, Earth began to cool, forming its first crust, called a primary or primordial crust.
This crust was likely repeatedly destroyed by large impacts, then reformed from the magma ocean left by the impact.
None of Earth's primary crust has survived to today; all was destroyed by erosion, impacts, and plate tectonics over the past several billion years.

Since then, Earth has been forming secondary and tertiary crust.
Secondary crust forms at mid-ocean spreading centers, where partial-melting of the underlying mantle yields basaltic magmas and new ocean crust forms.
This "ridge push" is one of the driving forces of plate tectonics, and it is constantly creating new ocean crust.
That means that old crust must be destroyed somewhere, so, opposite a spreading center, there is usually a subduction zone: a trench where an ocean plate is being shoved back into the mantle.
This constant process of creating new ocean crust and destroying old ocean crust means that the oldest ocean crust on Earth today is only about 200 million years old.

In contrast, the bulk of the continental crust is much older.
The oldest continental crustal rocks on Earth have ages in the range from about 3.7 to 4.28 billion years and have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on other cratonic regions such as those on the Fennoscandian Shield.

For more information, see Formation and Evolution (of the Crust) .

For more information, see Planetary Differentiation. close

Formation of Continents, Continental Drift

The surface of the earth had cooled and solidified into a solid crust sometime between 3.0 and 3.7 billion years ago.
This thin crust (3-5 miles thick under oceans, 20-30 miles thick for the continents) sits on top of the mantle.
The mantle is mostly solid, but behaves like a liquid in geologic time (the temperature ranges from 500 - 900 ℃ near the crust to over 4000 ℃ at the boundary with the core).
The crust is not a single piece but is broken up into several large plates and numerous smaller plates (think of a cracked eggshell).
Convection currents (over geologic time) in the mantle, gravity, the Earth's rotation, and other minor forces, all contribute to cause these plates to move slowly over the Earth.
The speed of plate motion ranges from a typical 10 - 40 metres per thousand years to about 160 metres per thousand years.
The land continents have come together several times in the past to form "supercontinents", and then broke apart again.
"Seafloor spreading", the process whereby convection currents in the mantle bring material from the mantle to the surface along mid-ocean ridges, and then carry this material away from these ridges, means that the oldest ocean crust is about 200 million years old.
In contrast, the average age of the continental crust has been estimated to be around 2 billion years old, with some parts being around 4.3 billion years old.
The oldest areas of the continental crust are in areas termed "cratons".
Where plates collide, a process called "subduction" leads to the edge of one plate being forced under the edge of the other plate, with the underneath plate being slowly carried back down into the mantle.
Other consequences of interactions along plate boundaries include mountain formation, volcanoes, earthquakes and tsunamis.

Convergent Boundary From Wikipedia:
In plate tectonics, a convergent boundary, also known as a destructive plate boundary, is a region of active deformation where two or more tectonic plates or fragments of the lithosphere are near the end of their life cycle.
This is in contrast to a constructive plate boundary (also known as a mid-ocean ridge or spreading center).
As a result of pressure, friction, and plate material melting in the mantle, earthquakes and volcanoes are common near destructive boundaries, where subduction zones or an area of continental collision (depending on the nature of the plates involved) occurs.
The subducting plate in a subduction zone is normally oceanic crust, and moves beneath the other plate, which can be made of either oceanic or continental crust.
During collisions between two continental plates, large mountain ranges, such as the Himalayas are formed.
In other regions, a divergent boundary or transform faults may be present.

For more information, see Convergent Boundary .

Craton From Wikipedia:
A craton (from Greek: kratos "strength") is an old and stable part of the continental lithosphere, where the lithosphere consists of the Earth's two topmost layers, the crust and the uppermost mantle.
Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates.
They are characteristically composed of ancient crystalline basement rock, which may be covered by younger sedimentary rock.
They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into the Earth's mantle.

The term craton is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable.
Cratons can be described as shields, in which the basement rock crops out at the surface, and platforms, in which the basement is overlaid by sediments and sedimentary rock.

For more information, see Craton .

Divergent Boundary From Wikipedia:
In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other.
Divergent boundaries within continents initially produce rifts which eventually become rift valleys.
Most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges.
Divergent boundaries also form volcanic islands which occur when the plates move apart to produce gaps which molten lava rises to fill.

For more information, see Divergent Boundary .

Fracture Zone From Wikipedia:
A fracture zone is a linear oceanic feature—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments.
They are a consequence of plate tectonics.
Lithospheric plates on either side of an active transform fault move in opposite directions; here, strike-slip activity occurs.
Fracture zones extend past the transform faults, away from the ridge axis; seismically inactive (because both plate segments are moving in the same direction), they display evidence of past transform fault activity, primarily in the different ages of the crust on opposite sides of the zone.

For more information, see Fracture Zone .

List of Shields and Cratons Click on the link to see this list. From Wikipedia:
A craton is an ancient part of the Earth's continental crust which has been more or less stable since Precambrian times.
Cratons whose ancient rocks are widely exposed at the surface, often with relatively subdued relief, are known as shields.
If the ancient rocks are largely overlain by a cover of younger rocks then the 'hidden' craton may be referred to as a platform.

For more information, see List of Shields and Cratons .

List of Supercontinents Click on the link to see this list. From Wikipedia:
This is a list of supercontinents.

For more information, see List of Supercontinents .

List of Tectonic Plate Interactions Click on the link to see this list. From Wikipedia:
Tectonic plate interactions are of three different basic types: ...

For more information, see List of Tectonic Plate Interactions .

List of Tectonic Plates From Wikipedia:
This is a list of tectonic plates on the Earth's surface.
Tectonic plates are pieces of Earth's crust and uppermost mantle, together referred to as the lithosphere.
The plates are around 100 km (62 mi) thick and consist of two principal types of material: oceanic crust (also called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium).
The composition of the two types of crust differs markedly, with mafic basaltic rocks dominating oceanic crust, while continental crust consists principally of lower-density felsic granitic rocks.

For more information, see List of Tectonic Plates .

Mid-ocean Ridge From Wikipedia:
A mid-ocean ridge (MOR) is an underwater mountain system formed by plate tectonics.
It consists of various mountains linked in chains, typically having a valley known as a rift running along its spine.
This type of oceanic mountain ridge is characteristic of what is known as an 'oceanic spreading center', which is responsible for seafloor spreading.
The production of new seafloor results from mantle upwelling in response to plate spreading; this isentropic upwelling solid mantle material eventually exceeds the solidus and melts.
The buoyant melt rises as magma at a linear weakness in the oceanic crust, and emerges as lava, creating new crust upon cooling.
A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary.

For more information, see Mid-ocean Ridge .

Oceanic Trench From Wikipedia:
Oceanic trenches are topographic depressions of the sea floor, relatively narrow in width, but very long.
These oceanographic features are the deepest parts of the ocean floor.
Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year.
A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab.
Trenches are generally parallel to a volcanic island arc, and about 200 km (120 mi) from a volcanic arc.
Oceanic trenches typically extend 3 to 4 km (1.9 to 2.5 mi) below the level of the surrounding oceanic floor.
The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 11,034 m (36,201 ft) below sea level.
Oceanic lithosphere moves into trenches at a global rate of about 3 km²/yr.

For more information, see Oceanic Trench .

Platform (geology) From Wikipedia:
In geology, a platform is a continental area covered by relatively flat or gently tilted, mainly sedimentary strata, which overlie a basement of consolidated igneous or metamorphic rocks of an earlier deformation.
Platforms, shields and the basement rocks together constitute cratons.

For more information, see Platform (geology) .

Seafloor Spreading From Wikipedia:
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.

For more information, see Seafloor Spreading .

Shield (geology) From Wikipedia:
A shield is generally a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas.
In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to 3.5 billion years.
They have been little affected by tectonic events following the end of the Precambrian, and are relatively flat regions where mountain building, faulting, and other tectonic processes are greatly diminished compared with the activity that occurs at the margins of the shields and the boundaries between tectonic plates.

For more information, see Shield (geology) .

Subduction From Wikipedia:
Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle.
Regions where this process occurs are known as subduction zones.
Rates of subduction are typically in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries.

Plates include both oceanic crust and continental crust.
Stable subduction zones involve the oceanic lithosphere of one plate sliding beneath the continental or oceanic lithosphere of another plate due to the higher density of the oceanic lithosphere.
That is, the subducted lithosphere is always oceanic while the overriding lithosphere may or may not be oceanic.
Subduction zones are sites that usually have a high rate of volcanism and earthquakes.
Additionally, subduction zones develop belts of deformation in the overriding plate's crust in a processes called orogeny part of which leads to mountain building.

For more information, see Subduction .

Supercontinent Cycle From Wikipedia:
The supercontinent cycle is the quasi-periodic aggregation and dispersal of Earth's continental crust.
There are varying opinions as to whether the amount of continental crust is increasing, decreasing, or staying about the same, but it is agreed that the Earth's crust is constantly being reconfigured.
One complete supercontinent cycle is said to take 300 to 500 million years.
Continental collision makes fewer and larger continents while rifting makes more and smaller continents.

For more information, see Supercontinent Cycle .

Transform Fault From Wikipedia:
A transform fault or transform boundary is a plate boundary where the motion is predominantly horizontal.
It ends abruptly and is connected to another transform, a spreading ridge, or a subduction zone.

Most of these faults are hidden in the deep ocean, where they offset divergent boundaries in short zigzags resulting from seafloor spreading, the best-known (and most destructive) being those on land at the margins of continental tectonic plates.
A transform fault is the only type of strike-slip fault that is classified as a plate boundary.

For more information, see Transform Fault .

For more information, see Plate Tectonics. close

The Covering and Uncovering of the Land by the Sea over Millions of Years

Sea levels change considerably over time.
About 20,000 years ago, during the peak of the most recent ice-age, they were about 130 m lower than they are today.
However, average sea levels over the past 100 million years have been 50 - 100 m higher than they are today.
Sea levels have fluctuated for most of the Earth's 4.6 billion years of history, sometimes rising and partially or totally covering entire continents, and then uncovering them again.

From Wikipedia:
Global or eustatic sea level has fluctuated significantly over the Earth's history.
The main factors affecting sea level are the amount and volume of available water and the shape and volume of the ocean basins.
The primary influences on water volume are the temperature of the seawater, which affects density, and the amounts of water retained in other reservoirs like rivers, aquifers, lakes, glaciers, polar ice caps and sea ice.
Over geological timescales, changes in the shape of the oceanic basins and in land/sea distribution affect sea level.
In addition to eustatic changes, local changes in sea level are caused by tectonic uplift and subsidence.

Over geologic time sea level has fluctuated by hundreds of meters.
Today's interglacial level is near the historic highs of the last 1,000,000 years and is 130 meters above the low level reached during the Last Glacial Maximum 19,000–20,000 years ago.
However, for almost the entirety of the last 100,000,000 years sea levels have been up to 50-100 meters higher than present, mainly due to smaller Antarctic ice caps and the concomitant Antarctic post-glacial rebound.

Observational and modeling studies of mass loss from glaciers and ice caps indicate a contribution to sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century.

Changes through Geologic Time From Wikipedia:
Sea level has changed over geologic time. As the graph shows, sea level today is very near the lowest level ever attained (the lowest level occurred at the Permian-Triassic boundary about 250 million years ago).

During the most recent ice age (at its maximum about 20,000 years ago) the world's sea level was about 130 m lower than today, due to the large amount of sea water that had evaporated and been deposited as snow and ice, mostly in the Laurentide ice sheet.
Most of this had melted by about 10,000 years ago.

Hundreds of similar glacial cycles have occurred throughout the Earth's history.

For more information, see Changes through Geologic Time .

Future Sea Level From Wikipedia:
The rate of global mean sea-level rise (~3 mm/year; SLR) has accelerated compared to the mean of the 20th century (~2 mm/year), but the rate of rise is locally variable.
Factors contributing to SLR include decreased global ice volume and warming of the ocean.
On Greenland, the deficiency between annual ice gained and lost tripled between 1996 and 2007.
On Antarctica the deficiency increased by 75%.
Mountain glaciers are retreating and the cumulative mean thickness change has accelerated from about -1.8 to -4 m in 1965 to 1970 to about -12 to -14 m in the first decade of the 21st century.
From 1961 to 2003, ocean temperatures to a depth of 700 m increased and portions of the deeper ocean are warming.

For more information, see Future Sea Level .

Geological Influences From Wikipedia:
At times during Earth's long history, the configuration of the continents and sea floor has changed due to plate tectonics.
This affects global sea level by altering the depths of various ocean basins and also by altering glacier distribution with resulting changes in glacial-interglacial cycles.
Changes in glacial-interglacial cycles are at least partially affected by changes in glacier distributions across the a Earth.

The depth of the ocean basins is a function of the age of oceanic lithosphere (the tectonic plates beneath the floors of the world's oceans).
As older plates age, they become denser and sink, allowing newer plates to rise and take their place.
Therefore, a configuration with many small oceanic plates that rapidly recycle the oceanic lithosphere would produce shallower ocean basins and (all other things being equal) higher sea levels.
A configuration with fewer plates and more cold, dense oceanic lithosphere, on the other hand, would result in deeper ocean basins and lower sea levels. ...

... Over most of geologic time, the long-term mean sea level has been higher than today (see graph above).
Only at the Permian-Triassic boundary ~250 million years ago was the long-term mean sea level lower than today.
Long term changes in the mean sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term.

During the glacial-interglacial cycles over the past few million years, the mean sea level has varied by somewhat more than a hundred metres.
This is primarily due to the growth and decay of ice sheets (mostly in the northern hemisphere) with water evaporated from the sea.

For more information, see Geological Influences .

Rise in Sea Level Associated with Global Warming From Wikipedia:
A sea level rise is an increase in global mean sea level as a result of an increase in the volume of water in the world’s oceans.
Sea level rise is usually attributed to global climate change by thermal expansion of the water in the oceans and by melting of ice sheets and glaciers on land.
The melting of floating ice shelves and icebergs at sea would raise sea levels only by about 4 cm (1.6 in).

Sea level rise at specific locations may be more or less than the global average.
Local factors might include tectonic effects, subsidence of the land, tides, currents, storms, etc.
Sea level rise is expected to continue for centuries.
Because of long response times for parts of the climate system, it has been estimated that we are already committed to a sea-level rise within the next 2,000 years of approximately 2.3 metres (7.5 ft) for each degree Celsius of temperature rise.

For more information, see Sea Level Rise .

For more information, see Past Sea Level. close

Formation of Mountains, and their Erosion over Time

Mountain formation, primarily due to tectonic plates colliding and interacting at their edges, has been going on for most of the Earth's history.
For example, the Napier Mountains in Antarctica first formed around 3.8 billion years ago.
New mountain chains form as movement of tectonic plates cause plates to collide and the land to uplift along the edges of the collision.
The processes of erosion gradually wear mountain chains down over millions of years.
The youngest mountain range is the Himalayas, which formed when the Indian tectonic plate collided with the Eurasian plate around 50 million years ago, with the subduction of the Indian plate under Eurasian plate.
The Indian plate is continuing to move northwards at about 67 m per thousand years, with the Himalayas continuing to rise at a rate of about 5 m per thousand years.
The movement of the Indian tectonic plate northward will continue for at least another 10 million years.

From Wikipedia:
Mountain formation refers to the geological processes that underlie the formation of mountains.
These processes are associated with large-scale movements of the Earth's crust (tectonic plates).
Folding, faulting, volcanic activity, igneous intrusion and metamorphism can all be parts of the orogenic process of mountain building.
The formation of mountains is not necessarily related to the geological structures found on it.

The understanding of specific landscape features in terms of the underlying tectonic processes is called tectonic geomorphology, and the study of geologically young or ongoing processes is called neotectonics.

Continental Collision From Wikipedia:
Continental collision is a phenomenon of the plate tectonics of Earth that occurs at convergent boundaries.
Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together.
Continental collision is known only to occur on Earth.

Continental collision is not an instantaneous event, but may take several tens of millions of years before the faulting and folding caused by collisions stops.
The collision between India and Asia has been ongoing for about 50 million years already and shows no signs of abating.
Collision between East and West Gondwana to form the East African Orogen took about 100 million years from beginning (610 Ma) to end (510 Ma).
Collision between Gondwana and Laurasia to form Pangea occurred in a relatively brief interval, about 50 million years long.

For more information, see Continental Collision .

Fault Block From Wikipedia:
Fault blocks are very large blocks of rock, sometimes hundreds of kilometres in extent, created by tectonic and localized stresses in the Earth's crust.
Large areas of bedrock are broken up into blocks by faults.
Blocks are characterized by relatively uniform lithology.
The largest of these fault blocks are called crustal blocks.
Large crustal blocks broken off from tectonic plates are called terranes.
Those terranes which are the full thickness of the lithosphere are called microplates.
Continent-sized blocks are called variously microcontinents, continental ribbons, H-blocks, extensional allochthons and outer highs.

Because most stresses relate to the tectonic activity of moving plates, most motion between blocks is horizontal, that is parallel to the Earth's crust by strike-slip faults.
However vertical movement of blocks produces much more dramatic results.
Landforms (mountains, hills, ridges, lakes, valleys, etc.) are sometimes formed when the faults have a large vertical displacement.
Adjacent raised blocks (horsts) and down-dropped blocks (grabens) can form high escarpments.
Often the movement of these blocks is accompanied by tilting, due to compaction or stretching of the crust at that point.

For more information, see Fault Block .

Fault-block Mountains From Wikipedia:
Fault-block mountains often result from rifting, another indicator of tensional tectonic forces.
These can be small or form extensive rift valley systems, such as the East African Rift zone.
Death Valley in California is a smaller example.
There are two types of block mountains; lifted and sloped.

Lifted type block mountains have two steep sides exposing both sides scarps, leading to the horst and graben terrain seen in northern Europe, including the Upper Rhine valley, a graben between two horsts, the Vosges mountains (in France) and Black Forest (in Germany).
Another graben is the basin of the Narmada River in India, between the Vindhya and Satpura horsts.

Tilted type block mountains have one gently sloping side and one steep side with an exposed scarp, and are common in the Basin and Range region of the western United States.

For more information, see Fault-block Mountains .

Fold and Thrust Belt From Wikipedia:
A fold and thrust belt is a series of mountainous foothills adjacent to an orogenic belt, which forms due to contractional tectonics.
Fold and thrust belts commonly form in the forelands adjacent to major orogens as deformation propagates outwards.
Fold and thrust belts usually comprise both folds and thrust faults, commonly interrelated.
They are commonly also known as thrust-and-fold belts, or simply thrust-fold belts.

For more information, see Fold and Thrust Belt .

Fold Mountains From Wikipedia:
Fold mountains are mountains that form mainly by the effects of folding on layers within the upper part of the Earth's crust.
Before either plate tectonic theory developed, or the internal architecture of thrust belts became well understood, the term was used for most mountain belts, such as the Himalayas.
The term is still fairly common in physical geography literature but has otherwise generally fallen out of use except as described below.
The forces responsible for formation of fold mountains are called orogenic movements.
The term orogenic has derived from a Greek word meaning mountain building.
These forces act at tangent to the surface of the earth and are primarily a result of plate tectonics.

For more information, see Fold Mountains .

List of Mountain Ranges Follow the link for this list.
From Wikipedia:
This is a list of mountain ranges on Earth and a few other astronomical bodies.
First, the highest and longest mountain ranges on Earth are listed, followed by more comprehensive alphabetical lists organized by continent.
Ranges in the oceans and on other celestial bodies are listed afterwards.

For more information, see List of Mountain Ranges .

List of Mountain Types From Wikipedia:
Mountains and hills can be characterized in several ways.
Some mountains are volcanoes and can be characterized by the type of lava and eruptive history.
Other mountains are shaped by glacial processes and can be characterized by their shape.
Finally, many mountains can be characterized by the type of rock that make up their composition.

For more information, see List of Mountain Types .

List of Orogenies From Wikipedia:
The following is a list of known orogenies organised by continent, starting with the oldest at the top.
The organization of this article is along present-day continents that do not necessarily reflect the geography contemporary to the orogenies.
Note that some orogenies encompass more than one continent and might have different names in each continent.
Like-wise some very large orogenies include a number of sub-orogenies.
As with other geological phenomena orogenies are often subject to different and changing interpretations regarding to their age, type and associated paleogeography.

For more information, see List of Orogenies .

Mountain From Wikipedia:
A mountain is a large landform that stretches above the surrounding land in a limited area, usually in the form of a peak.
A mountain is generally steeper than a hill.
Mountains are formed through tectonic forces or volcanism.
These forces can locally raise the surface of the earth.
Mountains erode slowly through the action of rivers, weather conditions, and glaciers.
A few mountains are isolated summits, but most occur in huge mountain ranges.

High elevations on mountains produce colder climates than at sea level.
These colder climates strongly affect the ecosystems of mountains: different elevations have different plants and animals.
Because of the less hospitable terrain and climate, mountains tend to be used less for agriculture and more for resource extraction and recreation, such as mountain climbing.

The highest mountain on Earth is Mount Everest in the Himalayas of Asia, whose summit is 8,850 m (29,035 ft) above mean sea level.
The highest known mountain on any planet in the Solar System is Olympus Mons on Mars at 21,171 m (69,459 ft).

For more information, see Mountain .

Orogen Erosion From Wikipedia:
Erosion represents a subsequent phase of the orogenic cycle.
Erosion inevitably removes much of the mountains, exposing the core or mountain roots (metamorphic rocks brought to the surface from a depth of several kilometres).
Isostatic movements may help such exhumation by balancing out the buoyancy of the evolving orogen.
Scholars debate about the extent to which erosion modifies the patterns of tectonic deformation (see erosion and tectonics).
Thus, the final form of the majority of old orogenic belts is a long arcuate strip of crystalline metamorphic rocks sequentially below younger sediments which are thrust atop them and which dip away from the orogenic core.
An orogen may be almost completely eroded away, and only recognizable by studying (old) rocks that bear traces of orogenesis.
Orogens are usually long, thin, arcuate tracts of rock that have a pronounced linear structure resulting in terranes or blocks of deformed rocks, separated generally by suture zones or dipping thrust faults.
These thrust faults carry relatively thin slices of rock (which are called nappes or thrust sheets, and differ from tectonic plates) from the core of the shortening orogen out toward the margins, and are intimately associated with folds and the development of metamorphism.

For more information, see Orogen Erosion .

Orogeny From Wikipedia:
An orogeny is an event that leads to a large structural deformation of the Earth's lithosphere (crust and uppermost mantle) due to the interaction between plate tectonics.
An orogen or orogenic belt develops when a continental plate crumples and is pushed upwards to form one or more mountain ranges; this involves many geological processes collectively called orogenesis.

Orogeny is the primary mechanism by which mountains are built on continents.

For more information, see Orogeny .

Residual Mountains From Wikipedia:
Residual mountains are formed as a result of erosion of an existing elevated area.
They are also called mountains of denudation.

Examples in India include: the Aravalli Range, the Nilgiri Mountains in Tamilnadu, the Rajmahal Hills, and the Eastern and Western Ghats.

For more information, see Residual Mountains .

Seamount From Wikipedia:
A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock.
Seamounts are typically formed from extinct volcanoes that rise abruptly and are usually found rising from the seafloor to 1,000–4,000 m (3,300–13,100 ft) in height.
They are defined by oceanographers as independent features that rise to at least 1,000 m (3,281 ft) above the seafloor, characteristically of conical form.
The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea.
During their evolution over geologic time, the largest seamounts may reach the sea surface where wave action erodes the summit to form a flat surface.
After they have subsided and sunk below the sea surface such flat-top seamounts are called "guyots" or "tablemounts".

A total of 9,951 seamounts and 283 guyots, covering a total of 8,796,150 km² (3,396,210 sq mi) have been mapped but only a few have been studied in detail by scientists.
Seamounts and guyots are most abundant in the North Pacific Ocean, and follow a distinctive evolutionary pattern of eruption, build-up, subsidence and erosion.
In recent years, several active seamounts have been observed, for example Loihi in the Hawaiian Islands.

For more information, see Seamount .

Tectonics From Wikipedia:
Tectonics (from Latin tectonicus; from Ancient Greek (tektonikos), meaning 'pertaining to building') is the process that controls the structure and properties of the Earth's crust and its evolution through time.
In particular, it describes the processes of mountain building, the growth and behavior of the strong, old cores of continents known as cratons, and the ways in which the relatively rigid plates that constitute the Earth's outer shell interact with each other.
Tectonics also provides a framework for understanding the earthquake and volcanic belts that directly affect much of the global population.
Tectonic studies are important as guides for economic geologists searching for fossil fuels and ore deposits of metallic and nonmetallic resources.
An understanding of tectonic principles is essential to geomorphologists to explain erosion patterns and other Earth surface features.

For more information, see Tectonics .

Uplifted Passive Margins From Wikipedia:
Unlike orogenic mountains there is no widely accepted geophysical model that explains elevated passive continental margins such as the Scandinavian Mountains, Eastern Greenland, the Brazilian Highlands or Australia's Great Dividing Range.
Different elevated passive continental margins most likely share the same mechanism of uplift.
This mechanism is possibly related to far-field stresses in Earth’s lithosphere.
According to this view elevated passived margins can be likened to giant anticlinal lithospheric folds, where folding is caused by horizontal compression acting on a thin to thick crust transition zone (as are all passive margins).

For more information, see Uplifted Passive Margins .

Volcanic Mountains From Wikipedia:
Movements of tectonic plates create volcanoes along the plate boundaries, which erupt and form mountains.
A volcanic arc system is a series of volcanoes that form near a subduction zone where the crust of a sinking oceanic plate melts and drags water down with the subducting crust.

Most volcanoes occur in a band encircling the Pacific Ocean (the Pacific Ring of Fire), and in another that extends from the Mediterranean across Asia to join the Pacific band in the Indonesian Archipelago.
The most important types of volcanic mountain are composite cones or stratovolcanoes (Vesuvius, Kilimanjaro and Mount Fuji are examples) and shield volcanoes (such as Mauna Loa on Hawaii, a hotspot volcano).

For more information, see Volcanic Mountains .

For more information, see Mountain Formation. close

Creation of Rocks and Rock Strata

The purpose of this section are to relate the present rocky structure and composition of the Earth's crust with the origin of the Earth, and various long-term dynamical processes that it is undergoing.
Some of the topics that might be covered are listed below.

From Wikipedia:
The three main ways rocks are formed:

- Sedimentary rocks are formed through the gradual accumulation of sediments: for example, sand on a beach or mud on a river bed.
As the sediments are buried they get compacted as more and more material is deposited on top.
Eventually the sediments will become so dense that they would essentially form a rock.
This process is known as lithification.

- Igneous rocks are rocks which have crystallized from a melt or magma.
The melt is made up of various components of pre-existing rocks which have been subjected to melting either at subduction zones or within the Earth's mantle.
The melt is hot and so passes upward through cooler country rock.
As it moves, it cools and various rock types will form through a process known as fractional crystallization.
Igneous rocks can be seen at mid ocean ridges, areas of island arc volcanism or in intra-plate hotspots.

- Metamorphic rocks are rocks which once existed as igneous or sedimentary rocks but have been subjected to varying degrees of pressure and heat within the Earth's crust.
The processes involved will change the composition and fabric of the rock and their original nature is often hard to distinguish.
Metamorphic rocks are typically found in areas of mountain building.

Assise From Wikipedia:
An assise (from the Fr., derived from Latin assidere, "to sit beside"), is a geological term for two or more beds or strata of rock united by the occurrence of the same characteristic species or genera.
In the hierarchy of stratigraphic units, an assise lies between a stage (or sub-stage) and a stratum.

For more information, see Assise .

Basement (geology) From Wikipedia:
In geology, basement and crystalline basement are the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin.
In the same way, the sediments or sedimentary rocks on top of the basement can be called a "cover" or "sedimentary cover". ... ... Basement rock is the thick foundation of ancient, and oldest metamorphic and igneous rock that forms the crust of continents, often in the form of granite.
Basement rock is contrasted to overlying sedimentary rocks which are laid down on top of the basement rocks after the continent was formed, such as sandstone and limestone.
The sedimentary rocks which may be deposited on top of the basement usually form a relatively thin veneer, but can be more than 3 miles thick.
The basement rock of the crust can be 20–30 miles thick, or more.
The basement rock can be located under layers of sedimentary rock, or be visible at the surface.
Basement rock is visible at the bottom of the Grand Canyon, consisting of 1.7-2 billion year old granite (Zoroaster granite) and schist (Vishnu Schist).
The Vishnu Schist is believed to be highly metamorphosed igneous rocks and shale, from basalt, mud and clay laid from volcanic eruptions, and the granite is the result of magma intrusions into the Vishnu schist.
An extensive cross section of sedimentary rocks laid down on top of it through the ages is visible as well.

For more information, see Basement (geology) .

Bed (geology) From Wikipedia:
Beds are the layers of sedimentary rocks that are distinctly different from overlying and underlying subsequent beds of different sedimentary rocks.
Layers of beds are called stratigraphy or strata.
They are formed from sedimentary rocks being deposited on the Earth's solid surface over a long periods of time.
The stratigraphy are layered in the same order that they were deposited, allowing a differentiation of which beds are younger and which ones are older (the Law of Superposition).
The structure of a bed is determined by its bedding plane.
Beds can be differentiated in various ways, including rock or mineral type and particle size.
The term is generally applied to sedimentary strata, but may also be used for volcanic flows or ash layers.

For more information, see Bed (geology) .

Carbonate Rock From Wikipedia:
Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals.
The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3) and dolostone, which is composed of the mineral dolomite (CaMg(CO₃)₂).

Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations.
Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases.

When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures.

Karst topography and caves develop in carbonate rocks because of their solubility in dilute acidic groundwater.
Cooling groundwater or mixing of different groundwaters will also create conditions suitable for cave formation.

Marble is the metamorphic carbonate rock.
Rare igneous carbonate rocks exist as intrusive carbonatites and even rarer volcanic carbonate lava.

For more information, see Carbonate Rock .

Cementation (geology) From Wikipedia:
Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains.
The new pore-filling minerals forms "bridges" between original sediment grains, thereby binding them together.
In this way sand becomes "sandstone", and gravel becomes "conglomerate" or "breccia".
Cementation occurs as part of the diagenesis or lithification of sediments.
Cementation occurs primarily below the water table regardless of sedimentary grain sizes present.
Large volumes of pore water must pass through sediment pores for new mineral cements to crystallize and so millions of years are generally required to complete the cementation process.
Common mineral cements include calcite, quartz or silica phases like cristobalite, iron oxides, and clay minerals, but other mineral cements also occur.

Cementation is continuous in the groundwater zone, so much so that the term "zone of cementation" is sometimes used interchangeably.
Cementation occurs in fissures or other openings of existing rocks and is a dynamic process more or less in equilibrium with a dissolution or dissolving process.

Cement found on the sea floor is commonly aragonite and can take different textural forms.
These textural forms include pendant cement, meniscus cement, isopachous cement, needle cement, botryoidal cement, blocky cement, syntaxial rim cement, and coarse mosaic cement.
The environment in which each of the cements is found depends on the pore space available.
Cements that are found in phreatic zones include: isopachous, blocky, and syntaxial rim cements.
As for calcite cementation, which occurs in meteoric realms (freshwater sources), the cement is produced by the dissolution of less stable aragonite and high-Mg calcite. (Boggs, 2011)

For more information, see Cementation (geology) .

Clastic Rock From Wikipedia:
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock.
A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering.
Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

For more information, see Clastic Rock .

Concretion From Wikipedia:
A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil.
Concretions are often ovoid or spherical in shape, although irregular shapes also occur.
The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'.
Concretions form within layers of sedimentary strata that have already been deposited.
They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock.
This concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum.

For more information, see Concretion .

Conglomerate (geology) From Wikipedia:
Conglomerate is a coarse-grained clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts, e.g., granules, pebbles, cobbles, and boulders, larger than 2 mm (0.079 in) in diameter.
Conglomerates form by the consolidation and lithification of gravel.
Conglomerates typically contain finer grained sediment, e.g., either sand, silt, clay or combination of them, called matrix by geologists, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay.

The size and composition of the gravel-size fraction of a conglomerate may or may not vary in composition, sorting, and size.
In some conglomerates, the gravel-size class consist almost entirely of what were clay clasts at the time of deposition.
Conglomerates can be found in sedimentary rock sequences of all ages but probably make up less than 1 percent by weight of all sedimentary rocks.
In terms of origin and depositional mechanisms, they are closely related to sandstones and exhibit many of the same types of sedimentary structures, e.g., tabular and trough cross-bedding and graded bedding.

For more information, see Conglomerate (geology) .

Cross-bedding From Wikipedia:
In geology, cross-bedding is layering within a stratum and at an angle to the main bedding plane.
The sedimentary structures which result are roughly horizontal units composed of inclined layers.
The original depositional layering is tilted, such tilting not being the result of post-depositional deformation.
Cross-beds or "sets" are the groups of inclined layers, which are known as cross strata.

Cross-bedding forms during deposition on the inclined surfaces of bedforms such as ripples and dunes; it indicates that the depositional environment contained a flowing medium (typically water or wind).
Examples of these bedforms are ripples, dunes, anti-dunes, sand waves, hummocks, bars, and delta slopes.
Environments in which water movement is fast enough and deep enough to develop large-scale bed forms fall into three natural groupings: rivers, tide-dominated coastal and marine settings.

For more information, see Cross-bedding .

Deposition (geology) From Wikipedia:
Deposition is the geological process in which sediments, soil and rocks are added to a landform or land mass.
Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment.

Deposition occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of gravity and friction, creating a resistance to motion; this is known as the null-point hypothesis.
Deposition can also refer to the buildup of sediment from organically derived matter or chemical processes.
For example, chalk is made up partly of the microscopic calcium carbonate skeletons of marine plankton, the deposition of which has induced chemical processes (diagenesis) to deposit further calcium carbonate.
Similarly, the formation of coal begins with deposition of organic material, mainly from plants, in anaerobic conditions.

For more information, see Deposition (geology) .

Diagenesis From Wikipedia:
Diagenesis is the change of sediments or existing sedimentary rocks into a different sedimentary rock during and after rock formation (lithification), at temperatures and pressures less than that required for the formation of metamorphic rocks.
It does not include changes from weathering.
It is any chemical, physical, or biological change undergone by a sediment after its initial deposition, after its lithification.
This process excludes surface alteration (weathering) and metamorphism.
These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture.
There is no sharp boundary between diagenesis and metamorphism, but the latter occurs at higher temperatures and pressures.
Hydrothermal solutions, meteoric groundwater, porosity, permeability, solubility, and time are all influential factors.

After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution.
Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis.
Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization.

For more information, see Diagenesis .

Erosion Before material can be transported and eventually deposited as sediment from which sedimentery rocks will form, it has to be eroded from existing rocks.
From Wikipedia:
In earth science, erosion is the action of surface processes (such as water flow or wind) that remove soil, rock, or dissolved material from one location on the Earth's crust, and then transport it to another location (not to be confused with weathering which involves no movement).
This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, animals, and humans.
In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind (aeolic) erosion, zoogenic erosion, and anthropogenic erosion.
The particulate breakdown of rock or soil into clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by its dissolving into a solvent (typically water), followed by the flow away of that solution.
Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.

Natural rates of erosion are controlled by the action of geological weathering geomorphic drivers, such as rainfall; bedrock wear in rivers; coastal erosion by the sea and waves; glacial plucking, abrasion, and scour; areal flooding; wind abrasion; groundwater processes; and mass movement processes in steep landscapes like landslides and debris flows.
The rates at which such processes act control how fast a surface is eroded.
Typically, physical erosion proceeds fastest on steeply sloping surfaces, and rates may also be sensitive to some climatically-controlled properties including amounts of water supplied (e.g., by rain), storminess, wind speed, wave fetch, or atmospheric temperature (especially for some ice-related processes).
Feedbacks are also possible between rates of erosion and the amount of eroded material that is already carried by, for example, a river or glacier.
Processes of erosion that produce sediment or solutes from a place contrast with those of deposition, which control the arrival and emplacement of material at a new location.

For more information, see Erosion .

Evaporite From Wikipedia:
Evaporite is the term for a water-soluble mineral sediment that results from concentration and crystallization by evaporation from an aqueous solution.
There are two types of evaporite deposits: marine, which can also be described as ocean deposits, and non-marine, which are found in standing bodies of water such as lakes.
Evaporites are considered sedimentary rocks and are formed by chemical sediments.

For more information, see Evaporite .

Igneous Rock From Wikipedia:
Igneous rock (derived from the Latin word ignis meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic.
Igneous rock is formed through the cooling and solidification of magma or lava.
The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust.
Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition.
Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks.
Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. ... ... Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume.
Igneous rocks form about 15% of the Earth's current land surface.
Most of the Earth's oceanic crust is made of igneous rock.

For more information, see Igneous Rock .

Iron-rich Sedimentary Rocks From Wikipedia:
Iron-rich sedimentary rocks are sedimentary rocks which contain 15% or more iron.
However, most sedimentary rocks contain iron in varying degrees.
The majority of these rocks were deposited during specific geologic time periods: The Precambrian (3800 to 570 million years ago), the early Paleozoic (570 to 410 million years ago), and the middle to late Mesozoic (205 to 66 million years ago).
Overall, they make up a very small portion of the total sedimentary record.

Iron-rich sedimentary rocks have economic uses as iron ores.
Iron deposits have been located on all major continents with the exception of Antarctica.
They are a major source of iron and are mined for commercial use.
The main iron ores are from the oxide group consisting of hematite, goethite, and magnetite.
The carbonate siderite is also typically mined. A productive belt of iron formations is known as an iron range.

For more information, see Iron-rich Sedimentary Rocks.

Key Bed From Wikipedia:
In geology, a key bed (syn marker bed) is a relatively thin layer of sedimentary rock that is readily recognized on the basis of either its distinct physical characteristics or fossil content and can be mapped over a very large geographic area.
As a result, a key bed is useful for correlating sequences of sedimentary rocks over a large area.
Typically, key beds were created as the result of either instantaneous events or (geologically speaking) very short episodes of the widespread deposition of a specific types of sediment.
As the result, key beds often can be used for both mapping and correlating sedimentary rocks and dating them.
Volcanic ash beds (tonsteins and bentonite beds) and impact spherule beds, and specific megaturbidites are types of key beds created by instantaneous events.
The widespread accumulation of distinctive sediments over a geologically short period of time have created key beds in the form of peat beds, coal beds, shell beds, marine bands, black shales in cyclothems, and oil shales.
A well-known example of a key bed is the global layer of iridium-rich impact ejecta that marks the Cretaceous–Paleogene boundary (K–T boundary).

For more information, see Key Bed .

Lagerstätte From Wikipedia:
A Lagerstätte (German: from Lager 'storage, lair' Stätte 'place'; plural Lagerstätten) is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues.
These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus delaying decomposition.
Lagerstätten span geological time from the Neoproterozoic era to the present.
Worldwide, some of the best examples of near-perfect fossilization are the Cambrian Maotianshan shales and Burgess Shale, the Devonian Hunsrück Slates and Gogo Formation, the Carboniferous Mazon Creek, the Jurassic Solnhofen limestone, the Cretaceous Santana and Yixian formations, and the Eocene Green River Formation.

For more information, see Lagerstätte.

Lamination (geology) From Wikipedia:
In geology, lamination is a small-scale sequence of fine layers (laminae; singular: lamina) that occurs in sedimentary rocks.
Laminae are normally smaller and less pronounced than bedding.
Lamination is often regarded as planar structures one centimetre or less in thickness, whereas bedding layers are greater than one centimetre.
However, structures from several millimetres to many centimetres have been described as laminae.
A single sedimentary rock can have both laminae and beds. ... ... Lamination develops in fine grained sediment when fine grained particles settle, which can only happen in quiet water.
Examples of sedimentary environments are deep marine (at the seafloor) or lacustrine (at the bottom of a lake), or mudflats, where the tide creates cyclic differences in sediment supply.

Laminae formed in glaciolacustrine environments (in glacier lakes) are a special case.
They are called varves.
Quaternary varves are used in stratigraphy and palaeoclimatology to reconstruct climate changes during the last few hundred thousand years.

Lamination in sandstone is often formed in a coastal environment, where wave energy causes a separation between grains of different sizes.

For more information, see Lamination (geology) .

List of Minerals From Wikipedia:
This is a list of minerals for which there are articles on Wikipedia.
Mineral variety names and mineraloids are to be listed after the valid minerals for each letter.

For a complete listing (about 5,000) of all mineral names, see List of minerals (complete).

For more information, see List of Minerals .

List of Rock Formations From Wikipedia:
A rock formation is an isolated, scenic, or spectacular surface rock outcrop.
Rock formations are usually the result of weathering and erosion sculpting the existing rock.
The term 'rock formation' can also refer to specific sedimentary strata or other rock unit in stratigraphic and petrologic studies.

A rock structure can be created in any rock type or combination:

Sedimentary rocks are created by a variety of processes but usually involving deposition, grain by grain, layer by layer, in water or, in the case of terrestrial sediments, on land through the action of wind or sometimes moving ice.
Erosion later exposes them in their current form. An example is Bryce Canyon National Park in Utah.

Metamorphic rocks are created by rocks that have been transformed into another kind of rock, usually by some combination of heat, pressure and chemical alteration. An example are the Seneca Rocks in West Virginia.

Igneous rocks are created when molten rock cools and solidifies, with or without crystallization.
They may be either plutonic bodies or volcanic extrusives.
Again, erosive forces sculpt their current forms.
An example is Half Dome in Yosemite National Park.

Geologists have created a number of terms to describe different rock structures in the landscape that can be formed by natural processes:

Inselberg or monadnock is a residual relief feature. It can be an isolated hill, a knob, ridge or small mountain that rises abruptly, like an island, from a gently sloping or virtually level surrounding plain. Inselberg is a German word that means "island mountain". Uluru/Ayers Rock in Australia is a noted example.
Peak
Stack
Mesa
Butte
Escarpment
Gorge
Sea cliff
River cliff
Stone run
Crag
Tor
Promontory is a prominent mass of land which overlooks lower lying land or a body of water.

Here is a list of rock formations by continent.

For more information, see List of Rock Formations .

List of Rock Types From Wikipedia:
The following is a list of rock types recognized by petrologists.
There is no agreed number of specific types of rocks.
Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can describe rock types.
Additionally, different classification systems exist for each major type of rock.
There are three major types of rock, sedimentary, metamorphic, and igneous.
They are all identified by their texture, streak, and location, among other factors.

For more information, see List of Rock Types .

Lithification From Wikipedia:
Lithification (from the Ancient Greek word lithos meaning 'rock' and the Latin-derived suffix -ific) is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock.
Essentially, lithification is a process of porosity destruction through compaction and cementation.
Lithification includes all the processes which convert unconsolidated sediments into sedimentary rocks.
Petrifaction, though often used as a synonym, is more specifically used to describe the replacement of organic material by silica in the formation of fossils.

For more information, see Lithification .

Metamorphic Rock From Wikipedia:
Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form".
The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (100 megapascals (1,000 bar) or more), causing profound physical or chemical change.
The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface.
They are classified by texture and by chemical and mineral assemblage (metamorphic facies).
They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it.
They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion.
They are also formed when rock is heated by the intrusion of hot molten rock called magma from the Earth's interior.
The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides information about the temperatures and pressures that occur at great depths within the Earth's crust.
Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.

For more information, see Metamorphic Rock .

Mineral From Wikipedia:
A mineral is a naturally occurring chemical compound, usually of crystalline form and not produced by life processes.
A mineral has one specific chemical composition, whereas a rock can be an aggregate of different minerals or mineraloids.
The study of minerals is called mineralogy.

As of March 2018, there are more than 5,500 known mineral species; 5,312 of these have been approved by the International Mineralogical Association (IMA).

Minerals are distinguished by various chemical and physical properties.
Differences in chemical composition and crystal structure distinguish the various species, which were determined by the mineral's geological environment when formed.
Changes in the temperature, pressure, or bulk composition of a rock mass cause changes in its minerals.
Within a mineral species there may be variation in physical properties or minor amounts of impurities that are recognized by mineralogists or wider society as a mineral variety, for example amethyst, a purple variety of the mineral species quartz.

For more information, see Mineral .

Mudrock From Wikipedia:
Mudrocks are a class of fine grained siliciclastic sedimentary rocks.
The varying types of mudrocks include: siltstone, claystone, mudstone, slate, and shale.
Most of the particles of which the stone is composed are less than 0.0625 mm (1/16th mm or 0.0025 inches) and are too small to study readily in the field.
At first sight the rock types look quite similar; however, there are important differences in composition and nomenclature.
There has been a great deal of disagreement involving the classification of mudrocks.
There are a few important hurdles to classification, including:

1. Mudrocks are the least understood, and one of the most understudied sedimentary rocks to date
2. It is difficult to study mudrock constituents, due to their diminutive size and susceptibility to weathering on outcrops
3. And most importantly, there is more than one classification scheme accepted by scientists

Mudrocks make up fifty percent of the sedimentary rocks in the geologic record, and are easily the most widespread deposits on Earth.
Fine sediment is the most abundant product of erosion, and these sediments contribute to the overall omnipresence of mudrocks.
With increased pressure over time the platey clay minerals may become aligned, with the appearance of parallel layering (fissility).
This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone.
The lack of fissility or layering in mudstone may be due either to the original texture or to the disruption of layering by burrowing organisms in the sediment prior to lithification.

For more information, see Mudrock .

Organic-rich Sedimentary Rocks From Wikipedia:
Organic-rich sedimentary rocks are a specific type of sedimentary rock that contains significant amounts (>3%) of organic carbon.
The most common types include coal, lignite, oil shale, or black shale.
The organic material may be disseminated throughout the rock giving it a uniform dark color, and/or may be present as discrete occurrences of tar, bitumen, asphalt, petroleum, coal or carbonaceous material.
Organic-rich sedimentary rocks may act as source rocks which generate hydrocarbons that accumulate in other sedimentary "reservoir" rocks (see oil sands and petroleum geology).
Potential source rocks are any type of sedimentary rock that the ability to dispel available carbon from within it (limestone is a classic example of a source rock).
Good reservoir rocks are any sedimentary rock that has high pore-space availability.
This allows the hydrocarbons to accumulate within the rock and be stored for long periods of time (a sandstone commonly makes a good source rock).
Highly permeable reservoir rocks are also of interest to industry professionals, as they allow for the easy extraction of the hydrocarbons within.
The hydrocarbon reservoir system is not complete however without a "cap rock".
Cap rocks are rock units which have very low porosity and permeability, which trap the hydrocarbons within the units below as they try to migrate upwards.

For more information, see Organic-rich Sedimentary Rocks .

Petrifaction From Wikipedia:
In geology, petrifaction or petrification is the process by which organic material becomes a fossil through the replacement of the original material and the filling of the original pore spaces with minerals.
Petrified wood typifies this process, but all organisms, from bacteria to vertebrates, can become petrified (although harder, more durable matter such as bone, beaks, and shells survive the process better than softer remains such as muscle tissue, feathers, or skin).
Petrifaction takes place through a combination of two similar processes: permineralization and replacement.
These processes create replicas of the original specimen that are similar down to the microscopic level.

For more information, see Petrifaction .

Rock Cycle From Wikipedia:
The rock cycle is a basic concept in geology that describes the time-consuming transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous.
As the adjacent diagram illustrates, each of the types of rocks is altered or destroyed when it is forced out of its equilibrium conditions.
An igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent.
Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments.
The rock cycle is an illustration that explains how the three rock types are related to each other, and how processes change from one type to another over time.
This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

For more information, see Rock Cycle .

Sandstone From Wikipedia:
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) mineral particles or rock fragments.

Most sandstone is composed of quartz or feldspar (both silicates) because they are the most resistant minerals to weathering processes at the Earth's surface, as seen in Bowen's reaction series.
Like uncemented sand, sandstone may be any color due to impurities within the minerals, but the most common colors are tan, brown, yellow, red, grey, pink, white, and black.
Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone have been strongly identified with certain regions.

Rock formations that are primarily composed of sandstone usually allow the percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs.
Fine-grained aquifers, such as sandstones, are better able to filter out pollutants from the surface than are rocks with cracks and crevices, such as limestone or other rocks fractured by seismic activity.

Quartz-bearing sandstone can be converted into quartzite through metamorphism, usually related to tectonic compression within orogenic belts.

For more information, see Sandstone .

Sediment From Wikipedia:
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice, and/or by the force of gravity acting on the particles.
For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation and if buried, may eventually become sandstone and siltstone (sedimentary rocks).

Sediments are most often transported by water (fluvial processes), but also wind (aeolian processes) and glaciers.
Beach sands and river channel deposits are examples of fluvial transport and deposition, though sediment also often settles out of slow-moving or standing water in lakes and oceans.
Desert sand dunes and loess are examples of aeolian transport and deposition.
Glacial moraine deposits and till are ice-transported sediments.

For more information, see Sediment .

Sedimentary Rock From Wikipedia:
Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the Earth's surface and within bodies of water.
Sedimentation is the collective name for processes that cause mineral or organic particles (detritus) to settle in place.
The particles that form a sedimentary rock by accumulating are called sediment.
Before being deposited, the sediment was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation.
Sedimentation may also occur as minerals precipitate from water solution or shells of aquatic creatures settle out of suspension.

The sedimentary rock cover of the continents of the Earth's crust is extensive (73% of the Earth's current land surface), but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust.
Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.
Sedimentary rocks are deposited in layers as strata, forming a structure called bedding.

For more information, see Sedimentary Rock .

Sedimentary Structures From Wikipedia:
Sedimentary structures include all kinds of features formed at the time of deposition.
Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different particle sizes are deposited on top of each other.
These beds range from millimeters to centimeters thick and can even go to meters or multiple meters thick.

Sedimentary structures such as cross bedding, graded bedding and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terrains and understand the depositional environment of the sediment.


Contents
1 Flow structures
2 Bedforms vs. flow
3 Ripple marks
4 Antidunes
5 Biological structures
6 Soft sediment deformation structures
7 Bedding plane structures
8 Within bedding structures
9 Secondary sedimentary structures

For more information, see Sedimentary Structures .

Sediment Transport The material that settles as sediment and eventually forms a layer of sedimentary rock is transported by various mechanisms from the region where it is eroded to the area where it eventually settles.
From Wikipedia:
Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained.
Sediment transport occurs in natural systems where the particles are clastic rocks (sand, gravel, boulders, etc.), mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting.
Sediment transport due to fluid motion occurs in rivers, oceans, lakes, seas, and other bodies of water due to currents and tides.
Transport is also caused by glaciers as they flow, and on terrestrial surfaces under the influence of wind.
Sediment transport due only to gravity can occur on sloping surfaces in general, including hillslopes, scarps, cliffs, and the continental shelf—continental slope boundary.

For more information, see Sediment Transport .

Siliceous Rock Siliceous rocks are sedimentary rocks that have silica (SiOsub>2) as the principal constituent.
The most common siliceous rock is chert; other types include diatomite.
They commonly form from silica-secreting organisms such as radiolarians, diatoms, or some types of sponges.

For more information, see Siliceous Rock .

Speleothem Speleothems include various natural formations found in caves, such as stalactites and stalagmites.
From Wikipedia:
Speleothems (Ancient Greek: "cave deposit"), commonly known as cave formations , are secondary mineral deposits formed in a cave.
Speleothems typically form in limestone or dolostone solutional caves.
The term "speleothem" as first introduced by Moore (1952), is derived from the Greek words spelaion "cave" + théma "deposit".
The definition of "speleothem" in most publications, specifically excludes secondary mineral deposits in mines, tunnels and on man-made structures. ...

... 319 variations of cave mineral deposits have been identified.
The vast majority of speleothems are calcareous, composed of calcium carbonate in the form of calcite or aragonite, or calcium sulfate in the form of gypsum.
Calcareous speleothems form via carbonate dissolution reactions.
Rainwater in the soil zone reacts with soil CO₂ to create weakly acidic water via the reaction:

H₂O + CO₂ → H₂CO₃

As the lower pH water travels through the calcium carbonate bedrock from the surface to the cave ceiling, it dissolves the bedrock via the reaction:

CaCO₃ + H₂CO₃ → Ca²⁺ + 2 HCO₃^-

When the solution reaches a cave, degassing due to lower cave pCO₂ drives precipitation of CaCO₃:

Ca²⁺ + 2 HCO₃^- → CaCO₃ + H₂O + CO₂

Over time the accumulation of these precipitates form stalagmites, stalactites, and flowstones, which compose the major categories of speleothems.

For more information, see Speleothem .

Stratum In geology and related fields, a stratum (plural: strata) is a layer of sedimentary rock or soil, or igneous rock that were formed at the Earth's surface, with internally consistent characteristics that distinguish it from other layers.
The "stratum" is the fundamental unit in a stratigraphic column and forms the basis of the study of stratigraphy. ... ... Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural processes.
They may extend over hundreds of thousands of square kilometers of the Earth's surface.
Strata are typically seen as bands of different colored or differently structured material exposed in cliffs, road cuts, quarries, and river banks. Individual bands may vary in thickness from a few millimeters to a kilometer or more.
Each band represents a specific mode of deposition: river silt, beach sand, coal swamp, sand dune, lava bed, etc.

For more information, see Stratum .

For more information, see Formation of Rocks. close

Changing Extent of Snow/Ice Coverage, Past Ice Ages

Large areas of the Earth's surface have been covered in thick sheets of ice, up to 2 miles or more thick, at various times in the past, including two occasions when it is believed that there was a "Snowball Earth", when the Earth's surface was entirely covered in ice.
The earlies ice age began about 2.4 billion years ago, and lasted about 300 million years.
The current ice age began about 2.6 million years ago.
The Earth is presently in an interglacial period (period when the ice cover has retreated) that commenced about 11,700 years ago.
The expansion of the ice cover again will likely not start for at least 50,00 years.

From Wikipedia:
An ice age is a period of long-term reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers.
Within a long-term ice age, individual pulses of cold climate are termed "glacial periods" (or, alternatively, "glacials" or "glaciations", or colloquially "ice ages"), and intermittent warm periods are called "interglacials".

In the terminology of glaciology, ice age implies the presence of extensive ice sheets in both northern and southern hemispheres.
By this definition, we are in an interglacial period—the Holocene—of the ice age.

The ice age began 2.6 million years ago at the start of the Pleistocene epoch, because the Greenland, Arctic, and Antarctic ice sheets still exist. ...

... There have been at least five major ice ages in the Earth's history (the Huronian, Cryogenian, Andean-Saharan, Karoo Ice Age, and the current Quaternary Ice Age).
Outside these ages, the Earth seems to have been ice free even in high latitudes.

• 8.2 Kiloyear Event From Wikipedia:
  In climatology, the 8.2-kiloyear event was a sudden decrease in global temperatures that occurred approximately 8,200 years before the present, or c. 6,200 BC, and which lasted for the next two to four centuries.
  It defines the start of the Northgrippian age in the Holocene epoch.
  Milder than the Younger Dryas cold spell before it but more severe than the Little Ice Age after it, the 8.2-kiloyear cooling was a significant exception to general trends of the Holocene climatic optimum.
  During the event, atmospheric methane concentration decreased by 80 ppb, an emission reduction of 15%, by cooling and drying at a hemispheric scale.
  
  For more information, see 8.2 Kiloyear Event .
• Andean-Saharan Glaciation From Wikipedia:
  The Andean-Saharan glaciation occurred during the Paleozoic from 450 Ma to 420 Ma, during the late Ordovician and the Silurian period.
  For the Ordovician/Saharan part, see the more extensive article on the Late Ordovician glaciation.
  
  According to Eyles and Young, "A major glacial episode at c. 440 Ma, is recorded in Late Ordovician strata (predominantly Ashgillian) in West Africa (Tamadjert Formation of the Sahara), in Morocco (Tindouf Basin) and in west-central Saudi Arabia, all areas at polar latitudes at the time.
  From the Late Ordovician to the Early Silurian the centre of glaciation moved from northern Africa to southwestern South America."
  
  During this period glaciation is known from Arabia, Sahara, West Africa, the south Amazon, and the Andes.
  The center of glaciation migrated from Sahara in the Ordovician (450–440 Ma) to South America in the Silurian (440–420 Ma).
  The maximum extent of glaciation developed in Africa and eastern Brazil.
  
  A minor ice age, the Andean-Saharan was preceded by the Cryogenian ice ages (720–630 Ma, the Sturtian and Marinoan glaciations), often referred to as Snowball Earth, and followed by the Karoo Ice Age (350–260 Ma).
  
  For more information, see Andean-Saharan Glaciation .
• Antarctic Cold Reversal From Wikipedia:
  The Antarctic Cold Reversal (ACR) was an important episode of cooling in the climate history of the Earth during the deglaciation at the close of the last ice age.
  It illustrates the complexity of the climate changes at the transition from the Pleistocene to the Holocene Epochs.
  
  The Last Glacial Maximum and sea-level minimum occurred c. 21,000 years before present (BP).
  Antarctic ice cores show gradual warming beginning 3000 years later.
  At about 14,700 BP, there was a large pulse of meltwater, identified as Meltwater pulse 1A, probably from either the Antarctic ice sheet or the Laurentide ice sheet.
  Meltwater pulse 1A produced a marine transgression that raised global sea level about 20 meters in two to five centuries and is thought to have influenced the start of the Bølling/Allerød interstadial, the major break with glacial cold in the Northern Hemisphere.
  Meltwater pulse 1A was followed in Antarctica and the Southern Hemisphere by a renewed cooling, the Antarctic Cold Reversal, in c. 14,500 BP, which lasted for two millennia — an instance of warming causing cooling.
  The ACR brought an average cooling of perhaps 3 °C.
  The Younger Dryas cooling, in the Northern Hemisphere, began while the Antarctic Cold Reversal was still ongoing, and the ACR ended in the midst of the Younger Dryas.
  
  For more information, see Antarctic Cold Reversal .
• Antarctic Ice Sheet From Wikipedia:
  The Antarctic ice sheet is one of the two polar ice caps of the Earth.
  It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth.
  It covers an area of almost 14 million square kilometres (5.4 million square miles) and contains 26.5 million cubic kilometres (6,400,000 cubic miles) of ice.
  Approximately 61 percent of all fresh water on the Earth is held in the Antarctic ice sheet, an amount equivalent to about 58 m of sea-level rise.
  In East Antarctica, the ice sheet rests on a major land mass, while in West Antarctica the bed can extend to more than 2,500 m below sea level.
  
  In contrast to the melting of the Arctic sea ice, sea ice around Antarctica was expanding as of 2013.
  The reasons for this are not fully understood, but suggestions include the climatic effects on ocean and atmospheric circulation of the ozone hole, and/or cooler ocean surface temperatures as the warming deep waters melt the ice shelves.
  
  For more information, see Antarctic Ice Sheet .
• Antarctic Sea Ice From Wikipedia:
  Antarctic sea ice is the sea ice of the Southern Ocean.
  It extends far north in winter and retreats almost to the coastline each summer.
  Sea ice is frozen seawater that is usually less than a few meters thick.
  This is in contrast to ice shelves, which are formed by glaciers, float in the sea, and are up to a kilometer thick.
  There are two subdivisions of sea ice: fast ice, which is attached to land; and ice floes, which are not.
  
  Sea ice in the Southern Ocean melts from the bottom instead of from the surface like Arctic ice because it is covered in snow.
  As a result, melt ponds are rarely observed.
  On average, Antarctic sea ice is younger, thinner, warmer, saltier, and more mobile than Arctic sea ice.
  Due to its inaccessibility, it is not as well-studied as Arctic ice.
  
  For more information, see Antarctic Sea Ice .
• Arctic Ice Pack From Wikipedia:
  The Arctic ice pack is the sea ice cover of the Arctic Ocean and its vicinity.
  The Arctic ice pack undergoes a regular seasonal cycle in which ice melts in spring and summer, reaches a minimum around mid-September, then increases during fall and winter.
  Summer ice cover in the Arctic is about 50% of winter cover.
  Some of the ice survives from one year to the next. Currently 28% of Arctic basin sea ice is multi-year ice, thicker than seasonal ice: up to 3–4 m (9.8–13.1 ft) thick over large areas, with ridges up to 20 m (65.6 ft) thick.
  As well as the regular seasonal cycle there has been an underlying trend of declining sea ice in the Arctic in recent decades.
  
  For more information, see Arctic Ice Pack .
• Arctic Sea Ice Decline From Wikipedia:
  Arctic sea ice decline is the sea ice loss observed in recent decades in the Arctic Ocean.
  The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report states that greenhouse gas forcing is largely, but not wholly, responsible for the decline in Arctic sea ice extent.
  
  For more information, see Arctic Sea Ice Decline .
• Causes of Ice Ages From Wikipedia:
  The causes of ice ages are not fully understood for either the large-scale ice age periods or the smaller ebb and flow of glacial–interglacial periods within an ice age.
  The consensus is that several factors are important: atmospheric composition, such as the concentrations of carbon dioxide and methane (the specific levels of the previously mentioned gases are now able to be seen with the new ice core samples from EPICA Dome C in Antarctica over the past 800,000 years); changes in the earth's orbit around the Sun known as Milankovitch cycles; the motion of tectonic plates resulting in changes in the relative location and amount of continental and oceanic crust on the earth's surface, which affect wind and ocean currents; variations in solar output; the orbital dynamics of the Earth–Moon system; the impact of relatively large meteorites and volcanism including eruptions of supervolcanoes.
  
  Some of these factors influence each other.
  For example, changes in Earth's atmospheric composition (especially the concentrations of greenhouse gases) may alter the climate, while climate change itself can change the atmospheric composition (for example by changing the rate at which weathering removes CO₂).
  
  For more information, see Causes of Ice Ages .
• Cryogenian Glaciations From Wikipedia:
  The Cryogenian ( from Greek (krýos), meaning "cold" and (génesis), meaning "birth") is a geologic period that lasted from 720 to 635 million years ago.
  It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran.
  
  The Sturtian and Marinoan glaciations occurred during the Cryogenian period, which are the greatest ice ages known to have occurred on Earth.
  These events are the subject of much scientific controversy.
  The main debate contests whether these glaciations covered the entire planet (the so-called "Snowball Earth") or a band of open sea survived near the equator (termed "slushball Earth").
  
  For more information, see Cryogenian .
• Deglaciation From Wikipedia:
  Deglaciation describes the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume (IPCC AR5).
  Thus, it refers to the retreat of a glacier, an ice sheet or frozen surface layer, and the resulting exposure of the Earth's surface.
  The decline of the cryosphere due to ablation can occur on any scale from global to localized to a particular glacier.
  After the Last Glacial Maximum (ca. 21,000 years ago), the last deglaciation begun, which lasted until the early Holocene.
  Around much of Earth, deglaciation during the last 100 years has been accelerating as a result of climate change, partly brought on by anthropogenic changes to greenhouse gases.
  
  The previous deglaciation took place between approximately 22ka until 11.5ka.
  This occurred when there was an annual mean atmospheric temperature on the earth that increased by roughly 5 °C, which was also accompanied by regional high-latitude warming that exceeded 10 °C.
  This was also followed by noteworthy deep-sea and tropical-sea warming, between about 1-2 °C (deep-sea) and 2-4 °C (tropical sea).
  Not only did this warming occur, but the global hydrological budget also experienced noticeable changes and regional precipitation patterns changed.
  As a result of all of this, the world's main ice sheets, including the ones located in Eurasia, North America and parts of the Antarctic melted.
  As a consequence, sea levels rose roughly 120 metres.
  These processes did not occur steadily, and they also did not occur at the same time.
  
  For more information, see Deglaciation .
• Eemian Interglacial From Wikipedia:
  The Eemian (also called the last interglacial, Sangamonian, Ipswichian, Mikulin, Kaydaky, Valdivia or Riss-Würm) was the interglacial period which began about 130,000 years ago and ended about 115,000 years ago.
  It corresponds to Marine Isotope Stage 5e.
  Although sometimes referred to as the "last interglacial" (in the "most recent previous" sense of "last"), it was the second-to-latest interglacial period of the current Ice Age, the most recent being the Holocene which extends to the present day (having followed the last glacial period).
  The prevailing Eemian climate is believed to have been warmer than that of the Holocene.
  
  The Eemian is known as the Ipswichian in the UK, the Mikulin interglacial in Russia, the Valdivia interglacial in Chile and the Riss-Würm interglacial in the Alps.
  Depending on how a specific publication defines the Sangamonian Stage of North America, the Eemian is equivalent to either all or part of it.
  
  For more information, see Eemian .
• Glacial Period From Wikipedia:
  A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances.
  Interglacials, on the other hand, are periods of warmer climate between glacial periods.
  The last glacial period ended about 15,000 years ago.
  The Holocene epoch is the current interglacial.
  A time when there are no glaciers on Earth is considered a greenhouse climate state.
  
  For more information, see Glacial Period .
• Glacier From Wikipedia:
  A glacier is a persistent body of dense ice that is constantly moving under its own weight; it forms where the accumulation of snow exceeds its ablation (melting and sublimation) over many years, often centuries.
  Glaciers slowly deform and flow due to stresses induced by their weight, creating crevasses, seracs, and other distinguishing features.
  They also abrade rock and debris from their substrate to create landforms such as cirques and moraines.
  Glaciers form only on land and are distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.
  
  On Earth, 99% of glacial ice is contained within vast ice sheets (also known as "continental glaciers") in the polar regions, but glaciers may be found in mountain ranges on every continent including Oceania's high-latitude oceanic islands such as New Zealand and Papua New Guinea.
  Between 35°N and 35°S, glaciers occur only in the Himalayas, Andes, Rocky Mountains, a few high mountains in East Africa, Mexico, New Guinea and on Zard Kuh in Iran.
  Glaciers cover about 10 percent of Earth's land surface.
  Continental glaciers cover nearly 13,000,000 km2 (5×10⁶ sq mi) or about 98 percent of Antarctica's 13,200,000 km² (5.1×10<sup>6/ sq mi), with an average thickness of 2,100 m (7,000 ft).
  Greenland and Patagonia also have huge expanses of continental glaciers.
  
  For more information, see Glacier .</sup>
• Greenland Ice Sheet From Wikipedia:
  The Greenland ice sheet (Danish: Grønlands indlandsis, Greenlandic: Sermersuaq) is a vast body of ice covering 1,710,000 square kilometres (660,000 sq mi), roughly 80% of the surface of Greenland.
  
  It is the second largest ice body in the world, after the Antarctic ice sheet.
  The ice sheet is almost 2,400 kilometres (1,500 mi) long in a north-south direction, and its greatest width is 1,100 kilometres (680 mi) at a latitude of 77°N, near its northern margin.
  The mean altitude of the ice is 2,135 metres (7,005 ft).
  The thickness is generally more than 2 km (1.2 mi) and over 3 km (1.9 mi) at its thickest point.
  It is not the only ice mass of Greenland - isolated glaciers and small ice caps cover between 76,000 and 100,000 square kilometres (29,000 and 39,000 sq mi) around the periphery.
  If the entire 2,850,000 cubic kilometres (684,000 cu mi) of ice were to melt, it would lead to a global sea level rise of 7.2 m (24 ft).
  The Greenland Ice Sheet is sometimes referred to under the term inland ice, or its Danish equivalent, indlandsis.
  It is also sometimes referred to as an ice cap.
  
  For more information, see Greenland Ice Sheet .
• Holocene Climatic Optimum From Wikipedia:
  The Holocene Climate Optimum (HCO) was a warm period during roughly the interval 9,000 to 5,000 years BP.
  It has also been known by many other names, such as Altithermal, Climatic Optimum, Holocene Megathermal, Holocene Optimum, Holocene Thermal Maximum, Hypsithermal, and Mid-Holocene Warm Period. ...
  
  ... The Holocene Climate Optimum warm event consisted of increases of up to 4 °C near the North Pole (in one study, winter warming of 3 to 9 °C and summer of 2 to 6 °C in northern central Siberia).
  Northwestern Europe experienced warming, but there was cooling in Southern Europe.
  The average temperature change appears to have declined rapidly with latitude and so essentially no change in mean temperature is reported at low and middle latitudes.
  Tropical reefs tend to show temperature increases of less than 1 °C; the tropical ocean surface at the Great Barrier Reef about 5350 years ago was 1 °C warmer and enriched in 18O by 0.5 per mil relative to modern seawater.
  In terms of the global average, temperatures were probably warmer than now (depending on estimates of latitude dependence and seasonality in response patterns).
  
  This warm period was followed by a gradual decline until about two millennia ago.
  
  For more information, see Holocene Climatic Optimum .
• Holocene Glacial Retreat From Wikipedia:
  The Holocene glacial retreat is a geographical phenomenon that involved the global deglaciation of glaciers that previously had advanced during the Last Glacial Maximum.
  Ice sheet retreat initiated ca. 19,000 years ago and accelerated after ca. 15,000 years ago.
  The Holocene, starting with abrupt warming 11,700 years ago, resulted in rapid melting of the remaining ice sheets of North America and Europe.
  The retreat of glaciers altered landscapes in many ways and is currently still acting as a result of climate change.
  
  For more information, see Holocene Glacial Retreat .
• Huronian Glaciation From Wikipedia:
  The Huronian glaciation (or Makganyene glaciation) was a glaciation that extended from 2.4 billion years ago (Ga) to 2.1 Ga, during the Siderian and Rhyacian periods of the Paleoproterozoic era.
  The Huronian glaciation followed the Great Oxygenation Event (GOE), a time when increased atmospheric oxygen decreased atmospheric methane.
  The oxygen combined with the methane to form carbon dioxide and water, which do not retain heat as well as methane does.
  
  It is the oldest and longest ice age, occurring at a time when only simple, unicellular life existed on Earth.
  This ice age led to a mass extinction on Earth.
  
  For more information, see Huronian Glaciation .
• Ice Shelf From Wikipedia:
  An ice shelf is a thick floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface.
  Ice shelves are only found in Antarctica, Greenland, Canada, and the Russian Arctic.
  The boundary between the floating ice shelf and the anchor ice (resting on bedrock) that feeds it is called the grounding line.
  The thickness of ice shelves can range from about 100 m (330 ft) to 1,000 m (3,300 ft).
  
  In contrast, sea ice is formed on water, is much thinner (typically less than 3 m (9.8 ft)), and forms throughout the Arctic Ocean.
  It also is found in the Southern Ocean around the continent of Antarctica.
  
  For more information, see Ice Shelf .
• Ice Sheet From Wikipedia:
  An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² (19,000 sq mi), this is also known as continental glacier.
  The only current ice sheets are in Antarctica and Greenland; during the last glacial period at Last Glacial Maximum (LGM) the Laurentide ice sheet covered much of North America, the Weichselian ice sheet covered northern Europe and the Patagonian Ice Sheet covered southern South America.
  
  Ice sheets are bigger than ice shelves or alpine glaciers.
  Masses of ice covering less than 50,000 km² are termed an ice cap.
  An ice cap will typically feed a series of glaciers around its periphery.
  
  Although the surface is cold, the base of an ice sheet is generally warmer due to geothermal heat.
  In places, melting occurs and the melt-water lubricates the ice sheet so that it flows more rapidly.
  This process produces fast-flowing channels in the ice sheet — these are ice streams.
  
  For more information, see Ice Sheet .
• Ice Sheet Dynamics From Wikipedia:
  Ice sheet dynamics describe the motion within large bodies of ice, such those currently on Greenland and Antarctica.
  Ice motion is dominated by the movement of glaciers, whose gravity-driven activity is controlled by two main variable factors: the temperature and strength of their bases.
  A number of processes alter these two factors, resulting in cyclic surges of activity interspersed with longer periods of inactivity, on both hourly and centennial time scales.
  Ice-sheet dynamics are of interest in modelling future sea level rise. ...
  
  ... The main cause of flow within glaciers can be attributed to an increase in the surface slope, brought upon by an imbalance between the amounts of accumulation vs. ablation.
  This imbalance increases the shear stress on a glacier until it begins to flow.
  The flow velocity and deformation will increase as the equilibrium line between these two processes is approached, but are also affected by the slope of the ice, the ice thickness and temperature.
  
  For more information, see Ice Sheet Dynamics.
• Interglacial From Wikipedia:
  An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age.
  The current Holocene interglacial began at the end of the Pleistocene, about 11,700 years ago. ...
  
  ... During the 2.5 million year span of the Pleistocene, numerous glacials, or significant advances of continental ice sheets in North America and Europe, have occurred at intervals of approximately 40,000 to 100,000 years.
  These long glacial periods were separated by more temperate and shorter interglacials.
  
  During interglacials, such as the present one, the climate warms and the tundra recedes polewards following the ice sheets.
  Forests return to areas that once supported tundra vegetation.
  Interglacials are identified on land or in shallow epicontinental seas by their paleontology.
  
  For more information, see Interglacial.
• Karoo Ice Age From Wikipedia:
  The Karoo Ice Age, more commonly referred to as the Late Paleozoic Ice Age, from 360–260 million years ago (Mya) was the second major ice age of the Phanerozoic Eon.
  It is named after the tillite (Dwyka Group) found in the Karoo Basin of South Africa, where evidence for this ice age was first clearly identified in the 19th century.
  
  The tectonic assembly of the continents of Euramerica (later with the Uralian orogeny, into Laurasia) and Gondwana into Pangaea, in the Hercynian-Alleghany Orogeny, made a major continental land mass within the Antarctic region, and the closure of the Rheic Ocean and Iapetus Ocean saw disruption of warm-water currents in the Panthalassa Ocean and Paleotethys Sea, which led to progressive cooling of summers, and the snowfields accumulating in winters, causing mountainous alpine glaciers to grow, and then spread out of highland areas, making continental glaciers which spread to cover much of Gondwana.
  
  At least two major periods of glaciation have been discovered:
  
  - The first glacial period was associated with the Mississippian subperiod (359.2–318.1 Mya): ice sheets expanded from a core in southern Africa and South America.
  - The second glacial period was associated with the Pennsylvanian subperiod (318.1–299 Mya); ice sheets expanded from a core in Australia and India.
  
  For more information, see Karoo Ice Age.
• Last Glacial Maximum From Wikipedia:
  In the Earth's climate history the Last Glacial Maximum (LGM) was the last time period during the last glacial period when ice sheets were at their greatest extension.
  Vast ice sheets covered much of North America, northern Europe, and Asia.
  The ice sheets profoundly affected Earth's climate by causing drought, desertification, and a dramatic drop in sea levels.
  Growth of the ice sheets reached its maximum at about 26.5 kBP.
  Deglaciation commenced in the Northern Hemisphere at approximately 20 kBP and in Antarctica approximately at 14.5 kBP, which is consistent with evidence that it was the primary source for an abrupt rise in the sea level at about 14.5 kBP.
  
  The LGM is referred to in Britain as the Dimlington Stadial i.
  It is dated by Nick Ashton to between 31,000 and 16,000 BP.
  
  The LGM was followed by the Late Glacial.
  
  For more information, see Last Glacial Maximum .
• Last Glacial Period From Wikipedia:
  The last glacial period occurred from the end of the Eemian interglacial to the end of the Younger Dryas, encompassing the period c. 115,000 - c. 11,700 years ago.
  This most recent glacial period is part of a larger pattern of glacial and interglacial periods known as the Quaternary glaciation extending from c. 2,588,000 years ago to present.
  
  During this last glacial period there were alternating episodes of glacier advance and retreat.
  Within the last glacial period the Last Glacial Maximum was approximately 22,000 years ago.
  While the general pattern of global cooling and glacier advance was similar, local differences in the development of glacier advance and retreat make it difficult to compare the details from continent to continent (see picture of ice core data below for differences).
  Approximately 13,000 years ago, the Late Glacial Maximum began.
  The end of the Younger Dryas about 11,700 years ago marked the beginning of the Holocene geological epoch, which includes the Holocene glacial retreat.
  
  For more information, see Last Glacial Period .
• Late Glacial Maximum From Wikipedia:
  The Late Glacial climate warming (c. 13,000–10,000 years ago), or Tardiglacial ("Late Glacial"), is defined primarily by the beginning of the modern warm period, in which temperatures in the Northern Hemisphere rose substantially, causing a process of accelerated deglaciation following the Last Glacial Maximum (c. 25,000–13,000 years ago).
  At this time, human populations, previously forced into refuge areas as a result of Last Glacial Maximum (LGM) climatic conditions, gradually begin to repopulate the Northern Hemisphere's Eurasian landmass and eventually populate North America via Beringia for the first time. The Late Glacial Interstadial c.14,670 to c.12,890 BP represents the first pronounced warming since the end of the LGM.
  
  For more information, see Late Glacial Maximum .
• Late Ordovician Glaciation From Wikipedia:
  The Late Ordovician Glaciation is the first part of the Andean-Saharan glaciation.
  It was centered on the Sahara region in late Ordovician, about 440-460 Ma (million years ago).
  The major glaciation during this period is widely considered to be the leading cause of the Ordovician-Silurian extinction event.
  Evidence of this glaciation can be seen in places such as Morocco, South Africa, Libya, and Wyoming.
  More evidence derived from isotopic data is that during the Late Ordovician, Tropical ocean temperatures were about 5 °C cooler than present day, this would have been a major factor that aided in the glaciation process.
  
  The Late Ordovician is the only glacial episode that appears to have coincided with a major mass extinction of nearly 61% of marine life.
  
  Estimates of peak ice sheet volume range from 50 to 250 million cubic kilometers, and its duration from 35 million to less than 1 million years.
  There were also two peaks of glaciation.
  Also, glaciation of the Northern Hemisphere was minimal because a large amount of the land was in the southern hemisphere.
  
  For more information, see Late Ordovician Glaciation .
• Laurentide Ice Sheet From Wikipedia:
  The Laurentide Ice Sheet was a massive sheet of ice that covered millions of square kilometers, including most of Canada and a large portion of the northern United States, multiple times during the Quaternary glacial epochs— from 2.588 ± 0.005 million years ago to the present.
  The last advance covered most of northern North America between c. 95,000 and c. 20,000 years before the present day, and among other geomorphological effects, gouged out the five Great Lakes and the hosts of smaller lakes of the Canadian shield.
  These lakes extend from the eastern Northwest Territories, through most of northern Canada, and the upper Midwestern United States (Minnesota, Wisconsin, and Michigan) to the Finger Lakes, through Lake Champlain and Lake George areas of New York, across the northern Appalachians into and through all of New England and Nova Scotia.
  At times, the ice sheet's southern margin included the present-day sites of northeastern coastal towns and cities such as Portsmouth, New Hampshire, Boston, and New York City, and Great Lakes coastal cities and towns as far south as Chicago and St. Louis, Missouri—and then followed quite precisely the present course of the Missouri River up to the northern slopes of the Cypress Hills, beyond which it merged with the Cordilleran Ice Sheet.
  The ice coverage extended approximately as far south as 38 degrees latitude in the mid-continent. ... ... This ice sheet was the primary feature of the Pleistocene epoch in North America, commonly referred to as the ice age.
  It was up to 2 mi (3.2 km) thick in Nunavik, Quebec, Canada but much thinner at its edges where nunataks were common in hilly areas.
  It created much of the surface geology of southern Canada and the northern United States, leaving behind glacially scoured valleys, moraines, eskers and glacial till.
  It also caused many changes to the shape, size, and drainage of the Great Lakes.
  As but one of many examples, near the end of the last ice age, Lake Iroquois extended well beyond the boundaries of present-day Lake Ontario, and drained down the Hudson River into the Atlantic Ocean.
  
  For more information, see Laurentide Ice Sheet .
• Little Ice Age From Wikipedia:
  The Little Ice Age (LIA) was a period of cooling that occurred after the Medieval Warm Period.
  Although it was not a true ice age, the term was introduced into scientific literature by François E. Matthes in 1939.
  It has been conventionally defined as a period extending from the 16th to the 19th centuries, but some experts prefer an alternative timespan from about 1300 to about 1850.
  Climatologists and historians working with local records no longer expect to agree on either the start or end dates of the period, which varied according to local conditions.
  The NASA Earth Observatory notes three particularly cold intervals: one beginning about 1650, another about 1770, and the last in 1850, all separated by intervals of slight warming.
  
  For more information, see Little Ice Age .
• Major Ice Ages From Wikipedia:
  There have been at least five major ice ages in the Earth's history (the Huronian, Cryogenian, Andean-Saharan, Karoo Ice Age, and the current Quaternary Ice Age).
  Outside these ages, the Earth seems to have been ice free even in high latitudes.
  
  Rocks from the earliest well established ice age, called the Huronian, formed around 2.4 to 2.1 Ga (billion years) ago during the early Proterozoic Eon. ...
  
  ...The Huronian ice age was caused by the elimination of atmospheric methane, a greenhouse gas, during the Great Oxygenation Event.
  
  The next well-documented ice age, and probably the most severe of the last billion years, occurred from 850 to 630 million years ago (the Cryogenian period) and may have produced a Snowball Earth in which glacial ice sheets reached the equator, possibly being ended by the accumulation of greenhouse gases such as CO2 produced by volcanoes. ...
  
  ... The Andean-Saharan occurred from 460 to 420 million years ago, during the Late Ordovician and the Silurian period.
  
  The evolution of land plants at the onset of the Devonian period caused a long term increase in planetary oxygen levels and reduction of CO₂ levels, which resulted in the Karoo Ice Age.
  It is named after the glacial tills found in the Karoo region of South Africa, where evidence for this ice age was first clearly identified.
  There were extensive polar ice caps at intervals from 360 to 260 million years ago in South Africa during the Carboniferous and early Permian Periods. ...
  
  ... The Quaternary Glaciation / Quaternary Ice Age, started about 2.58 million years ago at the beginning of the Quaternary Period when the spread of ice sheets in the Northern Hemisphere began.
  Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales called glacial periods, glacials or glacial advances, and interglacial periods, interglacials or glacial retreats.
  The earth is currently in an interglacial, and the last glacial period ended about 10,000 years ago.
  All that remains of the continental ice sheets are the Greenland and Antarctic ice sheets and smaller glaciers such as on Baffin Island.
  
  For more information, see Major Ice Ages .
• Medieval Warm Period From Wikipedia:
  The Medieval Warm Period (MWP) also known as the Medieval Climate Optimum, or Medieval Climatic Anomaly was a time of warm climate in the North Atlantic region that may have been related to other warming events in other regions during that time, including China and other areas, lasting from c. 950 to c. 1250.
  Other regions were colder, such as the tropical Pacific.
  Averaged global mean temperatures have been calculated to be similar to early-mid 20th century warming.
  Possible causes of the Medieval Warm Period include increased solar activity, decreased volcanic activity, and changes to ocean circulation.
  
  The period was followed by a cooler period in the North Atlantic and elsewhere termed the Little Ice Age.
  Some refer to the event as the Medieval Climatic Anomaly as this term emphasizes that climatic effects other than temperature were important.
  
  It is thought that between c. 950 and c. 1100 was the Northern Hemisphere's warmest period since the Roman Warm Period.
  It was only in the 20th and 21st centuries that the Northern Hemisphere experienced warmer temperatures.
  Climatic Proxy records show peak warmth occurred at different times for different regions, indicating that the Medieval Warm Period was not a globally uniform event.
  
  For more information, see Medieval Warm Period .
• Neoglaciation From Wikipedia:
  The neoglaciation ("renewed glaciation") describes the documented cooling trend in the Earth's climate during the Holocene, following the retreat of the Wisconsin glaciation, the most recent glacial period.
  Neoglaciation has followed the hypsithermal or Holocene Climatic Optimum, the warmest point in the Earth's climate during the current interglacial stage.
  The neoglaciation has no well-marked universal beginning: local conditions and ecological inertia affected the onset of detectably cooler (and wetter) conditions.
  
  Driven inexorably by the Milankovitch cycle, cooler summers in higher latitudes of North America, which would cease to completely melt the annual snowfall, were masked at first by the presence of the slowly disappearing continental ice sheets, which persisted long after the astronomically calculated moment of maximum summer warmth: "the neoglaciation can be said to have begun when the cooling caught up with the warming", remarked E. C. Pielou.
  With the close of the "Little Ice Age" (mid-14th to late 19th centuries), neoglaciation appears to have been stalled in the late 20th century, assumed to be caused by anthropogenic global warming.
  Whether it has been temporarily or semi-permanently stalled, neoglaciation has been marked by a retreat from the warm conditions of the Climatic Optimum and the advance or reformation of glaciers that had not existed since the last ice age.
  In the mountains of western North America, montane glaciers that had completely melted reformed shortly before 5000 BP.
  The most severe part of the best documented neoglacial period, especially in Europe and the North Atlantic, is termed the "Little Ice Age".
  
  For more information, see Neoglaciation .
• Post-glacial Rebound From Wikipedia:
  Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the lifting of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression.
  Post-glacial rebound and isostatic depression are phases of glacial isostasy (glacial isostatic adjustment, glacioisostasy), the deformation of the Earth's crust in response to changes in ice mass distribution.
  The direct raising effects of post-glacial rebound are readily apparent in parts of Northern Eurasia, Northern America, Patagonia, and Antarctica.
  However, through the processes of ocean siphoning and continental levering, the effects of post-glacial rebound on sea level are felt globally far from the locations of current and former ice sheets.
  
  For more information, see Post-glacial Rebound .
• Quaternary Glaciation From Wikipedia:
  The Quaternary glaciation, also known as the Quaternary Ice Age or Pleistocene glaciation, is a series of glacial events separated by interglacial events during the Quaternary period from 2.58 Ma (million years ago) to present.
  It constitutes the intense last 10% of the Current Ice Age, during which the previous long-term glaciation of Antarctica has spread to the northern hemisphere.
  During this period, ice sheets expanded, notably from out of Antarctica and Greenland, and fluctuating ice sheets occurred elsewhere (for example, the Laurentide ice sheet).
  The major effects of the ice age were the erosion of land and the deposition of material, both over large parts of the continents; the modification of river systems; the creation of millions of lakes, changes in sea level, the development of pluvial lakes far from the ice margins, the isostatic adjustment of the earth's crust, flooding, and abnormal winds.
  The ice sheets themselves, by raising the albedo (extent to which the energy of the sun is reflected from Earth) created significant feedback to further cool the climate.
  These effects re-shaped entire environments on land and in the oceans, and their associated biological communities.
  
  For more information, see Quaternary Glaciation .
• Snowball Earth From Wikipedia:
  The Snowball Earth hypothesis proposes that Earth surface's became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago).
  Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record.
  Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean and emphasize the difficulty of escaping an all-frozen condition.
  A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water.
  
  The snowball-Earth episodes are proposed to have occurred before the sudden radiation of multicellular bioforms, known as the Cambrian explosion.
  The most recent snowball episode may have triggered the evolution of multicellularity.
  Another, much earlier and longer snowball episode, the Huronian glaciation, which would have occurred 2400 to 2100 Mya, may have been triggered by the first appearance of oxygen in the atmosphere, the "Great Oxygenation Event".
  
  For more information, see Snowball Earth .
• Timeline of Glaciation From Wikipedia:
  There have been five or six major ice ages in the history of Earth over the past 3 billion years.
  The Current Ice Age began 34 million years ago, its latest phase being the Quaternary glaciation, in progress since 2.58 million years ago.
  
  Within ice ages, there exist periods of more severe glacial conditions and more temperate referred to as glacial periods and interglacial periods, respectively.
  The Earth is currently in such an interglacial period of the Quaternary glaciation, with the last glacial period of the Quaternary having ended approximately 11,700 years ago, the current interglacial being known as the Holocene epoch.
  Based on climate proxies, paleoclimatologists study the different climate states originating from glaciation.
  
  For more information, see Timeline of Glaciation .
• Younger Dryas From Wikipedia:
  The Younger Dryas (c. 12,900 to c. 11,700 years BP) was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum started receding around 20,000 BP.
  It is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala, as its leaves are occasionally abundant in the Late Glacial, often minerogenic-rich, like the lake sediments of Scandinavian lakes. ...
  
  The Younger Dryas was the most recent and longest of several interruptions to the gradual warming of the Earth's climate since the severe Last Glacial Maximum, c. 27,000 to 24,000 years BP.
  The change was relatively sudden, taking place in decades, and it resulted in a decline of 2 to 6 degrees Celsius and advances of glaciers and drier conditions, over much of the temperate northern hemisphere.
  It is thought to have been caused by a decline in the strength of the Atlantic meridional overturning circulation, which transports warm water from the Equator towards the North Pole, in turn thought to have been caused by an influx of fresh cold water from North America to the Atlantic.
  
  For more information, see Younger Dryas .

For more information, see Ice Age. close

Weathering and Erosion of Rocky Material to Create a Soil Covering and the variety of features found at the Earth's surface

As soon as parts of the Earth's crust first solidified after differentiation (possibly around 3.7 billion years ago), it began to be shaped by the agents of weathering and material transport.
The aim in this module is to explain the processes that led to the current Earth having its present natural topographical features.

From Wikipedia:
Earth's surface is modified by a combination of surface processes that shape landscapes, and geologic processes that cause tectonic uplift and subsidence, and shape the coastal geography.
Surface processes comprise the action of water, wind, ice, fire, and living things on the surface of the Earth, along with chemical reactions that form soils and alter material properties, the stability and rate of change of topography under the force of gravity, and other factors, such as (in the very recent past) human alteration of the landscape.
Many of these factors are strongly mediated by climate.
Geologic processes include the uplift of mountain ranges, the growth of volcanoes, isostatic changes in land surface elevation (sometimes in response to surface processes), and the formation of deep sedimentary basins where the surface of the Earth drops and is filled with material eroded from other parts of the landscape.
The Earth's surface and its topography therefore are an intersection of climatic, hydrologic, and biologic action with geologic processes, or alternatively stated, the intersection of the Earth's lithosphere with its hydrosphere, atmosphere, and biosphere.

Weathering Processes and Systems of Material Transport:

• Abrasion (geology) From Wikipedia:
  Abrasion is a process of erosion which occurs when material being transported wears away at a surface over time.
  It is the process of friction caused by scuffing, scratching, wearing down, marring, and rubbing away of materials.
  The intensity of abrasion depends on the hardness, concentration, velocity and mass of the moving particles.
  Abrasion generally occurs four ways.
  Glaciation slowly grinds rocks picked up by ice against rock surfaces.
  Solid objects transported in river channels make abrasive surface contact with the bed and walls.
  Objects transported in waves breaking on coastlines cause abrasion.
  And, finally, abrasion can be caused by wind transporting sand or small stones against surface rocks.
  
  Abrasion, under its strictest definition, is commonly confused with attrition.
  Both abrasion and attrition refer to the wearing down of an object.
  Abrasion occurs as a result of two surfaces rubbing against each other resulting in the wearing down of one or both of the surfaces.
  However, attrition refers to the breaking off of particles (erosion) which occurs as a result of objects hitting against each other.
  Abrasion leads to surface-level destruction over time, whereas attrition results in more change at a faster rate.
  Today, the geomorphology community uses the term “abrasion” in a looser way, often interchangeably with the term “wear”.
  
  For more information, see Abrasion (geology) .
• Aeolian Processes From Wikipedia:
  Aeolian processes, also spelled eolian or æolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets).
  Winds may erode, transport, and deposit materials and are effective agents in regions with sparse vegetation, a lack of soil moisture and a large supply of unconsolidated sediments.
  Although water is a much more powerful eroding force than wind, aeolian processes are important in arid environments such as deserts.
  
  The term is derived from the name of the Greek god Aeolus, the keeper of the winds.
  
  For more information, see Aeolian Processes .
• Attrition (erosion) From Wikipedia:
  Attrition is a form of coastal or river erosion, when the bed load is eroded by itself and the bed.
  As rocks are transported downstream along a riverbed, the regular impacts between the grains themselves and between the grains and the bed cause them to be broken up into smaller fragments.
  This process also makes them rounder and smoother.
  Attrition can also occur in glaciated regions, where it is caused by the movement of ice with embedded boulders over surface sediments.
  
  Pebbles are more affected by attrition further upstream, as the rivers' velocity tends to be higher, and therefore its competence (ability to carry sediment) is increased.
  This means that the load rubs against itself more and with more force when suspended in the river, thus increasing erosion by attrition, though there is a point after transport over a certain distance that pebbles reach a size that is relatively immune to further attrition.
  Grain-size distribution of sediments produced by attrition will also be controlled by the lithology of the rock from which they are derived.
  
  The effects of attrition can be mistaken for the effects of sorting, in which the grain size of sediments is affected by sediment transport mechanisms e.g. suspension vs. bed load.
  This affects pebble beaches the most as the pebbles smash into each other, which causes them to smooth.
  
  For more information, see Attrition (erosion) .
• Avalanche From Wikipedia:
  An avalanche (also called a snowslide) is a cohesive slab of snow lying upon a weaker layer of snow in the snowpack that fractures and slides down a steep slope when triggered.
  Avalanches are typically triggered in a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces of the snow exceed its strength but sometimes only with gradual widening (loose snow avalanche).
  After initiation, avalanches usually accelerate rapidly and grow in mass and volume as they entrain more snow.
  If the avalanche moves fast enough, some of the snow may mix with the air forming a powder snow avalanche, which is a type of gravity current.
  
  Slides of rocks or debris, behaving in a similar way to snow, are also referred to as avalanches (see rockslide).
  The remainder of this article refers to snow avalanches.
  
  For more information, see Avalanche .
• Bank Erosion From Wikipedia:
  Bank erosion is the wearing away of the banks of a stream or river.
  This is distinguished from erosion of the bed of the watercourse, which is referred to as scour.
  
  The roots of trees growing by a stream are undercut by such erosion.
  As the roots bind the soil tightly, they form abutments which jut out over the water.
  These have a significant effect upon the rate and progress of the erosion.
  
  For more information, see Bank Erosion .
• Beach Evolution From Wikipedia:
  The shoreline is where the land meets the sea and it is continually changing.
  Over the long term, the water is eroding the land.
  Beaches represent a special case, in that they exist where sand accumulated from the same processes that strip away rocky and sedimentary material.
  That is, they can grow as well as erode.
  River deltas are another exception, in that silt that erodes up river can accrete at the river's outlet and extend ocean shorelines.
  Catastrophic events such as tsunamis, hurricanes and storm surges accelerate beach erosion, potentially carrying away the entire sand load.
  Human activities can be as catastrophic as hurricanes, albeit usually over a longer time interval.
  
  For more information, see Beach Evolution .
• Beach Nourishment From Wikipedia:
  Beach nourishment (also referred to as beach renourishment, beach replenishment, or sand replenishment) describes a process by which sediment, usually sand, lost through longshore drift or erosion is replaced from other sources.
  A wider beach can reduce storm damage to coastal structures by dissipating energy across the surf zone, protecting upland structures and infrastructure from storm surges, tsunamis and unusually high tides.
  Beach nourishment is typically part of a larger coastal defense scheme.
  Nourishment is typically a repetitive process since it does not remove the physical forces that cause erosion but simply mitigates their effects.
  
  For more information, see Beach Nourishment .
• Bioerosion From Wikipedia:
  Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms.
  Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and fish; it can occur on coastlines, on coral reefs, and on ships; its mechanisms include biotic boring, drilling, rasping, and scraping.
  On dry land, bioerosion is typically performed by pioneer plants or plant-like organisms such as lichen, and mostly chemical (e.g. by acidic secretions on limestone) or mechanical (e.g. by roots growing into cracks) in nature.
  
  Bioerosion of coral reefs generates the fine and white coral sand characteristic of tropical islands.
  The coral is converted to sand by internal bioeroders such as algae, fungi, bacteria (microborers) and sponges (Clionaidae), bivalves (including Lithophaga), sipunculans, polychaetes, acrothoracican barnacles and phoronids, generating extremely fine sediment with diameters of 10 to 100 micrometres.
  External bioeroders include sea urchins (such as Diadema) and chitons.
  These forces in concert produce a great deal of erosion.
  Sea urchin erosion of calcium carbonate has been reported in some reefs at annual rates exceeding 20 kg/m².
  
  For more information, see Bioerosion .
• Chemical Weathering From Wikipedia:
  Chemical weathering changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions.
  Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment.
  New or secondary minerals develop from the original minerals of the rock.
  In this the processes of oxidation and hydrolysis are most important.
  Chemical weathering is enhanced by such geological agents as the presence of water and oxygen, as well as by such biological agents as the acids produced by microbial and plant-root metabolism.
  
  The process of mountain block uplift is important in exposing new rock strata to the atmosphere and moisture, enabling important chemical weathering to occur; significant release occurs of Ca²⁺ and other ions into surface waters.
  
  For more information, see Chemical Weathering .
• Coastal Erosion From Wikipedia:
  Coastal erosion is the wearing away of material from a coastal profile including the removal of beach, sand dunes, or sediment by wave action, tidal currents, wave currents, drainage or high winds (see also beach evolution).
  Waves, generated by storms, wind, or fast moving motor craft, can cause coastal erosion, which may take the form of long-term losses of sediment and rocks, or merely the temporary redistribution of coastal sediments; erosion in one location may result in accretion nearby.
  The study of erosion and sediment redistribution is called coastal morphodynamics.
  It may be caused by hydraulic action, abrasion, impact and corrosion by wind, water, and other forces, natural or unnatural.
  
  On non-rocky coasts, coastal erosion results in rock formations in areas where the coastline contains rock layers or fracture zones with varying resistance to erosion.
  Softer areas become eroded much faster than harder ones, which typically result in landforms such as tunnels, bridges, columns, and pillars.
  Over time the coast generally evens out.
  The softer areas fill up with sediment eroded from hard areas, and rock formations are eroded away.
  Also abrasion commonly happens in areas where there are strong winds, loose sand, and soft rocks.
  The blowing of millions of sharp sand grains creates a sandblasting effect.
  This effect helps to erode, smooth and polish rocks.
  The definition of abrasion is grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles.
  
  For more information, see Coastal Erosion .
• Coastal Sediment Supply From Wikipedia:
  Coastal sediment supply is the transport of sediment to the beach environment by both fluvial and aeolian transport.
  While aeolian transport plays a role in the overall sedimentary budget for the coastal environment, it is paled in comparison to the fluvial supply which makes up 95% of sediment entering the ocean.
  When sediment reaches the coast it is then entrained by longshore drift and littoral cells until it is accreted upon the beach or dunes.
  
  While it is acknowledged that storm systems are the driver behind coastal erosion.
  There is a general consensus that human activity, mainly dam and reservoir impoundments on rivers are the cause of indirect human related coastal erosion, along with other local scale effects such as: land use change, irrigation, gravel extraction and river re-alignment.
  
  For more information, see Coastal Sediment Supply .
• Debris Flow From Wikipedia:
  Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors.
  They generally have bulk densities comparable to those of rock avalanches and other types of landslides (roughly 2000 kilograms per cubic meter), but owing to widespread sediment liquefaction caused by high pore-fluid pressures, they can flow almost as fluidly as water.
  Debris flows descending steep channels commonly attain speeds that surpass 10 m/s (more than 20 mph), although some large flows can reach speeds that are much greater.
  Debris flows with volumes ranging up to about 100,000 cubic meters occur frequently in mountainous regions worldwide.
  The largest prehistoric flows have had volumes exceeding 1 billion cubic meters (i.e., 1 cubic kilometer).
  As a result of their high sediment concentrations and mobility, debris flows can be very destructive.
  
  Notable debris-flow disasters of the twentieth century involved more than 20,000 fatalities in Armero, Colombia in 1985 and tens of thousands in Vargas State, Venezuela in 1999.
  
  For more information, see Debris Flow .
• Denudation From Wikipedia:
  In geology, denudation involves the processes that cause the wearing away of the Earth's surface by moving water, by ice, by wind and by waves, leading to a reduction in elevation and in relief of landforms and of landscapes.
  Endogenous processes such as volcanoes, earthquakes, and plate tectonics uplift and expose continental crust to the exogenous processes of weathering, of erosion, and of mass wasting.
  
  For more information, see Denudation .
• Deposition of Wind-borne Material From Wikipedia:
  Wind-deposited materials hold clues to past as well as to present wind directions and intensities.
  These features help us understand the present climate and the forces that molded it.
  Wind-deposited sand bodies occur as sand sheets, ripples, and dunes.
  
  Sand sheets are flat, gently undulating sandy plots of sand surfaced by grains that may be too large for saltation.
  They form approximately 40 percent of aeolian depositional surfaces.
  The Selima Sand Sheet in the eastern Sahara Desert, which occupies 60,000 square kilometers in southern Egypt and northern Sudan, is one of the Earth's largest sand sheets.
  The Selima is absolutely flat in a few places; in others, active dunes move over its surface.
  
  Wind blowing on a sand surface ripples the surface into crests and troughs whose long axes are perpendicular to the wind direction.
  The average length of jumps during saltation corresponds to the wavelength, or distance between adjacent crests, of the ripples.
  In ripples, the coarsest materials collect at the crests causing inverse grading.
  This distinguishes small ripples from dunes, where the coarsest materials are generally in the troughs.
  This is also a distinguishing feature between water laid ripples and aeolian ripples.
  
  Accumulations of sediment blown by the wind into a mound or ridge, dunes have gentle upwind slopes on the windward side.
  The downwind portion of the dune, the lee slope, is commonly a steep avalanche slope referred to as a slipface.
  Dunes may have more than one slipface.
  The minimum height of a slipface is about 30 centimeters.
  
  For more information, see Deposition .
• Development of Drainage Systems From Wikipedia:
  In geomorphology, drainage systems, also known as river systems, are the patterns formed by the streams, rivers, and lakes in a particular drainage basin.
  They are governed by the topography of the land, whether a particular region is dominated by hard or soft rocks, and the gradient of the land.
  Geomorphologists and hydrologists often view streams as being part of drainage basins.
  A drainage basin is the topographic region from which a stream receives runoff, throughflow, and groundwater flow.
  The number, size, and shape of the drainage basins found in an area vary and the larger the topographic map, the more information on the drainage basin is available.
  
  For more information, see Drainage system (geomorphology) .
• Downcutting From Wikipedia:
  Downcutting, also called erosional downcutting, downward erosion or vertical erosion is a geological process by hydraulic action that deepens the channel of a stream or valley by removing material from the stream's bed or the valley's floor.
  The speed of downcutting depends on the stream's base level, the lowest point to which the stream can erode.
  Sea level is the ultimate base level, but many streams have a higher "temporary" base level because they empty into another body of water that is above sea level or encounter bedrock that resists erosion.
  A concurrent process called lateral erosion refers to the widening of a stream channel or valley.
  When a stream is high above its base level, downcutting will take place faster than lateral erosion; but as the level of the stream approaches its base level, the rate of lateral erosion increases.
  This is why streams in mountainous areas tend to be narrow and swift, forming V-shaped valleys, while streams in lowland areas tend to be wide and slow-moving, with valleys that are correspondingly wide and flat-bottomed.
  The term gradient refers to the elevation of a stream relative to its base level.
  The steeper the gradient, the faster the stream flows.
  Sometimes geological uplift will increase the gradient of a stream even while the stream downcuts toward its base level, a process called "rejuvenation."
  This happened in the case of the Colorado River in the western United States, resulting in the process that created the Grand Canyon.
  
  For more information, see Downcutting .
• Downhill Creep From Wikipedia:
  Downhill creep, also known as soil creep or commonly just creep, is the slow downward progression of rock and soil down a low grade slope; it can also refer to slow deformation of such materials as a result of prolonged pressure and stress.
  Creep may appear to an observer to be continuous, but it really is the sum of numerous minute, discrete movements of slope material caused by the force of gravity.
  Friction, being the primary force to resist gravity, is produced when one body of material slides past another offering a mechanical resistance between the two which acts to hold objects (or slopes) in place.
  As slope on a hill increases, the gravitational force that is perpendicular to the slope decreases and results in less friction between the material that could cause the slope to slide.
  
  For more information, see Downhill Creep .
• Earthflow From Wikipedia:
  An earthflow (earth flow) is a downslope viscous flow of fine-grained materials that have been saturated with water and moves under the pull of gravity.
  It is an intermediate type of mass wasting that is between downhill creep and mudflow.
  The types of materials that are susceptible to earthflows are clay, fine sand and silt, and fine-grained pyroclastic material.
  
  When the ground materials become saturated with enough water, they will start flowing (soil liquefaction).
  Its speed can range from being barely noticeable to rapid movement.
  The velocity of the flow is dictated by water content: the higher the water content is, the higher the velocity will be.
  Because of the dependency on water content for the velocity of the flow, it can take minutes or years for the materials to move down the slope.
  
  For more information, see Earthflow .
• Erosion From Wikipedia:
  In earth science, erosion is the action of surface processes (such as water flow or wind) that remove soil, rock, or dissolved material from one location on the Earth's crust, and then transport it to another location (not to be confused with weathering which involves no movement).
  This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, animals, and humans.
  In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind (aeolic) erosion, zoogenic erosion, and anthropogenic erosion.
  The particulate breakdown of rock or soil into clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by its dissolving into a solvent (typically water), followed by the flow away of that solution.
  Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.
  
  For more information, see Erosion .
• Fluvial Processes From Wikipedia:
  In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them.
  When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used. ...
  
  ... Fluvial processes include the motion of sediment and erosion or deposition on the river bed.
  
  Erosion by moving water can happen in two ways.
  Firstly, the movement of water across the stream bed exerts a shear stress directly onto the bed.
  If the cohesive strength of the substrate is lower than the shear exerted, or the bed is composed of loose sediment which can be mobilized by such stresses, then the bed will be lowered purely by clearwater flow.
  However, if the river carries significant quantities of sediment, this material can act as tools to enhance wear of the bed (abrasion).
  At the same time the fragments themselves are ground down, becoming smaller and more rounded (attrition).
  
  Sediment in rivers is transported as either bedload (the coarser fragments which move close to the bed) or suspended load (finer fragments carried in the water).
  There is also a component carried as dissolved material.
  
  For more information, see Fluvial Processes .
• Frost Heaving From Wikipedia:
  Frost heaving (or a frost heave) is an upwards swelling of soil during freezing conditions caused by an increasing presence of ice as it grows towards the surface, upwards from the depth in the soil where freezing temperatures have penetrated into the soil (the freezing front or freezing boundary).
  Ice growth requires a water supply that delivers water to the freezing front via capillary action in certain soils.
  The weight of overlying soil restrains vertical growth of the ice and can promote the formation of lens-shaped areas of ice within the soil.
  Yet the force of one or more growing ice lenses is sufficient to lift a layer of soil, as much as 1 foot (0.30 metres) or more.
  The soil through which water passes to feed the formation of ice lenses must be sufficiently porous to allow capillary action, yet not so porous as to break capillary continuity.
  Such soil is referred to as "frost susceptible".
  The growth of ice lenses continually consumes the rising water at the freezing front.
  Differential frost heaving can crack road surfaces—contributing to springtime pothole formation—and damage building foundations.
  Frost heaves may occur in mechanically refrigerated cold-storage buildings and ice rinks.
  
  Needle ice is essentially frost heaving that occurs at the beginning of the freezing season, before the freezing front has penetrated very far into the soil and there is no soil overburden to lift as a frost heave.
  
  For more information, see Frost Heaving .
• Frost Weathering From Wikipedia:
  Frost weathering is a collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice.
  The term serves as an umbrella term for a variety of processes such as frost shattering, frost wedging and cryofracturing.
  The process may act on a wide range of spatial and temporal scales, from minutes to years and from dislodging mineral grains to fracturing boulders.
  It is most pronounced in high-altitude and high-latitude areas and is especially associated with alpine, periglacial, subpolar maritime and polar climates, but may occur anywhere at sub-freezing temperatures (between -3 and -8 °C) if water is present.
  
  For more information, see Frost Weathering .
• Glacial Lake Outburst Flood From Wikipedia:
  A glacial lake outburst flood (GLOF) is a type of outburst flood that occurs when the dam containing a glacial lake fails.
  An event similar to a GLOF, where a body of water contained by a glacier melts or overflows the glacier, is called a Jökulhlaup.
  The dam can consist of glacier ice or a terminal moraine.
  Failure can happen due to erosion, a buildup of water pressure, an avalanche of rock or heavy snow, an earthquake or cryoseism, volcanic eruptions under the ice, or if a large enough portion of a glacier breaks off and massively displaces the waters in a glacial lake at its base.
  
  For more information, see Glacial Lake Outburst Flood .
• Glacial Erosion From Wikipedia:
  Glaciers erode predominantly by three different processes: abrasion/scouring, plucking, and ice thrusting.
  In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood.
  Scientists have shown that, in addition to the role of temperature played in valley-deepening, other glaciological processes, such as erosion also control cross-valley variations.
  In a homogeneous bedrock erosion pattern, curved channel cross section beneath the ice is created.
  Though the glacier continues to incise vertically, the shape of the channel beneath the ice eventually remain constant, reaching a U-shaped parabolic steady-state shape as we now see in glaciated valleys.
  Scientists also provide numerical estimate of the time required for the ultimate formation of a steady-shaped U-shaped valley - approximately 100,000 years.
  In a weak bedrock (containing material more erodible than the surrounding rocks) erosion pattern, on the contrary, the amount of overdeepening is limited because ice velocities and erosion rates are reduced. ...
  
  ... The erosion caused by glaciers worldwide erodes mountains so effectively that the term glacial buzzsaw has become widely used, which describes the limiting effect of glaciers on the height of mountain ranges.
  As mountains grow higher, they generally allow for more glacial activity (especially in the accumulation zone above the glacial equilibrium line altitude), which causes increased rates of erosion of the mountain, decreasing mass faster than isostatic rebound can add to the mountain. ...
  
  ... These processes, combined with erosion and transport by the water network beneath the glacier, leave behind glacial landforms such as moraines, drumlins, ground moraine (till), kames, kame deltas, moulins, and glacial erratics in their wake, typically at the terminus or during glacier retreat.
  
  For more information, see Glaciers .
• Groundwater Sapping From Wikipedia:
  Groundwater sapping is a geomorphic erosion process that results in the headward migration of channels in response to near constant fluid discharge at a fixed point.
  The consistent flow of water displaces fine sediments which physically and chemically weathers rocks.
  Valleys that appear to have been created by groundwater sapping occur throughout the world in areas such as England, Colorado, Hawai’i, New Zealand, and many other places.
  However, it is difficult to characterize a landform as being formed exclusively by groundwater sapping due to phenomena such as pluvial runoff, plunge-pool undercutting, changes in water table level, and inconsistent groundwater flow.
  An example of drainage ways created purely by the outflow of subsurface fluids can be seen on the foreshores of beaches.
  As the surge of water and sand brought to land by a wave retreats seaward, the film of water becomes thinner until it forms rhomboid shaped patterns in the sand.
  Small fans form at the apex of the rhombic features, which are eventually fed by the remaining backflow of water traveling downslope.
  Channels begin to form headward in the form of millimeter wide rills along the sides of the fans; the creation of these small channel networks culminates when the last of the backwash dissipates.
  
  This is one of the processes involved in the formation of gullies, such as lavaka.
  Erosion by sapping tends to produce steep-sided U-shaped valleys of fairly uniform width with box-like, "theater-shaped" headwalls.
  This contrasts with the more common branching or dendritic pattern of V-shaped valleys produced by overland flows that become wider with distance from their source.
  Groundwater sapping has been suggested as the cause for erosion of the valley and channel networks on Mars, although studies show that groundwater alone can not excavate and transport the material required to create these canyons.
  
  For more information, see Groundwater Sapping .
• Haloclasty From Wikipedia:
  Haloclasty is a type of physical weathering caused by the growth of salt crystals.
  The process is first started when saline water seeps into cracks and evaporates depositing salt crystals, when the rocks are then heated, the crystals will expand putting pressure on the surrounding rock which will over time splinter the stone into fragments.
  
  Salt crystallization may also take place when solutions decompose rocks (for example, limestone and chalk) to form salt solutions of sodium sulfate or sodium carbonate, from which water evaporates to form their respective salt crystals.
  
  The salts which have proved most effective in disintegrating rocks are sodium sulfate, magnesium sulfate, and calcium chloride.
  Some of these salts can expand up to three times or more in volume.
  
  It is normally associated with arid climates where strong heating causes strong evaporation and therefore salt crystallization.
  It is also common along coasts.
  An example of salt weathering can be seen in the honeycombed stones in sea walls.
  
  For more information, see Haloclasty .
• Headward Erosion From Wikipedia:
  Headward erosion is erosion at the origin of a stream channel, which causes the origin to move back away from the direction of the stream flow, and so causes the stream channel to lengthen.
  It can also refer to widening of a canyon by erosion along its very top edge, when sheets of water first enter the canyon from a more roughly planar surface above it, such as at Canyonlands National Park in Utah.
  When sheets of water on a roughly planar surface first enter a depression in it, this erodes the top edge of the depression.
  This either causes the stream to grow longer at the very top of the stream, which moves its origin back, or causes the canyon formed by the stream to grow wider, by erosion along the length of its top side edge as sheets of water flow over the edge.
  Widening of the canyon by erosion inside the canyon, below the canyon side top edge, or origin or the stream, such as erosion caused by the streamflow inside it, is not called headwall erosion. ...
  
  Headward erosion is a fluvial process of erosion that lengthens a stream, a valley or a gully at its head and also enlarges its drainage basin.
  The stream erodes away at the rock and soil at its headwaters in the opposite direction that it flows.
  Once a stream has begun to cut back, the erosion is sped up by the steep gradient the water is flowing down.
  As water erodes a path from its headwaters to its mouth at a standing body of water, it tries to cut an ever-shallower path.
  This leads to increased erosion at the steepest parts, which is headward erosion.
  If headward erosion continues long enough, it can cause a stream to break through into a neighboring watershed and capture drainage that previously flowed to another stream.
  
  For more information, see Headward Erosion .
• Hydraulic Action From Wikipedia:
  Hydraulic action is the erosion that occurs when the motion of water against a rock surface produces mechanical weathering.
  Most generally, it is the ability of moving water (flowing or waves) to dislodge and transport rock particles.
  Within this rubric are a number of specific erosional processes, including abrasion, attrition, corrasion, saltation, and scouring (downcutting).
  Hydraulic action is distinguished from other types of water facilitated erosion, such as static erosion where water leaches salts and floats off organic material from unconsolidated sediments, and from chemical erosion more often called chemical weathering.
  It is a mechanical process, in which the moving water current flows against the banks and bed of a river, thereby removing rock particles.
  
  A primary example of hydraulic action is a wave striking a cliff face which compresses the air in cracks of the rocks.
  This exerts pressure on the surrounding rock which can progressively crack, break, splinter and detach rock particles.
  This is followed by the decompression of the air as the wave retreats which can occur suddenly with explosive force which additionally weakens the rock.
  Cracks are gradually widened so each wave compresses more air, increasing the explosive force of its release.
  Thus, the effect intensifies in a 'positive feedback' system.
  Over time, as the cracks may grow they sometimes form a sea cave.
  The broken pieces that fall off produce two additional types of erosion, abrasion (sandpapering) and attrition.
  In corrasion, the newly formed chunks are thrown against the rock face.
  Attrition is a similar effect caused by eroded particles after they fall to the sea bed where they are subjected to further wave action.
  In coastal areas wave hydraulic action is often the most important form of erosion.
  
  Similarly, where hydraulic action is strong enough to loosen sediment along a stream bed and its banks; this will take rocks and particles from the banks and bed of the stream and add this to the stream's load.
  This process is the result of friction between the moving water and the static stream bed and banks.
  This friction increases with the speed of the water and the roughness of the bed.
  Once loosened the smaller particles are actually held in suspension by the force of the flowing water, these suspended particles can scour the sides and bottom of the stream.
  The scouring action produces distinctive markings on streams beds such as ripple marks, fluting, and crescent marks.
  The larger particles and even large rocks are scooted (dragged) along the bottom in a process known as traction which causes attrition, and are often "bounced" along in a process known as saltation where the force of the water temporarily lifts the rock particle which then crashes back into the bed dislodging other particles.
  
  For more information, see Hydraulic Action .
• Ice Segregation From Wikipedia:
  Ice segregation is the geological phenomenon produced by the formation of ice lenses, which induce erosion when moisture, diffused within soil or rock, accumulates in a localized zone.
  The ice initially accumulates within small collocated pores or pre-existing cracks, and, as long as the conditions remain favorable, continues to collect in the ice layer or ice lens, wedging the soil or rock apart.
  Ice lenses grow parallel to the surface and several centimeters to several decimeters (inches to feet) deep in the soil or rock.
  Studies between 1990 and present have demonstrated that rock fracture by ice segregation (i.e., the fracture of intact rock by ice lenses that grow by drawing water from their surroundings during periods of sustained subfreezing temperatures) is a more effective weathering process than the freeze-thaw process which older texts proposed.
  
  Ice lenses play the key role in fracture of bedrock and frost induced heaving of soils, which are fundamental to weathering in cold regions.
  Frost heaving creates debris and dramatically shapes landscapes into complex patterns.
  Rock fracture in periglacial regions (alpine, subpolar and polar) has often been attributed to the freezing and volumetric expansion of water trapped within pores and cracks.
  However the majority of frost heaving and of bedrock fracture results instead from ice segregation in ice lenses in the near-surface frozen regions. Ice segregation results in rock fracture and frost heave.
  
  For more information, see Ice Segregation .
• Ice Wedging From Wikipedia:
  An ice wedge is a crack in the ground formed by a narrow or thin piece of ice that measures up to 3-4 meters in length at ground level and extends downwards into the ground up to several meters.
  During the winter months, the water in the ground freezes and expands.
  Once temperatures reach -17 degrees Celsius or lower, the ice that has already formed acts like a solid and expands to form cracks in the surface known as ice wedges.
  As this process continues over many years ice wedges can grow, up to the size of a swimming pool.
  Ice wedges usually appear in a polygonal pattern known as ice wedge polygons.
  The cracks can also be filled with materials other than ice, especially sand, and then they're called sand wedges.
  
  For more information, see Ice Wedging .
• Jökulhlaup From Wikipedia:
  A jökulhlaup (Icelandic (literally "glacial run") is a type of glacial outburst flood.
  It is an Icelandic term that has been adopted in glaciological terminology in many languages.
  It originally referred to the well-known subglacial outburst floods from Vatnajökull, Iceland, which are triggered by geothermal heating and occasionally by a volcanic subglacial eruption, but it is now used to describe any large and abrupt release of water from a subglacial or proglacial lake/reservoir.
  
  Since jökulhlaups emerge from hydrostatically-sealed lakes with floating levels far above the threshold, their peak discharge can be much larger than that of a marginal or extra-marginal lake burst.
  The hydrograph of a jökulhlaup from Vatnajökull typically either climbs over a period of weeks with the largest flow near the end, or it climbs much faster during the course of some hours.
  These patterns are suggested to reflect channel melting, and sheet flow under the front, respectively.
  Similar processes on a very large scale occurred during the deglaciation of North America and Europe after the last ice age (e.g., Lake Agassiz and the English Channel), and presumably at earlier times, although the geological record is not well preserved.
  
  For more information, see Jökulhlaup .
• Lahar From Wikipedia:
  A lahar ( from Javanese: translit. wlahar) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water.
  The material flows down from a volcano, typically along a river valley.
  
  Lahars are extremely destructive: they can flow tens of metres per second (22 mph or more), they have been known to be up to 140 metres (460 ft) deep, and large flows tend to destroy any structures in their path.
  They have even been known to decimate entire settlements.
  Notable lahars include those at Mount Pinatubo and Nevado del Ruiz, the latter of which killed thousands of people and caused extensive damage to infrastructure.
  
  For more information, see Lahar.
• Landslide From Wikipedia:
  The term landslide or, less frequently, landslip, refers to several forms of mass wasting that include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows and debris flows.
  Landslides can occur underwater, in which case they are called submarine landslides, and in coastal and onshore environments.
  Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting slope stability.
  Typically, pre-conditional factors build up specific surface or sub-surface conditions that make a slope prone to failure, whereas the actual landslide often requires a trigger before being released.
  
  For more information, see Landslide.
• Lava Flow From Wikipedia:
  Lava is molten rock generated by geothermal energy and expelled through fractures in planetary crust or in an eruption, usually at temperatures from 700 to 1,200 °C (1,292 to 2,192 °F).
  The resulting structures after solidification and cooling are also sometimes described as lava.
  The molten rock is formed in the interior of some planets, including Earth, and some of their satellites, though such material located below the crust is referred to by other terms.
  
  A lava flow is a moving outpouring of lava created during a non-explosive effusive eruption.
  When it has stopped moving, lava solidifies to form igneous rock.
  The term lava flow is commonly shortened to lava.
  Although lava can be up to 100,000 times more viscous than water, lava can flow great distances before cooling and solidifying because of its thixotropic and shear thinning properties.
  
  Explosive eruptions produce a mixture of volcanic ash and other fragments called tephra, rather than lava flows.
  
  For more information, see Lava Flow.
• Longshore Drift From Wikipedia:
  Longshore drift is a geological process that consists of the transportation of sediments (clay, silt, sand and shingle) along a coast parallel to the shoreline, which is dependent on oblique incoming wind direction.
  Oblique incoming wind squeezes water along the coast, and so generates a water current which moves parallel to the coast.
  Longshore drift is simply the sediment moved by the longshore current.
  This current and sediment movement occur within the surf zone.
  
  Beach sand is also moved on such oblique wind days, due to the swash and backwash of water on the beach.
  Breaking surf sends water up the beach (swash) at an oblique angle and gravity then drains the water straight downslope (backwash) perpendicular to the shoreline.
  Thus beach sand can move downbeach in a zig zag fashion many tens of meters per day.
  This process is called "beach drift" but some workers regard it as simply part of "longshore drift" because of the overall movement of sand parallel to the coast. Longshore drift affects numerous sediment sizes as it works in slightly different ways depending on the sediment (e.g. the difference in long shore drift of sediments from a sandy beach to that of sediments from a shingle beach).
  Sand is largely affected by the oscillatory force of breaking waves, the motion of sediment due to the impact of breaking waves and bed shear from long shore current.
  Because shingle beaches are much steeper than sandy ones, plunging breakers are more likely to form, causing the majority of long shore transport to occur in the swash zone, due to a lack of an extended surf zone.
  
  For more information, see Longshore Drift .
• Mass Wasting From Wikipedia:
  Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, sand, regolith, and rock move downslope typically as a solid, continuous or discontinuous mass, largely under the force of gravity, but frequently with characteristics of a flow as in debris flows and mudflows.
  Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place over timescales from seconds to hundreds of years.
  Mass wasting occurs on both terrestrial and submarine slopes, and has been observed on Earth, Mars, Venus, and Jupiter's moon Io.
  
  For more information, see Mass Wasting .
• Mudflow From Wikipedia:
  A mudflow or mud flow is a form of mass wasting involving "very rapid to extremely rapid surging flow" of debris that has become partially or fully liquified by the addition of significant amounts of water to the source material.
  
  Mudflows contain a significant proportion of clay, which makes them more fluid than debris flows; thus, they are able to travel farther and across lower slope angles.
  Both types are generally mixtures of various kinds of materials of different sizes, which are typically sorted by size upon deposition.
  
  Mudflows are often called mudslides, a term applied indiscriminately by the mass media to a variety of mass wasting events.
  Mudflows often start as slides, becoming flows as water is entrained along the flow path; such events are often called flow slides.
  
  Other types of mudflows include lahars (involving fine-grained pyroclastic deposits on the flanks of volcanoes) and jökulhlaups (outbursts from under glaciers or icecaps).
  
  For more information, see Mudflow .
• Outburst Flood From Wikipedia:
  In geomorphology, an outburst flood, which is a type of megaflood, is a high-magnitude, low-frequency catastrophic flood involving the sudden release of water.
  During the last deglaciation, numerous glacial lake outburst floods were caused by the collapse of either ice sheets or glaciers that formed the dams of proglacial lakes.
  Examples of older outburst floods are known from the geological past of the Earth and inferred from geomorphological evidence on Mars.
  Landslides, lahars, and volcanic dams can also block rivers and create lakes, which trigger such floods when the rock or earthen barrier collapses or is eroded.
  Lakes also form behind glacial moraines, which can collapse and create outburst floods.
  
  For more information, see Outburst Flood .
• Periglaciation From Wikipedia:
  Periglaciation (adjective: "periglacial," also referring to places at the edges of glacial areas) describes geomorphic processes that result from seasonal thawing of snow in areas of permafrost, the runoff from which refreezes in ice wedges and other structures.
  "Periglacial" suggests an environment located on the margin of past glaciers.
  However, freeze and thaw cycles influence landscapes outside areas of past glaciation.
  Therefore, periglacial environments are anywhere that freezing and thawing modify the landscape in a significant manner.
  
  Tundra is a common ecological community in periglacial areas.
  
  For more information, see Periglaciation .
• Post-orogenic Collapse From Wikipedia:
  A post-orogenic collapse is the loss of height and lateral spread out of mass of an orogen as consequence of the cessation or overcoming of the tectonic forces that formed the orogeny.
  It has been argued that extension during orogenic collapse is a more effective mechanism of lowering the mountains than erosion.
  
  The Scandinavian Caledonides is an example of an orogeny and mountain chain that reached heights of 8–9 km and then collapsed in the Devonian.
  The collapse was such that the modern Scandinavian Mountains do not owe their height to the former orogeny but to other processes that occurred in the Cenozoic.
  
  For more information, see Post-orogenic Collapse .
• Pyroclastic Fall From Wikipedia:
  A pyroclastic fall is a uniform deposit of material which has been ejected from a volcanic eruption or plume such as an ash fall or tuff.
  Pyroclastic air fall deposits are a result of:
  
  1. Ballistic transport of ejecta such as volcanic blocks, volcanic bombs and lapilli from volcanic explosions
  2. Deposition of material from convective clouds associated with pyroclastic flows such as coignimbrite falls
  3. Ejecta carried in gas streaming from a vent. The material under the action of gravity will settle out from an eruption plume or eruption column
  4. Ejecta settling from an eruptive plume or eruption column that is displaced laterally by wind currents and is dispersed over great distances.
  
  For more information, see Pyroclastic Fall .
• Pyroclastic Flow From Wikipedia:
  A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that moves away from a volcano about 100 km/h (62 mph) on average but is capable of reaching speeds up to 700 km/h (430 mph).
  The gases can reach temperatures of about 1,000 °C (1,830 °F).
  
  Pyroclastic flows are a common and devastating result of certain explosive eruptions; they normally touch the ground and hurtle downhill, or spread laterally under gravity.
  Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope.
  
  For more information, see Pyroclastic Flow .
• Pyroclastic Surge From Wikipedia:
  A pyroclastic surge is a fluidized mass of turbulent gas and rock fragments that is ejected during some volcanic eruptions.
  It is similar to a pyroclastic flow but it has a lower density or contains a much higher ratio of gas to rock, which makes it more turbulent and allows it to rise over ridges and hills rather than always travel downhill as pyroclastic flows do.
  
  The speed of pyroclastic density currents has been measured directly via photography only in the case of Mount St. Helens, where they reached 90–130 m/s (200–290 mph).
  Estimates of other modern eruptions are around 100 m/s.
  Pyroclastic flows may generate surges.
  For example, the city of Saint-Pierre in Martinique in 1902 was overcome by one.
  Pyroclastic surge include 3 types, which are base surge, ash-cloud surge, and ground surge.
  
  For more information, see Pyroclastic Surge .
• Rainfall and Surface Runoff From Wikipedia:
  Rainfall, and the surface runoff which may result from rainfall, produces four main types of soil erosion: splash erosion, sheet erosion, rill erosion, and gully erosion.
  Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by sheet erosion, then rill erosion and finally gully erosion (the most severe of the four).
  
  In splash erosion, the impact of a falling raindrop creates a small crater in the soil, ejecting soil particles.
  The distance these soil particles travel can be as much as 0.6 m (two feet) vertically and 1.5 m (five feet) horizontally on level ground.
  
  If the soil is saturated, or if the rainfall rate is greater than the rate at which water can infiltrate into the soil, surface runoff occurs.
  If the runoff has sufficient flow energy, it will transport loosened soil particles (sediment) down the slope.
  Sheet erosion is the transport of loosened soil particles by overland flow.
  
  Rill erosion refers to the development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes.
  Generally, where water erosion rates on disturbed upland areas are greatest, rills are active.
  Flow depths in rills are typically of the order of a few centimetres (about an inch) or less and along-channel slopes may be quite steep.
  This means that rills exhibit hydraulic physics very different from water flowing through the deeper, wider channels of streams and rivers.
  
  Gully erosion occurs when runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to a considerable depth.
  
  For more information, see Rainfall and Surface Runoff .
• Rivers and Streams From Wikipedia:
  Valley or stream erosion occurs with continued water flow along a linear feature.
  The erosion is both downward, deepening the valley, and headward, extending the valley into the hillside, creating head cuts and steep banks.
  In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V cross-section and the stream gradient is relatively steep.
  When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain.
  The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream meanders across the valley floor.
  In all stages of stream erosion, by far the most erosion occurs during times of flood, when more and faster-moving water is available to carry a larger sediment load.
  In such processes, it is not the water alone that erodes: suspended abrasive particles, pebbles, and boulders can also act erosively as they traverse a surface, in a process known as traction.
  
  Bank erosion is the wearing away of the banks of a stream or river.
  This is distinguished from changes on the bed of the watercourse, which is referred to as scour.
  Erosion and changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times.
  
  For more information, see Rivers and Streams .
• Saltation (geology) From Wikipedia:
  In geology, saltation (from Latin saltus, "leap") is a specific type of particle transport by fluids such as wind or water.
  It occurs when loose materials are removed from a bed and carried by the fluid, before being transported back to the surface.
  Examples include pebble transport by rivers, sand drift over desert surfaces, soil blowing over fields, and snow drift over smooth surfaces such as those in the Arctic or Canadian Prairies.
  
  For more information, see Saltation (geology) .
• Slaking (geology) From Wikipedia:
  Slaking is the process in which earth materials disintegrate and crumble when exposed to moisture.
  The term can be applied to natural geologic formations, land modified by or for human use, or to the use of earth materials in manufacturing or industry.
  
  This process can often lead to erosion if the geologic area is not flat or vegetated.
  The slaking property does not necessarily have to be in the A horizon, with B horizon slaking only becoming a problem when the A horizon is disturbed or eroded away.
  
  For more information, see Slaking (geology) .
• Slump (geology) From Wikipedia:
  A slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials or rock layers moves a short distance down a slope.
  Movement is characterized by sliding along a concave-upward or planar surface.
  Causes of slumping include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of a slope.
  
  Translational slumps occur when a detached landmass moves along a planar surface.
  Common planar surfaces of failure include joints or bedding planes, especially where a permeable layer overrides an impermeable surface.
  Block slumps are a type of translational slump in which one or more related block units move downslope as a relatively coherent mass.
  
  Rotational slumps occur when a slump block, composed of sediment or rock, slides along a concave-upward slip surface with rotation about an axis parallel to the slope.
  Rotational movement causes the original surface of the block to become less steep, and the top of the slump is rotated backward.
  This results in internal deformation of the moving mass consisting chiefly of overturned folds called sheath folds.
  
  For more information, see Slump (geology) .
• Slush Flow From Wikipedia:
  A slushflow is rapid mass movement of water and snow, and is categorized as a type of debris flow.
  Slushflows are caused when water reaches a critical concentration in the snowpack due to more water inflow than outflow.
  The high water concentration weakens the cohesion of the snow crystals and increases the weight of the snowpack.
  A slushflow is released when the component of the force of gravity parallel to the slope generates a hydraulic pressure gradient exceeding the tensile strength and basal friction of the snowpack.
  
  While frequently compared to an avalanche, they have some key differences.
  Slushflows are lower in frequency than avalanches, have higher water content, are more laminar in flow, and have lower velocity.
  They most commonly occur at higher latitudes in the winter during high precipitation and in the spring during high snowmelt.
  Because of their high water content, they can flow down gentle slopes of only a few degrees at low altitudes.
  They are a significant hazard in Norway and Iceland and are responsible for the deaths of dozens of people as well as the destruction of buildings and the closure of roads.
  
  For more information, see Slush Flow .
• Soil Erosion From Wikipedia:
  Soil erosion is the displacement of the upper layer of soil, one form of soil degradation.
  This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, animals, and humans.
  In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind (aeolean) erosion, zoogenic erosion, and anthropogenic erosion.
  Soil erosion may be a slow process that continues relatively unnoticed, or it may occur at an alarming rate causing a serious loss of topsoil.
  The loss of soil from farmland may be reflected in reduced crop production potential, lower surface water quality and damaged drainage networks.
  
  Human activities have increased by 10–40 times the rate at which erosion is occurring globally.
  
  For more information, see Soil Erosion .
• Soil Formation From Wikipedia:
  Pedogenesis (from the Greek pedo-, or pedon, meaning 'soil, earth,' and genesis, meaning 'origin, birth') (also termed soil development, soil evolution, soil formation, and soil genesis) is the process of soil formation as regulated by the effects of place, environment, and history.
  Biogeochemical processes act to both create and destroy order (anisotropy) within soils.
  These alterations lead to the development of layers, termed soil horizons, distinguished by differences in color, structure, texture, and chemistry.
  These features occur in patterns of soil type distribution, forming in response to differences in soil forming factors. ...
  
  ... Soil develops through a series of changes.
  The starting point is weathering of freshly accumulated parent material.
  Primitive microbes feed on simple compounds (nutrients) released by weathering, and produce acids which contribute to weathering.
  They also leave behind organic residues.
  New soils increase in depth by a combination of weathering, and further deposition.
  Gradually soil is able to support higher forms of plants and animals, starting with pioneer species, and proceeding to more complex plant and animal communities.
  
  Soils deepen with accumulation of organic matter primarily due to the activities of higher plants.
  Topsoil deepen through soil mixing.
  Soils develop layers as organic matter accumulates and leaching takes place.
  This development of layers is the beginning of the soil profile.
  
  A rate of pedogenesis of 1/10 mm per year is in order of magnitude of the global soil production estimates.
  
  For more information, see Pedogenesis .
• Soil Retrogression and Degradation From Wikipedia:
  Soil retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a stable soil.
  Retrogression is primarily due to soil erosion and corresponds to a phenomenon where succession reverts the land to its natural physical state.
  Degradation is an evolution, different from natural evolution, related to the local climate and vegetation.
  It is due to the replacement of primary plant communities (known as climax vegetation) by the secondary communities.
  This replacement modifies the humus composition and amount, and affects the formation of the soil.
  It is directly related to human activity.
  Soil degradation may also be viewed as any change or ecological disturbance to the soil perceived to be deleterious or undesirable.
  
  For more information, see Soil Retrogression and Degradation .
• Solifluction From Wikipedia:
  Solifluction is a collective name for gradual mass wasting slope processes related to freeze-thaw activity.
  This is the standard modern meaning of solifluction, which differs from the original meaning given to it by Johan Gunnar Andersson in 1906. ...
  
  ... In the original sense it meant the movement of waste saturated in water found in periglacial regions.
  However it was later discovered that various slow waste movements in periglacial regions did not required saturation in water, but were rather associated to freeze-thaw processes.
  The term solifluction was appropriated to refer to these slow processes, and therefore excludes rapid periglacial movements.
  In slow periglacial solifluction there are not clear gliding planes, and therefore skinflows and active layer detachments are not included in the concept.
  On the other hand, movement of waste saturated in water can occur in any humid climate, and therefore this kind of solifluction is not restricted to cold climates.
  
  Slow periglacial solifluction is classified into four types:
  
  - Ice creep
  - Frost creep
  - Gelifluction
  - Plug-like flow
  
  Slow solifluction acts much slower than some geochemical fluxes or than other erosion processes.
  The relatively low rates at which solifluction operates contrast with its occurrence over wide mountain areas and periglaciated lowlands.
  Since solifluction is associated with humidity and cold climates it can be used to infer past climates.
  
  For more information, see Solifluction .
• Submarine Landslide From Wikipedia:
  Submarine landslides are marine landslides that transport sediment across the continental shelf and into the deep ocean.
  A submarine landslide is initiated when the downwards driving stress (gravity and other factors) exceeds the resisting stress of the seafloor slope material causing movements along one or more concave to planar rupture surfaces.
  Submarine landslides take place in a variety of different settings including planes as low as 1° and can cause significant damage to both life and property.
  Recent advances have been made in understanding the nature and processes of submarine landslides through the use of sidescan sonar and other seafloor mapping technology.
  
  For more information, see Submarine Landslide .
• Transport by Wind From Wikipedia:
  Particles are transported by winds through suspension, saltation (skipping or bouncing) and creeping (rolling or sliding) along the ground.
  
  Small particles may be held in the atmosphere in suspension.
  Upward currents of air support the weight of suspended particles and hold them indefinitely in the surrounding air.
  Typical winds near Earth's surface suspend particles less than 0.2 millimeters in diameter and scatter them aloft as dust or haze.
  
  Saltation is downwind movement of particles in a series of jumps or skips.
  Saltation normally lifts sand-size particles no more than one centimeter above the ground and proceeds at one-half to one-third the speed of the wind.
  A saltating grain may hit other grains that jump up to continue the saltation.
  The grain may also hit larger grains that are too heavy to hop, but that slowly creep forward as they are pushed by saltating grains.
  Surface creep accounts for as much as 25 percent of grain movement in a desert.
  
  Aeolian turbidity currents are better known as dust storms.
  Air over deserts is cooled significantly when rain passes through it.
  This cooler and denser air sinks toward the desert surface.
  When it reaches the ground, the air is deflected forward and sweeps up surface debris in its turbulence as a dust storm.
  
  For more information, see Transport .
• Wave Action From Wikipedia:
  Hydraulic action
  Hydraulic Action occurs when waves striking a cliff face compress air in cracks on the cliff face.
  This exerts pressure on the surrounding rock, and can progressively splinter and remove pieces.
  Over time, the cracks can grow, sometimes forming a cave.
  The splinters fall to the sea bed where they are subjected to further wave action.
  
  Attrition
  Attrition occurs when waves cause loose pieces of rock debris (scree) to collide with each other, grinding and chipping each other, progressively becoming smaller, smoother and rounder.
  Scree also collides with the base of the cliff face, chipping small pieces of rock from the cliff or have a corrasion (abrasion) effect, similar to sandpapering.
  
  Solution
  Solution is the process in which acids contained in sea water will dissolve some types of rock such as chalk or limestone.
  
  Corrasion
  Corrasion or otherwise known as abrasion occurs when waves break on cliff faces and slowly erode it.
  As the sea pounds cliff faces it also uses the scree from other wave actions to batter and break off pieces of rock from higher up the cliff face which can be used for this same wave action and attrition.
  
  Corrosion
  Corrosion or solution/chemical weathering occurs when the sea's pH (anything below pH 7.0) corrodes rocks on a cliff face.
  Limestone cliff faces, which have a moderately high pH, are particularly affected in this way.
  Wave action also increases the rate of reaction by removing the reacted material.
  
  For more information, see Wave Action .
• Wave Pounding From Wikipedia:
  Wave pounding is the 'sledge hammer' effect of tonnes of water crashing against cliffs.
  It shakes and weakens the rocks leaving them open to attack from hydraulic action and abrasion.
  Eroded material gets carried away by the wave.
  Wave pounding is particularly fierce in a storm, where the waves are exceptionally large, and have a lot of energy.
  It is an important engineering consideration in the construction of structures such as seawalls and dams.
  
  For more information, see Wave Pounding .
• Weathering From Wikipedia:
  Weathering is the breaking down of rocks, soil, and minerals as well as wood and artificial materials through contact with the Earth's atmosphere, water, and biological organisms.
  Weathering occurs in situ (on site), that is, in the same place, with little or no movement, and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity and then being transported and deposited in other locations.
  
  Two important classifications of weathering processes exist – physical and chemical weathering; each sometimes involves a biological component.
  Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure.
  The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals also known as biological weathering in the breakdown of rocks, soils and minerals.
  
  For more information, see Weathering .
• Wind Erosion From Wikipedia:
  Wind erodes the Earth's surface by deflation (the removal of loose, fine-grained particles by the turbulent action of the wind) and by abrasion (the wearing down of surfaces by the grinding action and sandblasting by windborne particles).
  
  Regions which experience intense and sustained erosion are called deflation zones.
  Most aeolian deflation zones are composed of desert pavement, a sheet-like surface of rock fragments that remains after wind and water have removed the fine particles.
  Almost half of Earth's desert surfaces are stony deflation zones.
  The rock mantle in desert pavements protects the underlying material from deflation.
  
  A dark, shiny stain, called desert varnish or rock varnish, is often found on the surfaces of some desert rocks that have been exposed at the surface for a long period of time.
  Manganese, iron oxides, hydroxides, and clay minerals form most varnishes and provide the shine.
  
  Deflation basins, called blowouts, are hollows formed by the removal of particles by wind.
  Blowouts are generally small, but may be up to several kilometers in diameter.
  
  Wind-driven grains abrade landforms.
  In parts of Antarctica wind-blown snowflakes that are technically sediments have also caused abrasion of exposed rocks.
  Grinding by particles carried in the wind creates grooves or small depressions.
  Ventifacts are rocks which have been cut, and sometimes polished, by the abrasive action of wind.
  
  Sculpted landforms, called yardangs, are up to tens of meters high and kilometers long and are forms that have been streamlined by desert winds.
  The famous Great Sphinx of Giza in Egypt may be a modified yardang.
  
  For more information, see Wind Erosion .

The processes described above have contributed towards, or are wholly responsible for, the various features to be found at the Earth's surface. These features may be classified as follows (follow the links to arrive at links to Wikipedia articles about each of the listed landforms):

• Aeolian Landforms From Wikipedia:
  Aeolian landforms are features of the Earth's surface produced by either the erosive or constructive action of the wind.
  This process is not unique to the Earth, and it has been observed and studied on other planets, including Mars. ...
  
  ... In aeolian processes, wind transports and deposits particles of sediment.
  Aeolian features form in areas where wind is the primary source of erosion.
  The particles deposited are of sand, silt and clay size.
  The particles are entrained in by one of four processes.
  Creep occurs when a particle rolls or slides across the surface.
  Lift occurs when a particle rises off the surface due to the Bernoulli effect.
  If the airflow is turbulent, larger particles are transported by a process known as saltation.
  Finally, impact transport occurs which one particle strikes another causing the second particle to move. ...
  
  ... Wind eroded landforms are rarely preserved on the surface of the Earth except in arid regions.
  Elsewhere, moving water erases the aeolian landforms.
  There are several types of landforms associated with erosion:
  Dry lake Sandhill lag deposits, ventifacts, yardangs, pans.
  
  For more information, see Aeolian Landform ,
  List of Aeolian Landforms .
• Coastal and Oceanic Landforms From Wikipedia:
  Coastal and oceanic landforms include:
  Abyssal fan
  Abyssal plain
  Archipelago
  Atoll
  Arch
  Ayre
  Barrier bar and barrier island
  Bay
  Baymouth bar
  Beach and raised beach
  Beach cusps
  Beach ridge
  Bight
  Blowhole
  Channel
  Cape
  Calanque
  Cliff
  Coast
  Continental shelf
  Coral reef
  Cove
  Cuspate foreland
  Dune system
  Estuary
  Firth
  Fjard
  Fjord
  Geo
  Gulf
  Headland
  Inlet
  Island, Islet
  Isthmus
  Lagoon
  Machair
  Marine terrace
  Mid-ocean ridge
  Oceanic basin
  Oceanic plateau
  Oceanic ridge
  Oceanic trench
  Peninsula
  Ria
  River delta
  Salt marsh
  Sea cave
  Seamount, Seamount chains
  Shoal
  Shore
  Sound
  Spit
  Strait
  Strandflat
  Stack, and stump
  Submarine canyon
  Surge channel
  Tessellated pavement
  Tidal marsh
  Tide pool
  Tombolo
  Volcanic arc
  Wave cut platform
  
  For more information, see Coastal and Oceanic Landforms .
• Cryogenic Landforms From Wikipedia:
  Cryogenic landforms include:
  Cryoplanation terrace
  Earth hummocks
  Lithalsa
  Nivation hollow
  Palsa
  Permafrost plateau
  Pingo
  Rock glacier
  Solifluction lobe
  Solifluction sheet
  Thermokarst
  
  For more information, see Cryogenic Landforms .
• Erosion Landforms From Wikipedia:
  Erosion landforms include:
  Landforms produced by erosion and weathering usually occur in coastal or fluvial environments, and many also appear under those headings.
  
  Arête
  Badlands
  Bornhardt
  Butte
  Canyon
  Cave
  Cirque
  Cliff
  Cryoplanation terrace
  Cuesta
  Dissected plateau
  Erg
  Etchplain
  Exhumed river channel
  Fjord
  Flared slope
  Flatiron
  Gulch
  Gully
  Hogback
  Hoodoo
  Homoclinal ridge
  Inselberg
  Inverted relief
  Lavaka
  Limestone pavement
  Monadnock
  Mesa
  Natural arch
  Paleoplain
  Paleosurface
  Pediment
  Pediplain
  Peneplain
  Planation surface
  Potrero
  Ridge
  Roche moutonnée
  Rock formations
  Strike ridge
  Structural bench
  Structural terrace
  Tea table
  Tepui
  Tessellated pavement
  Truncated spur
  Tor
  Valley
  Wave-cut platform
  
  For more information, see Erosion Landforms .
• Fluvial Landforms From Wikipedia:
  Fluvial landforms include:
  Fluvial landforms include:
  
  Ait
  Alluvial fan
  Anabranch
  Arroyo and (wash)
  Asymmetric valley
  Backswamp
  Bar
  Bayou
  Bench
  Braided channel
  Cave
  Cliff
  Cut bank
  Crevasse splay
  Confluence
  Drainage basin
  Drainage divide
  Endorheic basin
  Entrenched meander
  Epigenetic valley
  Esker
  Exhumed river channel
  Floodplain
  Fluvial island
  Fluvial terrace
  Gorge and canyon
  Gully
  Natural levee
  Marsh
  Meander
  Oxbow lake
  Point bar
  Plunge pool
  Rapid
  Riffle
  River
  River delta
  River island
  Rock-cut basin
  Shut-in
  Thalweg
  Towhead
  Shoal
  Spring
  Strath
  Stream
  Stream pool
  Swamp
  Valley and Vale
  Wadi
  Waterfall
  Watershed
  Yazoo stream
  Mis-fit River
  V-Shaped Valley
  
  For more information, see Fluvial Landforms .
• Impact Landforms From Wikipedia:
  Landforms created by extraterrestrial impacts include:
  
  Central peak
  Complex crater
  Cratered landscape
  Ejecta blanket
  Impact crater
  Simple crater
  
  For more information, see Impact Landforms .
• Karst Landforms From Wikipedia:
  Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum.
  It is characterized by underground drainage systems with sinkholes and caves.
  It has also been documented for more weathering-resistant rocks, such as quartzite, given the right conditions.
  Subterranean drainage may limit surface water, with few to no rivers or lakes.
  However, in regions where the dissolved bedrock is covered (perhaps by debris) or confined by one or more superimposed non-soluble rock strata, distinctive karst features may occur only at subsurface levels and be totally missing above ground.
  
  Karst landforms include:
  
  Abîme
  Calanque
  Cave
  Cenote
  Foiba
  Karst fenster
  Limestone pavement
  Mogote
  Polje
  Scowle
  Sinkhole
  Turlough
  Uvala
  
  For more information, see
  ,
  Karst List of Karst Landforms .
• Lacustrine Landforms From Wikipedia:
  Lacustrine Plains (or lake plains) are lakes that get filled by incoming sediment.
  Over time, the water may drain from the lake, leaving the deposited sediments behind.
  This can be caused by natural drainage, evaporation or other geophysical processes.
  
  The soil of the plain left behind may constitute fertile and productive farm land, due to the previous accumulation of lacustrine sediments.
  In other cases it may become a wetland or a desert.
  
  Lacustrine landforms include:
  
  Beach and raised beach
  Carolina bay
  Dry lake
  Endorheic basin
  Lacustrine plain
  Lacustrine terraces
  Lake
  Oasis
  Oxbow lake
  Parallel Roads of Glen Roy
  Pond
  Proglacial lake
  Salt pan (salt flat)
  
  For more information, see
  ,
  Lacustrine Plain List of Lacustrine Landforms .
• Mountain and Glacial Landforms From Wikipedia:
  Mountain and glacial landforms include:
  
  Arête
  Cirque
  Col
  Crevasse
  Corrie or cwm
  Cove (mountain)
  Dirt cone
  Drumlin and drumlin field
  Esker
  Fjord
  Fluvial terrace
  Flyggberg
  Glacial horn
  Glacier
  Glacier cave
  Glacier foreland
  Hanging valley
  Hill
  Kame
  Kame delta
  Kettle
  Monadnock
  Moraine and Ribbed moraines
  Moulin
  Mountain
  Mountain pass
  Mountain range
  Nunatak
  Proglacial lake
  Outwash fan and outwash plain
  Rift valley
  Roche moutonnée
  Sandur
  Side valley
  Summit
  Trim line
  Truncated spur
  Tunnel valley
  Valley
  U-shaped valley
  
  For more information, see Mountain and Glacial Landforms .
• Slope Landforms From Wikipedia:
  Slope landforms include:
  
  Bluff
  Butte
  Cliff
  Col
  Cuesta
  Dale
  Defile
  Dell
  Doab
  Draw
  Escarpment (scarp)
  Flat (landform)
  Glen
  Gully
  Hill
  Knoll
  Interfluve
  Mesa
  Mountain pass
  Plain
  Plateau
  Ravine
  Ridge
  Rock shelter
  Scree
  Solifluction lobe
  Solifluction sheet
  Strath
  Summit
  Terrace
  Terracette
  Vale
  Valley
  Valley shoulder
  
  For more information, see Slope Landforms .
• Tectonic Landforms From Wikipedia:
  Landforms created by tectonic activity include:
  
  Asymmetric valley
  Dome
  Faceted spur
  Fault scarp
  Graben
  Horst
  Mid-ocean ridge
  Mud volcano
  Oceanic trench
  Pull-apart basin
  Rift valley
  Sand volcano
  
  For more information, see Tectonic Landforms .
• Volcanic Landforms From Wikipedia:
  Volcanic landforms include:
  
  Caldera
  Cinder cone
  Complex volcano
  Crater lake
  Cryptodome
  Cryovolcano
  Diatreme
  Dike
  Fissure vent
  Geyser
  Guyot
  Hornito
  Kipuka
  Lava dome
  Lava coulee
  Lava flow
  Lava lake
  Lava plain
  Lava spine
  Lava tube
  Maar
  Malpais
  Mamelon
  Mid-ocean ridge
  Pit crater
  Pseudocrater
  Pyroclastic shield
  Resurgent dome
  Seamount
  Shield volcano
  Stratocone
  Stratovolcano
  Somma volcano
  Spatter cone
  Subglacial mound
  Submarine volcano
  Supervolcano
  Tuff cone
  Tuya
  Vent
  Volcanic cone
  Volcanic crater
  Volcanic dam
  Volcanic field
  Volcanic group
  Volcanic island
  Volcanic plateau
  Volcanic plug
  Volcano
  
  For more information, see Volcanic Landforms .
• Weathering Landforms From Wikipedia:
  Weathering landforms include:
  
  Bornhardt
  Etchplain
  Flared slope
  Flute
  Honeycomb weathering
  Inselberg
  Karst
  Nubbin
  Panhole (Weathering pit)
  Tafoni
  Tor
  
  For more information, see Weathering Landforms .


• Category:Types of soil From Wikipedia:
  Subcategories
  This category has the following 6 subcategories, out of 6 total.
  
  A
  Alkaline soils (4 P)
  
  C
  Clay (4 C, 13 P)
  
  P
  Peat (2 C, 4 P)
  Permafrost (1 C, 25 P)
  
  S
  Serpentine soils (2 P)
  
  V
  Volcanic soils (5 P)
  
  Pages in category "Types of soil"
  The following 138 pages are in this category, out of 138 total. This list may not reflect recent changes (learn more).
  
  
  Soil type
  
  Soil series
  
  A
  Acid sulfate soil
  Acrisol
  Akadama
  Alfisol
  Residual sodium carbonate index
  Alvar
  Andisol
  Andosol
  Anthrepts
  Anthrosol
  Antigo (soil)
  Aridisol
  Australian Soil Classification
  
  B
  Bama (soil)
  Bay mud
  Berkshire (soil)
  Blandford (soil)
  Bog
  Brickearth
  Brown earth
  Brown podzolic
  
  C
  Calcid
  Calciorthid
  Calcisol
  Cambisol
  Casa Grande (soil)
  Cecil (soil)
  Charlottetown (soil series)
  Chernozem
  Cid (soil)
  Clay
  Claypan
  
  D
  Downer (soil)
  Drummer (soil)
  Durisol
  
  E
  Entisol
  Expansive clay
  
  F
  Fill dirt
  Flark
  Flatwood
  Fluvisol
  Fuller's earth
  
  G
  Gelisol
  Gleysol
  Greenwich (soil)
  Guelph soil
  Gumbo (soil)
  Gypsisols
  
  H
  Haplocambids
  Histosol
  Houdek (soil)
  Houston black (soil)
  Hume (soil)
  Hydric soil
  Hydrophobic soil
  
  I
  Inceptisol
  
  J
  Jory (soil)
  
  K
  Kalkaska sand
  Kastanozems
  
  L
  Latosol
  Leptosol
  List of bogs
  Lixisol
  Loam
  Loess
  
  M
  Menfro
  Mesotrophic soil
  Miami (soil)
  Mire
  Mollisol
  Muskeg
  Myakka (soil)
  
  N
  Narragansett (soil)
  Natchez silt loam
  Necrosol
  Nitisol
  
  O
  Orovada (soil)
  Orthent
  Oxisol
  
  P
  Paleosol
  Paxton (soil)
  Peat
  Pedalfer
  Pedocal
  Phaeozem
  Plaggen soil
  Planosol
  Plinthosol
  Podzol
  Polish Soil Classification
  Port Silt Loam
  Prime farmland
  Psamment
  
  R
  Rankers
  Red soil
  Referentiel pedologique
  Regosol
  Rendzina
  
  S
  San Joaquin (soil)
  Sapric
  Scobey (soil)
  Seitz (soil)
  Serpentine soil
  Shrub swamp
  Soil classification
  Solonchak
  Solonetz
  Spodic soil
  Stagnogley
  Stagnosol
  Stuttgart (soil)
  Subaqueous soil
  Subsoil
  
  T
  Talik
  Tanana (soil)
  Technosol
  Tepetate
  Terra fusca
  Terra preta
  Terra rossa (soil)
  Threebear (soil)
  Tifton (soil)
  Topsoil
  Tropical peat
  
  U
  Ultisol
  Umbrisol
  Unified Soil Classification System
  List of U.S. state soils
  USDA soil taxonomy
  Ustic
  Ustochrept
  
  V
  Vertisol
  List of vineyard soil types
  
  W
  Windsor (soil)
  World Reference Base for Soil Resources
  
  Y
  Yedoma
  
  For more information, see Category:Types of soil .
• Soil Type From Wikipedia:
  In terms of soil texture, soil type usually refers to the different sizes of mineral particles in a particular sample.
  Soil is made up in part of finely ground rock.
  Hard surface of base is called hard strata soil particles, grouped according to size as sand and silt in addition to clay, organic material such as decomposed plant matter.
  
  Each component, and their size, play an important role.
  For example, the large particles, sand, determine aeration and drainage characteristics, while the tiniest, sub-microscopic clay particles, are chemically active, binding with water and plant nutrients.
  The ratio of these sizes determines soil type: clay, loam, clay-loam, silt-loam, and so on.
  
  In addition to the mineral composition of soil, humus (organic material) also plays an important role in soil characteristics and fertility for plant life.
  Soil may be mixed with larger aggregate, such as pebbles or gravel.
  Not all types of soil are permeable, such as pure clay.
  
  There are many recognized soil classifications, both international and national.
  
  For more information, see Soil Type .

For more information, see Geomorphology . close

Development of Weather and Climate Patterns

From Wikipedia:
"Climate is the statistics of weather over long periods of time.
It is measured by assessing the patterns of variation in temperature, humidity, atmospheric pressure, wind, precipitation, atmospheric particle count and other meteorological variables in a given region over long periods of time.
Climate differs from weather, in that weather only describes the short-term conditions of these variables in a given region."
For more information, see Climate .

The Earth's climate pattern, at any particular time in its history, depends on a number of factors, including:
- the atmosphere
- the location, shape and extent of the landmasses
- the location, shape and extent of the Earth's oceans, seas, lakes and other bodies of water
- the location and extent of coverage (if any) of the Earth by ice sheets, glaciers and other long-lasting bodies of ice
- the totality of all the ecosystems (if any) on the Earth


Since, as has been seen in other modules, all of the above change over time, the Earth's pattern of climate also changes over time.
The aim of this module is to set the Earth's present pattern of climate in context over time, and to show how it is related to the features listed above.


Agents/Mechanisms/Processes of Change:

• Air Mass From Wikipedia:
  In meteorology, an air mass is a volume of air defined by its temperature and water vapor content.
  Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them.
  They are classified according to latitude and their continental or maritime source regions.
  Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical.
  Continental and superior air masses are dry while maritime and monsoon air masses are moist.
  Weather fronts separate air masses with different density (temperature and/or moisture) characteristics.
  Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character.
  Classification schemes tackle an air mass' characteristics, as well as modification.
  
  For more information, see Air Mass .
• Antarctic oscillation From Wikipedia:
  The Antarctic oscillation (AAO, to distinguish it from the Arctic oscillation or AO) is a low-frequency mode of atmospheric variability of the southern hemisphere.
  It is also known as the Southern Annular Mode (SAM).
  It is defined as a belt of westerly winds or low pressure surrounding Antarctica which moves north or south as its mode of variability.
  In its positive phase, the westerly wind belt that drives the Antarctic Circumpolar Current intensifies and contracts towards Antarctica, while its negative phase involves this belt moving towards the Equator.
  Winds associated with the Southern Annular Mode cause oceanic upwelling of warm circumpolar deep water along the Antarctic continental shelf, which has been linked to ice shelf basal melt, representing a possible wind-driven mechanism that could destabilize large portions of the Antarctic Ice Sheet.
  
  In 2014, Dr Nerilie Abram used a network of temperature-sensitive ice core and tree growth records to reconstruct a 1000-year history of the Southern Annular Mode.
  This work suggests that the Southern Annular Mode is currently in its most extreme positive phase over at least the last 1000 years, and that recent positive trends in the SAM are attributed to increasing greenhouse gas levels and later stratospheric ozone depletion.
  
  For more information, see Antarctic oscillation .
• Anticyclogenesis From Wikipedia:
  Anticyclogenesis is the development or strengthening of anticyclonic circulation in the atmosphere.
  It is the opposite of anticyclolysis, and has a cyclonic equivalent—cyclogenesis.
  Anticyclones are alternatively referred to as high pressure systems.
  High pressure areas form due to downward motion through the troposphere, the atmospheric layer where weather occurs.
  Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs.
  On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere.
  On weather maps, high pressure centers are associated with the letter H.
  On constant pressure upper level charts, it is located within the highest height line contour.
  
  Process
  High-pressure systems form due to downward motion through the troposphere, the atmospheric layer where weather occurs.
  Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs.
  On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere.
  High-pressure systems are alternatively referred to as anticyclones.
  On weather maps, high-pressure centers are associated with the letter H placed within the isobar with the highest pressure value.
  On constant pressure upper level charts, it is located within the highest height line contour.
  
  For more information, see Anticyclogenesis .
• Arctic oscillation From Wikipedia:
  The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic and Antarctic poles north (or south) of 20 degrees latitude.
  The index varies over time with no particular periodicity, and is characterized by non-seasonal sea-level pressure anomalies of one sign in the Arctic, balanced by anomalies of opposite sign centered at about 37–45N.
  
  The North Atlantic oscillation (NAO) is a close relative of the AO.
  There is debate over whether one or the other is more fundamentally representative of the atmosphere's dynamics.
  The NAO may be identified in a more physically meaningful way, which may carry more impact on measurable effects of changes in the atmosphere.
  
  For more information, see Arctic oscillation .
• Atlantic Equatorial mode From Wikipedia:
  The Atlantic Equatorial Mode or Atlantic Niño is a quasiperiodic interannual climate pattern of the equatorial Atlantic Ocean.
  It is the dominant mode of year-to-year variability that results in alternating warming and cooling episodes of sea surface temperatures accompanied by changes in atmospheric circulation.
  The term Atlantic Niño comes from its close similarity with the El Niño-Southern Oscillation (ENSO) that dominates the tropical Pacific basin.
  The Atlantic Niño is not the same as the Atlantic Meridional (Interhemispheric) Mode that consists of a north-south dipole and operates more on decadal timescales.
  The equatorial warming and cooling events associated with the Atlantic Niño are known to be strongly related to atmospheric climate anomalies, especially in African countries bordering the Gulf of Guinea.
  Therefore, understanding of the Atlantic Niño (or lack thereof) has important implications for climate prediction in those regions.
  Although the Atlantic Niño is an intrinsic mode to the equatorial Atlantic, there may be a tenuous causal relationship between ENSO and the Atlantic Niño in some circumstances.
  
  For more information, see Atlantic Equatorial mode .
• Atlantic multidecadal oscillation From Wikipedia:
  The Atlantic Multidecadal Oscillation (AMO) is a climate cycle that affects the sea surface temperature (SST) of the North Atlantic Ocean based on different modes on multidecadal timescales.
  While there is some support for this mode in models and in historical observations, controversy exists with regard to its amplitude, and in particular, the attribution of sea surface temperature change to natural or anthropogenic causes, especially in tropical Atlantic areas important for hurricane development.
  The Atlantic multidecadal oscillation is also connected with shifts in hurricane activity, rainfall patterns and intensity, and changes in fish populations.
  
  For more information, see Atlantic multidecadal oscillation .
• Atmospheric circulation From Wikipedia:
  Atmospheric circulation is the large-scale movement of air, and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth.
  
  The Earth's atmospheric circulation varies from year to year, but the large-scale structure of its circulation remains fairly constant.
  The smaller scale weather systems – mid-latitude depressions, or tropical convective cells – occur "randomly", and long-range weather predictions of those cannot be made beyond ten days in practice, or a month in theory (see Chaos theory and Butterfly effect).
  
  The Earth's weather is a consequence of its illumination by the Sun, and the laws of thermodynamics.
  The atmospheric circulation can be viewed as a heat engine driven by the Sun's energy, and whose energy sink, ultimately, is the blackness of space.
  The work produced by that engine causes the motion of the masses of air and in that process, it redistributes the energy absorbed by the Earth's surface near the tropics to the latitudes nearer the poles, and then to space.
  
  For more information, see Atmospheric circulation .
• Atmospheric river From Wikipedia:
  An atmospheric river (AR) is a narrow corridor or filament of concentrated moisture in the atmosphere.
  Atmospheric rivers consist of narrow bands of enhanced water vapor transport, typically along the boundaries between large areas of divergent surface air flow, including some frontal zones in association with extratropical cyclones that form over the oceans.
  Pineapple Express storms are the most commonly represented and recognized type of atmospheric rivers; they are given the name due to the warm water vapor plumes originating over the Hawaiian tropics that follow a path towards California.
  
  For more information, see Atmospheric river .
• Climate (see also the quote above) From Wikipedia:
  ... The climate of a location is affected by its latitude, terrain, and altitude, as well as nearby water bodies and their currents.
  Climates can be classified according to the average and the typical ranges of different variables, most commonly temperature and precipitation. ...
  
  ... Climate change may occur over long and short timescales from a variety of factors; recent warming is discussed in global warming.
  
  For more information, see Climate.
• Cloud feedback From Wikipedia:
  Cloud feedback is the coupling between cloudiness and surface air temperature where a surface air temperature change leads to a change in clouds, which could then amplify or diminish the initial temperature perturbation.
  Cloud feedbacks can affect the magnitude of internally generated climate variability or they can affect the magnitude of climate change resulting from external radiative forcings.
  
  Global warming is expected to change the distribution and type of clouds.
  Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect.
  Cloud representations vary among global climate models, and small changes in cloud cover have a large impact on the climate.
  Differences in planetary boundary layer cloud modeling schemes can lead to large differences in derived values of climate sensitivity.
  A model that decreases boundary layer clouds in response to global warming has a climate sensitivity twice that of a model that does not include this feedback.
  However, satellite data show that cloud optical thickness actually increases with increasing temperature.
  Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud; details that are difficult to represent in climate models.
  
  For more information, see Cloud feedback .
• Cold front From Wikipedia:
  A cold front is the leading edge of a cooler mass of air, replacing at ground level a warmer mass of air, which lies within a fairly sharp surface trough of low pressure.
  It forms in the wake of an extratropical cyclone, at the leading edge of its cold air advection pattern, which is also known as the cyclone's dry conveyor belt circulation.
  Temperature changes across the boundary can exceed 30 °C (54 °F).
  When enough moisture is present, rain can occur along the boundary.
  If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone.
  If instability is less, a broad shield of rain can move in behind the front, which increases the temperature difference across the boundary. Cold fronts are stronger in the fall and spring transition seasons and weakest during the summer.
  When a cold front catches up with the preceding warm front, the portion of the boundary that does so is then known as an occluded front.
  
  For more information, see Cold front.
• Cyclogenesis From Wikipedia:
  Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low-pressure area).
  Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale.
  
  - Tropical cyclones form due to latent heat driven by significant thunderstorm activity, developing a warm core. - Extratropical cyclones form as waves along weather fronts before occluding later in their life cycle as cold core cyclones. - Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.
  The process in which an extratropical cyclone undergoes a rapid drop in atmospheric pressure (24 millibars or more) in a 24-hour period is referred to as explosive cyclogenesis, and is usually present during the formation of a nor'easter.
  The anticyclonic equivalent, the process of formation of high pressure areas, is anticyclogenesis.
  The opposite of cyclogenesis is cyclolysis.
  
  For more information, see Cyclogenesis .
• El Niño–Southern Oscillation From Wikipedia:
  El Niño–Southern Oscillation (ENSO) is an irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting climate of much of the tropics and subtropics.
  The warming phase of the sea temperature is known as El Niño and the cooling phase as La Niña.
  Southern Oscillation is the accompanying atmospheric component, coupled with the sea temperature change: El Niño is accompanied with high, and La Niña with low air surface pressure in the tropical western Pacific.
  The two periods last several months each (typically occurring every few years) and their effects vary in intensity.
  
  The two phases relate to the Walker circulation, discovered by Gilbert Walker during the early twentieth century.
  The Walker circulation is caused by the pressure gradient force that results from a high pressure system over the eastern Pacific Ocean, and a low pressure system over Indonesia.
  When the Walker circulation weakens or reverses, an El Niño results, causing the ocean surface to be warmer than average, as upwelling of cold water occurs less or not at all.
  An especially strong Walker circulation causes a La Niña, resulting in cooler ocean temperatures due to increased upwelling.
  
  For more information, see El Niño–Southern Oscillation .
• Explosive cyclogenesis From Wikipedia:
  Explosive cyclogenesis (also referred to as a weather bomb, meteorological bomb, explosive development, bomb cyclone or bombogenesis) is the rapid deepening of an extratropical cyclonic low-pressure area.
  The change in pressure needed to classify something as explosive cyclogenesis is latitude dependent.
  For example, at 60° latitude, explosive cyclogenesis occurs if the central pressure decreases by 24 mbar (hPa) or more in 24 hours.
  This is a predominantly maritime, winter event, but also occurs in continental settings, even in the summer.
  This process is the extratropical equivalent of the tropical rapid deepening.
  Although their cyclogenesis is totally different from that of tropical cyclones, bombs can produce winds of the same order as the first categories of the Saffir-Simpson scale and give heavy rainfall.
  Even though only a minority of the bombs become so strong, some have caused significant damage.
  
  For more information, see Explosive cyclogenesis .
• Ferrel cell From Wikipedia:
  Part of the air rising at 60° latitude diverges at high altitude toward the poles and creates the polar cell.
  The rest moves toward the equator where it collides at 30° latitude with the high-level air of the Hadley cell.
  There it subsides and strengthens the high pressure ridges beneath.
  A large part of the energy that drives the Ferrel cell is provided by the polar and Hadley cells circulating on either side and that drag the Ferrel cell with it.
  The Ferrel cell, theorized by William Ferrel (1817–1891), is, therefore, a secondary circulation feature, whose existence depends upon the Hadley and polar cells on either side of it.
  It might be thought of as an eddy created by the Hadley and polar cells.
  
  The air of the Ferrel cell that descends at 30° latitude returns poleward at the ground level, and as it does so it deviates toward the east.
  In the upper atmosphere of the Ferrel cell, the air moving toward the equator deviates toward the west.
  Both of those deviations, as in the case of the Hadley and polar cells, are driven by conservation of angular momentum.
  As a result, just as the easterly Trade Winds are found below the Hadley cell, the Westerlies are found beneath the Ferrel cell.
  
  For more information, see Ferrel cell .
• Frontogenesis From Wikipedia:
  Frontogenesis is a meteorological process of tightening of horizontal temperature gradients to produce fronts.
  In the end, two types of fronts form: cold fronts and warm fronts.
  A cold front is a narrow line where temperature decreases rapidly.
  A warm front is a narrow line of warmer temperatures and essentially where much of the precipitation occurs.
  Frontogenesis occurs as a result of a developing baroclinic wave.
  According to Hoskins & Bretherton (1972, p. 11), there are eight mechanisms that influence temperature gradients:
  horizontal deformation,
  horizontal shearing,
  vertical deformation,
  differential vertical motion,
  latent heat release,
  surface friction,
  turbulence and mixing,
  and radiation.
  Semigeostrophic frontogenesis theory focuses on the role of horizontal deformation and shear.
  
  For more information, see Frontogenesis .
• Frontolysis From Wikipedia:
  Frontolysis in meteorology, is the dissipation or weakening of an atmospheric front.
  
  In contrary to areas of "Frontogenesis", the areas where air masses diverge are called areas of frontolysis.
  
  For more information, see Frontolysis .
• Greenhouse effect From Wikipedia:
  The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without its atmosphere.
  
  If a planet's atmosphere contains radiatively active gases (i.e., greenhouse gases) they will radiate energy in all directions.
  Part of this radiation is directed towards the surface, warming it.
  The intensity of the downward radiation – that is, the strength of the greenhouse effect – will depend on the atmosphere's temperature and on the amount of greenhouse gases that the atmosphere contains.
  
  Earth’s natural greenhouse effect is critical to supporting life.
  Human activities, mainly the burning of fossil fuels and clearing of forests, have strengthened the greenhouse effect and caused global warming.
  
  For more information, see Greenhouse effect .
• Hadley cell From Wikipedia:
  The Hadley cell, named after George Hadley, is a global scale tropical atmospheric circulation that features air rising near the Equator, flowing poleward at 10–15 kilometers above the surface, descending in the subtropics, and then returning equatorward near the surface.
  This circulation creates the trade winds, tropical rain-belts and hurricanes, subtropical deserts and the jet streams.
  
  In each hemisphere, there is one primary circulation cell known as a Hadley cell and two secondary circulation cells at higher latitudes; between 30° and 60° latitude known as the Ferrel cell and beyond 60° as the Polar cell.
  
  For more information, see Hadley cell .
• High-pressure area From Wikipedia:
  A high-pressure area, high or anticyclone is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment.
  
  Winds within high-pressure areas flow outward from the higher pressure areas near their centers towards the lower pressure areas further from their centers.
  Gravity adds to the forces causing this general movement, because the higher pressure compresses the column of air near the center of the area into greater density – and so greater weight compared to lower pressure, lower density, and lower weight of the air outside the center.
  
  However, because the planet is rotating underneath the atmosphere, and frictional forces arise as the planetary surface drags some atmosphere with it, the air flow from center to periphery is not direct, but is twisted due to the Coriolis effect, or the merely apparent force that arise when the observer is in a rotating frame of reference.
  Viewed from above this twist in wind direction is in the same direction as the rotation of the planet.
  
  For more information, see High-pressure area .
• Inflow (meteorology) From Wikipedia:
  Inflow is the flow of a fluid into a large collection of that fluid.
  Within meteorology, inflow normally refers to the influx of warmth and moisture from air within the Earth's atmosphere into storm systems.
  Extratropical cyclones are fed by inflow focused along their cold front and warm fronts.
  Tropical cyclones require a large inflow of warmth and moisture from warm oceans in order to develop significantly, mainly within the lowest 1 kilometre (0.62 mi) of the atmosphere.
  Once the flow of warm and moist air is cut off from thunderstorms and their associated tornadoes, normally by the thunderstorm's own rain-cooled outflow boundary, the storms begin to dissipate.
  Rear inflow jets behind squall lines act to erode the broad rain shield behind the squall line, and accelerate its forward motion.
  
  For more information, see Inflow (meteorology) .
• Interdecadal Pacific Oscillation From Wikipedia:
  The Interdecadal Pacific oscillation (IPO) is an oceanographic/meteorological phenomenon similar to the Pacific decadal oscillation (PDO), but occurring in a wider area of the Pacific.
  While the PDO occurs in mid-latitudes of the Pacific Ocean in the northern hemisphere, the IPO stretches from the southern hemisphere into the northern hemisphere.
  
  The period of oscillation is roughly 15-30 years.
  Positive phases of the IPO are characterized by a warmer than average tropical Pacific and cooler than average northern Pacific.
  Negative phases are characterized by an inversion of this pattern, with cool tropics and warm northern regions.
  
  The IPO had positive phases (southeastern tropical Pacific warm) from 1922 to 1946 and 1978 to 1998, and a negative phase between 1947 and 1976.
  
  For more information, see Interdecadal Pacific Oscillation .
• Intertropical Convergence Zone From Wikipedia:
  The Intertropical Convergence Zone (ITCZ), known by sailors as the doldrums, is the area encircling Earth near the Equator, where the northeast and southeast trade winds converge.
  
  The ITCZ was originally identified from the 1920s to the 1940s as the "Intertropical Front" ("ITF"), but after the recognition in the 1940s and 1950s of the significance of wind field convergence in tropical weather production, the term ITCZ was then applied.
  When it lies near the Equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough, a usage more common in Australia and parts of Asia.
  In the seamen's speech, the zone is referred to as the doldrums because of its erratic (monotonous) weather patterns with stagnant calms and violent thunderstorms.
  
  The ITCZ appears as a band of clouds, usually thunderstorms, that encircle the globe near the Equator.
  
  For more information, see Intertropical Convergence Zone .
• Jet stream From Wikipedia:
  Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of some planets, including Earth.
  On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds (flowing west to east).
  Their paths typically have a meandering shape.
  Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet.
  
  The strongest jet streams are the polar jets, at 9–12 km (30,000–39,000 ft) above sea level, and the higher altitude and somewhat weaker subtropical jets at 10–16 km (33,000–52,000 ft).
  The Northern Hemisphere and the Southern Hemisphere each have a polar jet and a subtropical jet.
  The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round.
  
  For more information, see Jet stream.
• Low-pressure area From Wikipedia:
  A low-pressure area, low, or depression, is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations.
  Low-pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere.
  The formation process of a low-pressure area is known as cyclogenesis.
  Within the field of meteorology, atmospheric divergence aloft occurs in two areas.
  The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies (a trough with large wavelength that extends through the troposphere).
  A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength.
  Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity.
  
  Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses.
  Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface.
  Large-scale thermal lows over continents help drive monsoon circulations.
  Low-pressure areas can also form due to organized thunderstorm activity over warm water.
  When this occurs over the tropics in concert with the Intertropical Convergence Zone, it is known as a monsoon trough.
  Monsoon troughs reach their northerly extent in August and their southerly extent in February.
  When a convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone.
  Tropical cyclones can form during any month of the year globally, but can occur in either the northern or southern hemisphere during November.
  
  For more information, see Low-pressure area.
• Madden–Julian oscillation From Wikipedia:
  The Madden–Julian oscillation (MJO) is the largest element of the intraseasonal (30- to 90-day) variability in the tropical atmosphere.
  It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric Research (NCAR).
  It is a large-scale coupling between atmospheric circulation and tropical deep convection.
  Unlike a standing pattern like the El Niño Southern Oscillation (ENSO), the Madden–Julian oscillation is a traveling pattern that propagates eastward at approximately 4 to 8 m/s (14 to 29 km/h, 9 to 18 mph), through the atmosphere above the warm parts of the Indian and Pacific oceans.
  This overall circulation pattern manifests itself most clearly as anomalous rainfall.
  
  The Madden–Julian oscillation is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian and Pacific Ocean.
  The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific.
  This pattern of tropical rainfall then generally becomes nondescript as it moves over the cooler ocean waters of the eastern Pacific (except over the region of warmer water off the west coast of Central America) but occasionally reappears at low amplitude over the tropical Atlantic and higher amplitude over the Indian Ocean.
  The wet phase of enhanced convection and precipitation is followed by a dry phase where thunderstorm activity is suppressed.
  Each cycle lasts approximately 30–60 days.
  Because of this pattern, the Madden–Julian oscillation is also known as the 30- to 60-day oscillation, 30- to 60-day wave, or intraseasonal oscillation.
  
  For more information, see Madden–Julian oscillation.
• Monsoon trough From Wikipedia:
  The monsoon trough is a portion of the Intertropical Convergence Zone, as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, is a convergence zone between the wind patterns of the southern and northern hemispheres.
  
  Westerly monsoon winds lie in its equatorward portion while easterly trade winds exist poleward of the trough.
  Right along its axis, heavy rains can be found which usher in the peak of a location's respective rainy season.
  As it passes poleward of a location, hot and dry conditions develop.
  The monsoon trough plays a role in creating many of the world's rainforests.
  
  The term "monsoon trough" is most commonly used in monsoonal regions of the Western Pacific such as Asia and Australia.
  The migration of the ITCZ/monsoon trough into a landmass heralds the beginning of the annual rainy season during summer months.
  Depressions and tropical cyclones often form in the vicinity of the monsoon trough, with each capable of producing a year's worth of rainfall in a matter of days.
  
  For more information, see Monsoon trough.
• North Atlantic oscillation From Wikipedia:
  The North Atlantic Oscillation (NAO) is a weather phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level (SLP) between the Icelandic low and the Azores high.
  Through fluctuations in the strength of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and location of storm tracks across the North Atlantic.
  It is part of the Arctic oscillation, and varies over time with no particular periodicity.
  
  The NAO was discovered through several studies in the late 19th and early 20th centuries.
  Unlike the El Niño-Southern Oscillation phenomenon in the Pacific Ocean, the NAO is a largely atmospheric mode.
  It is one of the most important manifestations of climate fluctuations in the North Atlantic and surrounding humid climates.
  
  The North Atlantic Oscillation is closely related to the Arctic oscillation (AO) (or Northern Annular Mode (NAM)), but should not be confused with the Atlantic Multidecadal Oscillation (AMO).
  
  For more information, see North Atlantic oscillation.
• Occluded front From Wikipedia:
  In meteorology, an occluded front is a weather front formed during the process of cyclogenesis when a cold front overtakes a warm front.
  When this occurs, the warm air is separated (occluded) from the cyclone center at the Earth's surface.
  The point where the warm front and the occluded front meet (and consequently the nearest location of warm air to the center of the cyclone) is called the triple point.
  
  The trowal (short for TROugh of Warm air ALoft) is the projection on the Earth's surface of the trough of warm air aloft formed during the occlusion process of the depression.
  
  For more information, see Occluded front.
• Ocean current From Wikipedia:
  An ocean current is a seasonal directed movement of sea water generated by forces acting upon this mean flow, such as wind, the Coriolis effect, breaking waves, cabbing, temperature and salinity differences, while tides are caused by the gravitational pull of the Sun and Moon.
  Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength.
  Ocean currents are primarily horizontal water movements.
  
  Ocean currents flow for great distances, and together, create the global conveyor belt which plays a dominant role in determining the climate of many of the Earth’s regions.
  More specifically, ocean currents influence the temperature of the regions through which they travel.
  For example, warm currents traveling along more temperate coasts increase the temperature of the area by warming the sea breezes that blow over them.
  Perhaps the most striking example is the Gulf Stream, which makes northwest Europe much more temperate than any other region at the same latitude.
  Another example is Lima, Peru, where the climate is cooler, being sub-tropical, than the tropical latitudes in which the area is located, due to the effect of the Humboldt Current.
  
  For more information, see Ocean current.
• Orographic lift From Wikipedia:
  Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain.
  As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.
  
  For more information, see Orographic lift.
• Outflow (meteorology) From Wikipedia:
  Outflow, in meteorology, is air that flows outwards from a storm system.
  It is associated with ridging, or anticyclonic flow.
  In the low levels of the troposphere, outflow radiates from thunderstorms in the form of a wedge of rain-cooled air, which is visible as a thin rope-like cloud on weather satellite imagery or a fine line on weather radar imagery.
  Low-level outflow boundaries can disrupt the center of small tropical cyclones.
  However, outflow aloft is essential for the strengthening of a tropical cyclone.
  If this outflow is undercut, the tropical cyclone weakens.
  If two tropical cyclones are in proximity, the upper level outflow from the system to the west can limit the development of the system to the east.
  
  For more information, see Outflow (meteorology).
• Outflow boundary From Wikipedia:
  An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump.
  Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin.
  New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.).
  Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery.
  From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.
  
  For more information, see Outflow boundary.
• Pacific decadal oscillation From Wikipedia:
  The Pacific Decadal Oscillation (PDO) is a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin.
  The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20°N.
  Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales (meaning time periods of a few years to as much as time periods of multiple decades).
  There is evidence of reversals in the prevailing polarity (meaning changes in cool surface waters versus warm surface waters within the region) of the oscillation occurring around 1925, 1947, and 1977; the last two reversals corresponded with dramatic shifts in salmon production regimes in the North Pacific Ocean.
  This climate pattern also affects coastal sea and continental surface air temperatures from Alaska to California.
  
  During a "warm", or "positive", phase, the west Pacific becomes cooler and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs.
  The Pacific Decadal Oscillation was named by Steven R. Hare, who noticed it while studying salmon production pattern results in 1997. ...
  
  ... The PDO spatial pattern and impacts are similar to those associated with ENSO events.
  During the positive phase the wintertime Aleutian low is deepened and shifted southward, warm/humid air is advected along the North American west coast and temperatures are higher than usual from the Pacific Northwest to Alaska but below normal in Mexico and the Southeastern United States.
  Winter precipitation is higher than usual in the Alaska Coast Range, Mexico and the Southwestern United States but reduced over Canada, Eastern Siberia and Australia.
  McCabe et al. showed that the PDO along with the AMO strongly influence multidecadal droughts pattern in the United States, drought frequency is enhanced over much of the Northern United States during the positive PDO phase and over the Southwest United States during the negative PDO phase in both cases if the PDO is associated with a positive AMO.
  The Asian Monsoon is also affected, increased rainfall and decreased summer temperature is observed over the Indian subcontinent during the negative phase.
  
  For more information, see Pacific decadal oscillation.
• Pacific–North American teleconnection pattern From Wikipedia:
  The Pacific–North American teleconnection pattern (PNA) is a climatological term for a large-scale weather pattern with two modes, denoted positive and negative, and which relates the atmospheric circulation pattern over the North Pacific Ocean with the one over the North American continent.
  
  The positive phase of the PNA pattern features above-average barometric pressure heights in the vicinity of Hawaii and over the inter-mountain region of North America, and below-average heights located south of the Aleutian Islands and over the southeastern United States.
  The PNA pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream.
  The positive phase is associated with an enhanced East Asian jet stream and with an eastward shift in the jet exit region toward the western United States.
  The negative phase is associated with a westward retraction of that jet stream toward eastern Asia, blocking activity over the high latitudes of the North Pacific, and a strong split-flow configuration over the central North Pacific.
  
  For more information, see Pacific–North American teleconnection pattern.
• Polar amplification From Wikipedia:
  Polar amplification is the phenomenon that any change in the net radiation balance (for example greenhouse intensification) tends to produce a larger change in temperature near the poles than the planetary average.
  On a planet with an atmosphere that can restrict longwave radiation to space (a greenhouse effect), surface temperatures will be warmer than a simple planetary equilibrium temperature calculation would predict.
  Where the atmosphere or an extensive ocean is able to convect heat polewards, the poles will be warmer and equatorial regions cooler than their local net radiation balances would predict.
  
  For more information, see Polar amplification.
• Polar cell From Wikipedia:
  The Polar cell is a simple system with strong convection drivers.
  Though cool and dry relative to equatorial air, the air masses at the 60th parallel are still sufficiently warm and moist to undergo convection and drive a thermal loop.
  At the 60th parallel, the air rises to the tropopause (about 8 km at this latitude) and moves poleward.
  As it does so, the upper level air mass deviates toward the east.
  When the air reaches the polar areas, it has cooled and is considerably denser than the underlying air.
  It descends, creating a cold, dry high-pressure area.
  At the polar surface level, the mass of air is driven toward the 60th parallel, replacing the air that rose there, and the polar circulation cell is complete.
  As the air at the surface moves toward the equator, it deviates toward the west.
  Again, the deviations of the air masses are the result of the Coriolis effect.
  The air flows at the surface are called the polar easterlies.
  
  The outflow of air mass from the cell creates harmonic waves in the atmosphere known as Rossby waves.
  These ultra-long waves determine the path of the polar jet stream, which travels within the transitional zone between the tropopause and the Ferrel cell.
  By acting as a heat sink, the polar cell moves the abundant heat from the equator toward the polar regions.
  
  For more information, see Polar cell.
• Polar High From Wikipedia:
  The polar highs are areas of high atmospheric pressure around the north and south poles; the north polar high being the stronger one because land gains and loses heat more effectively than sea.
  The cold temperatures in the polar regions cause air to descend to create the high pressure (a process called subsidence), just as the warm temperatures around the equator cause air to rise to create the low pressure intertropical convergence zone.
  Rising air also occurs along bands of low pressure situated just below the polar highs around the 50th parallels of latitude.
  These extratropical convergence zones are occupied by the polar fronts where air masses of polar origin meet and clash with those of tropical or subtropical origin.
  This convergence of rising air completes the vertical cycle around the polar cell in each latitudinal hemisphere.
  Closely related to this concept is the polar vortex.
  
  Surface temperatures under the polar highs are the coldest on Earth, with no month having an average temperature above freezing.
  Regions under the polar high also experience very low levels of precipitation, which leads them to be known as "polar deserts".
  
  Air flows outwards from the poles to create the polar easterlies in the arctic and antarctic areas.
  
  For more information, see Polar High.
• Polar low From Wikipedia:
  A polar low is a small-scale, short-lived atmospheric low pressure system (depression) that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres, as well as the Sea of Japan.
  The systems usually have a horizontal length scale of less than 1,000 kilometres (620 mi) and exist for no more than a couple of days.
  They are part of the larger class of mesoscale weather systems.
  Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms.
  Polar lows have been referred to by many other terms, such as polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air depression.
  Today the term is usually reserved for the more vigorous systems that have near-surface winds of at least 17 m/s (38 mph).
  
  For more information, see Polar low.
• Polar vortex From Wikipedia:
  A polar vortex is an upper level low-pressure area lying near the Earth's poles.
  There are two polar vortices in the Earth's atmosphere, overlying the North and South Poles.
  Each polar vortex is a persistent, large-scale, low pressure zone that rotates counter-clockwise at the North Pole (called a cyclone), and clockwise at the South Pole, i.e. both polar vortices rotate eastward around the poles.
  The bases of the two polar vortices are located in the middle and upper troposphere and extend into the stratosphere.
  Beneath that lies a large mass of cold, dense arctic air.
  The vortices weaken and strengthen from year to year.
  When the vortex of the arctic is strong, it is well defined, there is a single vortex, and the arctic air is well contained; when weaker, which it generally is, it will break into two or more vortices; when very weak, the flow of arctic air becomes more disorganized and masses of cold arctic air can push equatorward, bringing with it a rapid and sharp temperature drop.
  The interface between the cold dry air mass of the pole and the warm moist air mass further south defines the location of the polar front.
  The polar front is centered, roughly at 60° latitude.
  A polar vortex strengthens in the winter and weakens in the summer due to its dependence on the temperature difference between the equator and the poles.
  The vortices span less than 1,000 kilometers (620 miles) in diameter within which they rotate counter-clockwise in the Northern Hemisphere, and in a clockwise fashion in the Southern Hemisphere.
  As with other cyclones, their rotation is driven by the Coriolis effect.
  
  For more information, see Polar vortex.
• Pseudo-cold front From Wikipedia:
  In meteorology, a pseudo-cold front is a boundary between a supercell's inflow region and the rear flank downdraft.
  It usually extends outward from a mesocyclone center, usually toward the south or southwest, and is characterized by advancing of the downdraft air toward the inflow region.
  It is a particular form of gust front.
  
  For more information, see Pseudo-cold front.
• Pseudo-warm front From Wikipedia:
  A pseudo-warm front is a boundary between the in-flow region and the forward-flank downdraft of a supercell.
  It can either be stationary or move in a northeasterly direction.
  If it were stationary it would technically be a pseudo-stationary front.
  
  For more information, see Pseudo-warm front.
• Rapid intensification From Wikipedia:
  Rapid intensification is a meteorological condition that occurs when a tropical cyclone intensifies dramatically in a short period of time.
  The United States National Hurricane Center defines rapid intensification as an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24-hour period. ...
  
  ... Necessary conditions
  External
  In order for rapid intensification to occur, several conditions must be in place.
  Water temperatures must be extremely warm (near or above 30 °C, 86 °F), and water of this temperature must be sufficiently deep such that waves do not churn deeper cooler waters up to the surface.
  Wind shear must be low; when wind shear is high, the convection and circulation in the cyclone will be disrupted.
  
  Internal
  Usually, an anticyclone in the upper layers of the troposphere above the storm must be present as well—for extremely low surface pressures to develop, air must be rising very rapidly in the eyewall of the storm, and an upper-level anticyclone helps channel this air away from the cyclone efficiently.
  Hot towers have been implicated in tropical cyclone rapid intensification, though diagnostically has seen varied skill across basins.
  
  For more information, see Rapid intensification.
• Rear flank downdraft From Wikipedia:
  The rear flank downdraft or RFD is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm.
  These areas of descending air are thought to be essential in the production of many supercellular tornadoes.
  Large hail within the rear flank downdraft often shows up brightly as a hook on weather radar images, producing the characteristic hook echo, which often indicates the presence of a tornado.
  
  For more information, see Rear flank downdraft.
• Sea Surface Temperature From Wikipedia:
  Sea surface temperature (SST) is the water temperature close to the ocean's surface.
  The exact meaning of surface varies according to the measurement method used, but it is between 1 millimetre (0.04 in) and 20 metres (70 ft) below the sea surface.
  Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a short distance of the shore.
  Localized areas of heavy snow can form in bands downwind of warm water bodies within an otherwise cold air mass.
  Warm sea surface temperatures are known to be a cause of tropical cyclogenesis over the Earth's oceans.
  Tropical cyclones can also cause a cool wake, due to turbulent mixing of the upper 30 metres (100 ft) of the ocean.
  SST changes diurnally, like the air above it, but to a lesser degree.
  There is less SST variation on breezy days than on calm days.
  In addition, ocean currents such as the Atlantic Multidecadal Oscillation (AMO), can effect SST's on multi-decadal time scales, a major impact results from the global thermohaline circulation, which affects average SST significantly throughout most of the world's oceans.
  
  Coastal SSTs can cause offshore winds to generate upwelling, which can significantly cool or warm nearby landmasses, but shallower waters over a continental shelf are often warmer.
  Onshore winds can cause a considerable warm-up even in areas where upwelling is fairly constant, such as the northwest coast of South America.
  Its values are important within numerical weather prediction as the SST influences the atmosphere above, such as in the formation of sea breezes and sea fog.
  It is also used to calibrate measurements from weather satellites.
  
  For more information, see Sea Surface Temperature .
• South Pacific convergence zone From Wikipedia:
  The South Pacific Convergence Zone (SPCZ), a reverse-oriented monsoon trough, is a band of low-level convergence, cloudiness and precipitation extending from the Western Pacific Warm Pool at the maritime continent south-eastwards towards French Polynesia and as far as the Cook Islands (160W, 20S).
  The SPCZ is a portion of the Intertropical Convergence Zone (ITCZ) which lies in a band extending east-west near the Equator but can be more extratropical in nature, especially east of the International Date Line.
  It is considered the largest and most important piece of the ITCZ, and has the least dependence upon heating from a nearby landmass during the summer than any other portion of the monsoon trough.
  The SPCZ can affect the precipitation on Polynesian islands in the southwest Pacific Ocean, so it is important to understand how the SPCZ behaves with large-scale, global climate phenomenon, such as the ITCZ, El Niño–Southern Oscillation, and the Interdecadal Pacific oscillation (IPO), a portion of the Pacific decadal oscillation.
  
  For more information, see South Pacific convergence zone .
• Stationary front From Wikipedia:
  A stationary front is a pair of air masses, neither of which is strong enough to replace the other.
  On a weather map, this is shown by an inter-playing series of blue spikes pointing one direction and red domes pointing the other.
  They tend to remain essentially in the same area for extended periods of time, and waves sometimes propagate along the frontal boundary.
  
  A wide variety of weather can be found along a stationary front, but usually clouds, prolonged precipitation, and storm trains are found there.
  When there is a lot of water vapor in the warmer air mass, significant amounts of rain or freezing rain can occur.
  
  Stationary fronts will either dissipate after several days or devolve into shear lines, but can change into a cold or warm front if conditions aloft change.
  Additionally, if one air mass enters the other, then it will change into a cold or warm front, and the stationary front will be reclassified as moving.
  For instance, if a warm air mass enters a cold air mass, since the advancing air mass is warm, the stationary front will change into a warm front.
  
  For more information, see Stationary front .
• Subsidence (atmosphere) From Wikipedia:
  Subsidence, in the Earth's atmosphere, is most commonly caused by a low temperature.
  As the air cools, it becomes denser and moves towards the ground, as warm air becomes less dense and moves upwards.
  Subsiding air, when cool, is subject to adiabatic warming, which tends to cause the evaporation of any clouds that might be present.
  
  Subsidence generally causes high barometric pressure as more air moves into the same space: the polar highs are areas of almost constant subsidence, as are the horse latitudes, and the areas of subsidence are the sources of much of the world's prevailing wind.
  
  Subsidence also causes many smaller-scale weather phenomena, such as morning fog.
  An extreme form of subsidence is a downburst, which can result in damage similar to that produced by a tornado.
  A milder form of subsidence is referred to as downdraft.
  
  For more information, see Subsidence (atmosphere) .
• Subtropical ridge From Wikipedia:
  The subtropical ridge, also known as the subtropical high or horse latitudes, is a significant belt of atmospheric high pressure situated around the latitudes of 30°N in the Northern Hemisphere and 30°S in the Southern Hemisphere.
  It is the product of the global air circulation cell known as the Hadley Cell.
  The subtropical ridge is characterized by mostly calm winds, which act to reduce air quality under its axis by causing fog overnight, and haze during daylight hours as a result of the stable atmosphere found near its location.
  The air descending from the upper troposphere flows out from its center at surface level toward the upper and lower latitudes of each hemisphere, creating both the trade winds and the westerlies.
  The subtropical ridge moves poleward during the summer, reaching its most northern latitude in early fall, before moving equatorward during the cold season.
  The El Niño southern climate oscillation (ENSO) can displace the northern hemisphere subtropical ridge, with La Niñas allowing for a more northerly axis for the ridge, while El Niños show flatter, more southerly ridges.
  The change of the ridge position during ENSO cycles changes the tracks of tropical cyclones that form around their equatorward and western peripheries.
  As the subtropical ridge varies in position and strength, it can enhance or depress monsoon regimes around their low-latitude periphery.
  
  For more information, see Subtropical ridge .
• Thermal wind From Wikipedia:
  An updraft is a small-scale current of rising air, often within a cloud. ...
  
  ... The thermal wind is wind that varies with height due to a balance between the Coriolis and pressure-gradient forces in the atmosphere.
  It is the primary physical mechanism for the jet stream and plays an important role in other large-scale atmospheric phenomena.
  The thermal wind ensures the jet stream is typically strongest in the upper half of the troposphere, which is the atmospheric layer extending from the surface of the planet up to a height of 12 km to 15 km.
  
  For more information, see Thermal wind .
• Thermohaline circulation From Wikipedia:
  An updraft is a small-scale current of rising air, often within a cloud. ...
  
  Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes.
  The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water.
  Wind-driven surface currents (such as the Gulf Stream) travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes (forming North Atlantic Deep Water).
  This dense water then flows into the ocean basins.
  While the bulk of it upwells in the Southern Ocean, the oldest waters (with a transit time of around 1000 years) upwell in the North Pacific.
  Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system.
  On their journey, the water masses transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe.
  As such, the state of the circulation has a large impact on the climate of the Earth.
  
  The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt.
  On occasion, it is used to refer to the meridional overturning circulation (often abbreviated as MOC).
  The term MOC is more accurate and well defined, as it is difficult to separate the part of the circulation which is driven by temperature and salinity alone as opposed to other factors such as the wind and tidal forces.
  Moreover, temperature and salinity gradients can also lead to circulation effects that are not included in the MOC itself.
  
  For more information, see Thermohaline circulation .
• Tornadogenesis From Wikipedia:
  An updraft is a small-scale current of rising air, often within a cloud. ...
  
  Tornadogenesis is the process by which a tornado forms.
  There are many types of tornadoes, and each type of tornado can have several different methods of formation.
  Despite ongoing scientific study and high-profile research projects such as VORTEX, many of the mechanisms of tornado formation are still poorly understood.
  
  For more information, see Tornadogenesis .
• Tropical cyclogenesis From Wikipedia:
  Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere.
  The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs.
  Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.
  
  Tropical cyclogenesis requires six main factors: sufficiently warm sea surface temperatures (at least 26.5 °C (79.7 °F)), atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low-pressure center, a pre-existing low-level focus or disturbance, and low vertical wind shear.
  
  For more information, see Tropical cyclogenesis .
• Tropical wave From Wikipedia:
  Tropical waves, easterly waves, or tropical easterly waves, also known as African easterly waves in the Atlantic region, are a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms.
  West-moving waves can also form from the tail end of frontal zones in the subtropics and tropics, and may be referred to as easterly waves, but these waves are not properly called tropical waves; they are a form of inverted trough sharing many characteristics with fully tropical waves.
  All tropical waves form in the easterly flow along the equatorward side of the subtropical ridge or belt of high pressure which lies north and south of the Intertropical Convergence Zone (ITCZ).
  Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator.
  They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins.
  Tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.
  
  For more information, see Tropical wave .
• Trough (meteorology) From Wikipedia:
  A trough is an elongated (extended) region of relatively low atmospheric pressure, often associated with fronts.
  Troughs may be at the surface, or aloft, or both under various conditions.
  Most troughs bring clouds, showers, and a wind shift, particularly following the passage of the trough.
  This results from convergence or "squeezing" which forces lifting of moist air behind the trough line.
  
  Unlike fronts, there is not a universal symbol for a trough on a weather chart.
  The weather charts in some countries or regions mark troughs by a line.
  In the United States, a trough may be marked as a dashed line or bold line.
  In the UK, Hong Kong and Fiji, it is represented by a bold line extended from a low pressure center or between two low pressure centers; in Macau and Australia, it is a dashed line.
  If they are not marked, troughs may still be identified as an extension of isobars away from a low pressure center.
  
  For more information, see Trough (meteorology) .
• Upper tropospheric cyclonic vortex From Wikipedia:
  An upper tropospheric cyclonic vortex is a vortex, or a circulation with a definable center, that usually moves slowly from east-northeast to west-southwest and is prevalent across Northern Hemisphere's warm season.
  Its circulations generally do not extend below 6,080 metres (19,950 ft) in altitude, as it is an example of a cold-core low.
  A weak inverted wave in the easterlies is generally found beneath it, and it may also be associated with broad areas of high-level clouds.
  Downward development results in an increase of cumulus clouds and the appearance of circulation at ground level.
  In rare cases, a warm-core cyclone can develop in its associated convective activity, resulting in a tropical cyclone and a weakening and southwest movement of the nearby upper tropospheric cyclonic vortex.
  Symbiotic relationships can exist between tropical cyclones and the upper level lows in their wake, with the two systems occasionally leading to their mutual strengthening. When they move over land during the warm season, an increase in monsoon rains occurs.
  
  For more information, see Upper tropospheric cyclonic vortex .
• Vertical draft From Wikipedia:
  An updraft is a small-scale current of rising air, often within a cloud. ...
  
  ... Localized regions of warm or cool air will exhibit vertical movement.
  A mass of warm air will typically be less dense than the surrounding region, and so will rise until it reaches air that is either warmer or less dense than itself.
  The converse will occur for a mass of cool air, and is known as subsidence.
  This movement of large volumes of air, especially when regions of hot, wet air rise, can create large clouds, and is the central source of thunderstorms.
  Drafts can also be conceived by low or high pressure regions.
  A low pressure region will attract air from the surrounding area, which will move towards the center and then rise, creating an updraft.
  A high pressure region will then attract air from the surrounding area, which will move towards the center and sink, spawning a downdraft.
  
  For more information, see Vertical draft .
• Wake low From Wikipedia:
  A wake low, or wake depression, is a mesoscale low-pressure area which trails the mesoscale high following a squall line.
  Due to the subsiding warm air associated with the systems formation, clearing skies are associated with the wake low.
  Once difficult to detect in surface weather observations due to their broad spacing, the formation of mesoscale weather station networks, or mesonets, has increased their detection.
  Severe weather, in the form of high winds, can be generated by the wake low when the pressure difference between the mesohigh preceding it and the wake low is intense enough.
  When the squall line is in the process of decay, heat bursts can be generated near the wake low.
  Once new thunderstorm activity along the squall line concludes, the wake low associated with it weakens in tandem.
  
  For more information, see Wake low .
• Warm front From Wikipedia:
  A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient.
  Warm fronts lie within broader troughs of low pressure than cold fronts, and move more slowly than the cold fronts which usually follow because cold air is denser and less easy to remove from the Earth's surface.
  This also forces temperature differences across warm fronts to be broader in scale.
  Clouds ahead of the warm front are mostly stratiform, and rainfall gradually increases as the front approaches.
  Fog can also occur preceding a warm frontal passage.
  Clearing and warming is usually rapid after frontal passage.
  If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue.
  On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.
  
  For more information, see Warm front .
• Weather front From Wikipedia:
  A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena outside the tropics.
  In surface weather analyses, fronts are depicted using various colored triangles and half-circles, depending on the type of front.
  The air masses separated by a front usually differ in temperature and humidity.
  
  Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines.
  Warm fronts are usually preceded by stratiform precipitation and fog.
  The weather usually clears quickly after a front's passage.
  Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.
  
  Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward.
  Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions.
  Mountains and warm bodies of water can slow the movement of fronts.
  When a front becomes stationary—and the density contrast across the frontal boundary vanishes—the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline.
  This is most common over the open ocean.
  
  For more information, see Weather front .
• Wind shear From Wikipedia:
  Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere.
  Atmospheric wind shear is normally described as either vertical or horizontal wind shear.
  Vertical wind shear is a change in wind speed or direction with change in altitude.
  Horizontal wind shear is a change in wind speed with change in lateral position for a given altitude.
  
  Wind shear is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts.
  It is commonly observed near microbursts and downbursts caused by thunderstorms, fronts, areas of locally higher low-level winds referred to as low level jets, near mountains, radiation inversions that occur due to clear skies and calm winds, buildings, wind turbines, and sailboats.
  Wind shear has significant effects on control of an aircraft, and it has been a sole or contributing cause of many aircraft accidents.
  
  For more information, see Wind shear .

Weather Features:

• Air current From Wikipedia:
  Air currents are concentrated areas of winds.
  They are mainly due to differences in pressure and/or temperature.
  They are divided into horizontal and vertical currents: both are present at mesoscale while horizontal ones dominate at synoptic scale.
  Air currents are not only found in the troposphere, but extend to the stratosphere and mesosphere.
  
  For more information, see Air current .
• Air-mass thunderstorm From Wikipedia:
  An air-mass thunderstorm, also called an "ordinary", "single cell", or "garden variety" thunderstorm, is a thunderstorm that is generally weak and usually not severe.
  These storms form in environments where at least some amount of Convective Available Potential Energy (CAPE) is present, but very low levels of wind shear and helicity.
  The lifting source, which is a crucial factor in thunderstorm development, is usually the result of uneven heating of the surface, though they can be induced by weather fronts and other low-level boundaries associated with wind convergence.
  The energy needed for these storms to form comes in the form of insolation, or solar radiation.
  Air-mass thunderstorms do not move quickly, last no longer than an hour, and have the threats of lightning, as well as showery light, moderate, or heavy rainfall.
  Heavy rainfall can interfere with microwave transmissions within the atmosphere.
  
  For more information, see Air-mass thunderstorm .
• Anabatic wind From Wikipedia:
  An anabatic wind, from the Greek anabatos, verbal of anabainein meaning moving upward, is a warm wind which blows up a steep slope or mountain side, driven by heating of the slope through insolation.
  It is also known as an upslope flow.
  These winds typically occur during the daytime in calm sunny weather.
  A hill or mountain top will be radiatively warmed by the Sun which in turn heats the air just above it.
  Air at a similar altitude over an adjacent valley or plain does not get warmed so much because of the greater distance to the ground below it.
  The effect may be enhanced if the lower lying ground is shaded by the mountain and so receives less heat.
  
  The air over the hill top is now warmer than the air at a similar altitude around it and will rise through convection.
  This creates a lower pressure region into which the air at the bottom of the slope flows, causing the wind.
  It is common for the air rising from the tops of large mountains to reach a height where it cools adiabatically to below its dew point and forms cumulus clouds.
  These can then produce rain or even thunderstorms.
  
  For more information, see Anabatic wind .
• Antarctic sea ice From Wikipedia:
  Antarctic sea ice is the sea ice of the Southern Ocean.
  It extends far north in winter and retreats almost to the coastline each summer.
  Sea ice is frozen seawater that is usually less than a few meters thick.
  This is in contrast to ice shelves, which are formed by glaciers, float in the sea, and are up to a kilometer thick.
  There are two subdivisions of sea ice: fast ice, which is attached to land; and ice floes, which are not.
  
  Sea ice in the Southern Ocean melts from the bottom instead of from the surface like Arctic ice because it is covered in snow.
  As a result, melt ponds are rarely observed. On average, Antarctic sea ice is younger, thinner, warmer, saltier, and more mobile than Arctic sea ice.
  Due to its inaccessibility, it is not as well-studied as Arctic ice.
  
  For more information, see Antarctic sea ice .
• Anticyclonic storm From Wikipedia:
  An anticyclonic storm is a weather storm where winds around the storm flow in the direction opposite to that of the flow about a region of low pressure. ...
  
  ... In the Northern Hemisphere, anticyclonic storms involve clockwise wind flow; in the Southern Hemisphere, they involve counterclockwise wind flow.
  
  Anticyclonic storms usually form around high-pressure systems.
  These do not "contradict" the Coriolis effect; it predicts such anticyclonic flow about high-pressure regions.
  Anticyclonic storms, as high-pressure systems, usually accompany cold weather and are frequently a factor in large snowstorms.
  Jupiter's Great Red Spot is a well-known non-terrestrial example of an anticyclonic system.
  
  Anticyclonic tornadoes often occur; while tornadoes' vortices are low-pressure regions, this occurs because tornadoes occur on a small enough scale such that the Coriolis effect is negligible.
  
  For more information, see Anticyclonic storm .
• Anticyclonic tornado From Wikipedia:
  An anticyclonic tornado is a tornado which rotates in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere.
  The term is a naming convention denoting the anomaly from normal rotation which is cyclonic in upwards of 98 percent of tornadoes.
  Many anticyclonic tornadoes are smaller and weaker than cyclonic tornadoes, forming from a different process.
  
  For more information, see Anticyclonic tornado .
• April shower From Wikipedia:
  In the United Kingdom and Ireland, an April shower is rain during the month of April.
  One of the major causes of the often heavy downpours is the position of the jet stream.
  In early spring, the jet stream starts to move northwards, allowing large depressions to bring strong winds and rain in from the Atlantic.
  In one day the weather can change from springtime sunshine to winter sleet and snow.
  The track of these depressions can often be across Ireland and Scotland bringing bands of rain followed by heavy showers (often of hail or snow) and strong blustery winds.
  
  For more information, see April shower .
• Arctic ice pack From Wikipedia:
  The Arctic ice pack is the sea ice cover of the Arctic Ocean and its vicinity.
  The Arctic ice pack undergoes a regular seasonal cycle in which ice melts in spring and summer, reaches a minimum around mid-September, then increases during fall and winter.
  Summer ice cover in the Arctic is about 50% of winter cover.
  Some of the ice survives from one year to the next.
  Currently 28% of Arctic basin sea ice is multi-year ice, thicker than seasonal ice: up to 3–4 m (9.8–13.1 ft) thick over large areas, with ridges up to 20 m (65.6 ft) thick.
  As well as the regular seasonal cycle there has been an underlying trend of declining sea ice in the Arctic in recent decades.
  
  For more information, see Arctic ice pack .
• Atlantic hurricane From Wikipedia:
  An Atlantic hurricane or tropical storm is a tropical cyclone that forms in the Atlantic Ocean, usually in the summer or fall.
  A hurricane differs from a cyclone or typhoon only on the basis of location.
  A hurricane is a storm that occurs in the Atlantic Ocean and northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a cyclone occurs in the south Pacific or Indian Ocean.
  
  For more information, see Atlantic hurricane .
• Atlantic hurricane From Wikipedia:
  An Atlantic hurricane or tropical storm is a tropical cyclone that forms in the Atlantic Ocean, usually in the summer or fall.
  A hurricane differs from a cyclone or typhoon only on the basis of location.
  A hurricane is a storm that occurs in the Atlantic Ocean and northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a cyclone occurs in the south Pacific or Indian Ocean.
  
  For more information, see Atlantic hurricane .
• Atmospheric icing From Wikipedia:
  Atmospheric icing occurs when water droplets in the atmosphere freeze on objects they contact.
  This can be extremely dangerous to aircraft, as the built-up ice changes the aerodynamics of the flight surfaces, which can increase the risk of a subsequent stalling of the airfoil.
  For this reason, ice protection systems are often considered critical components of flight, and aircraft are often deiced prior to take-off in icy environments.
  
  Not all water freezes at 0 °C or 32 °F. Liquid water below this temperature is called supercooled, and such supercooled droplets cause the icing problems on aircraft.
  Below -20 °C (-4 °F), icing is rare because clouds at these temperatures usually consist of ice particles rather than supercooled water droplets.
  Below -48 °C (-54.4 °F), supercooled water cannot exist, therefore icing is impossible.
  
  For more information, see Atmospheric icing.
• Atmospheric pressure From Wikipedia:
  Atmospheric pressure, sometimes also called barometric pressure, is the pressure within the atmosphere of Earth (or that of another planet).
  In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point.
  As elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation.
  Pressure measures force per unit area, with SI units of Pascals (1 pascal = 1 newton per square metre, 1 N/m²). ...
  
  ... Atmospheric pressure is caused by the gravitational attraction of the planet on the atmospheric gases above the surface, and is a function of the mass of the planet, the radius of the surface, and the amount of gas and its vertical distribution in the atmosphere.
  It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition. ...
  
  ... Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate.
  See pressure system for the effects of air pressure variations on weather.
  
  Atmospheric pressure shows a diurnal or semidiurnal (twice-daily) cycle caused by global atmospheric tides.
  This effect is strongest in tropical zones, with an amplitude of a few millibars, and almost zero in polar areas.
  These variations have two superimposed cycles, a circadian (24 h) cycle and semi-circadian (12 h) cycle.
  
  For more information, see Atmospheric pressure.
• Black ice From Wikipedia:
  Black ice, sometimes called clear ice, is a thin coating of glaze ice on a surface, especially on roads.
  The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it.
  The typically low levels of noticeable ice pellets, snow, or sleet surrounding black ice means that areas of the ice are often practically invisible to drivers or people stepping on it.
  There is, thus, a risk of skidding and subsequent accident due to the unexpected loss of traction.
  
  For more information, see Black ice .
• Blizzard From Wikipedia:
  A blizzard is a severe snowstorm characterized by strong sustained winds of at least 35 mph (56 km/h) and lasting for a prolonged period of time—typically three hours or more.
  A ground blizzard is a weather condition where snow is not falling but loose snow on the ground is lifted and blown by strong winds.
  Blizzards can have an immense size, which can usually be larger than a few states in the United States.
  
  For more information, see Blizzard.
• Clear-air turbulence From Wikipedia:
  Clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual clues, such as clouds, and is caused when bodies of air moving at widely different speeds meet.
  
  The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around 7,000–12,000 metres (23,000–39,000 ft) as it meets the tropopause.
  Here CAT is most frequently encountered in the regions of jet streams.
  At lower altitudes it may also occur near mountain ranges.
  Thin cirrus clouds can also indicate high probability of CAT.
  
  CAT can be hazardous to the comfort, but rarely the safety, of air travelers.
  
  CAT in the jet stream is expected to become stronger and more frequent because of climate change, with transatlantic wintertime CAT increasing by 59% (light), 94% (moderate), and 149% (severe) by the time of CO2 doubling.
  
  For more information, see Clear-air turbulence .
• Clear ice From Wikipedia:
  Clear ice refers to a solid precipitation which forms when air temperature is between 0 °C (32 °F) and -3 °C (27 °F) and there are supercooled, relatively large drops of water (from freezing fog).
  A rapid accretion and a slow dissipation of latent heat of fusion favor the formation of a transparent ice coating, without air or other impurities.
  
  A similar phenomenon occurs when freezing rain or drizzle hit a surface and is called glaze.
  
  Clear ice, when formed on the ground, is often called black ice, and can be extremely hazardous.
  
  Clear ice is denser and more homogeneous than hard rime; like rime, however, clear ice accumulates on branches and overhead lines, where it is particularly dangerous due to its relative high density.
  
  For more information, see Clear ice .
• Cloud From Wikipedia:
  In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body.
  Water or various other chemicals may compose the droplets and crystals.
  On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture (usually in the form of water vapor) from an adjacent source to raise the dew point to the ambient temperature.
  They are seen in the Earth's homosphere (which includes the troposphere, stratosphere, and mesosphere).
  Nephology is the science of clouds, which is undertaken in the cloud physics branch of meteorology.
  
  For more information, see Cloud.
• Cloudburst From Wikipedia:
  A cloudburst is an extreme amount of precipitation in a short period of time, sometimes accompanied by hail and thunder, that is capable of creating flood conditions.
  A cloudburst can suddenly dump large amounts of water e.g. 25 mm of precipitation corresponds to 25,000 metric tons/km2 (1 inch corresponds to 72,300 short tons over one square mile).
  However, cloudbursts are infrequent as they occur only via orographic lift or occasionally when a warm air parcel mixes with cooler air, resulting in sudden condensation.
  At times, a large amount of runoff from higher elevations is mistakenly conflated with a cloudburst.
  The term "cloudburst" arose from the notion that clouds were akin to water balloons and could burst, resulting in rapid precipitation.
  Though this idea has since been disproven, the term remains in use.
  
  For more information, see Cloudburst.
• Cloud cover From Wikipedia:
  'Cloud cover' (also known as 'cloudiness', 'cloudage', or 'cloud amount') refers to the fraction of the sky obscured by clouds when observed from a particular location.
  Okta is the usual unit of measurement of the cloud cover.
  The cloud cover is correlated to the sunshine duration as the least cloudy locales are the sunniest ones while the cloudiest areas are the least sunny places.
  
  The global cloud cover averages around 0.68 when analyzing clouds with optical depth larger than 0.1.
  This value is lower (0.56) when considering clouds with an optical depth larger than 2, and higher when counting subvisible cirrus clouds.
  
  For more information, see Cloud cover .
• Cold-core low From Wikipedia:
  A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere.
  It is a low pressure system that strengthens with height in accordance with the thermal wind relationship.
  If a weak surface circulation forms in response to such a feature at subtropical latitudes of the eastern north Pacific or north Indian oceans, it is called a subtropical cyclone.
  Cloud cover and rainfall mainly occurs with these systems during the day.
  Severe weather, such as tornadoes, can occur near the center of cold-core lows.
  Cold lows can help spawn cyclones with significant weather impacts, such as polar lows, and Von Karman Vortices.
  Cold lows can lead directly to the development of tropical cyclones, owing to their associated cold pool of air aloft or by acting as additional outflow channels to aid in further development.
  
  For more information, see Cold-core low .
• Cyclone From Wikipedia:
  In meteorology, a cyclone is a large scale air mass that rotates around a strong center of low atmospheric pressure.
  Cyclones are characterized by inward spiraling winds that rotate about a zone of low pressure.
  The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale (the synoptic scale).
  Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale.
  Mesocyclones, tornadoes and dust devils lie within the smaller mesoscale.
  Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere.
  Cyclones have also been seen on extraterrestrial planets, such as Mars and Neptune.
  Cyclogenesis describes the process of cyclone formation and intensification.
  Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones.
  These zones contract and form weather fronts as the cyclonic circulation closes and intensifies.
  Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems.
  A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.
  
  For more information, see Cyclone .
• Derecho From Wikipedia:
  A derecho (from Spanish: derecho , "straight") is a widespread, long-lived, straight-line wind storm that is associated with a land-based, fast-moving group of severe thunderstorms.
  
  Derechos can cause hurricane-force winds, tornadoes, heavy rains, and flash floods.
  Convection-induced winds take on a bow echo (backward "C") form of squall line, forming in an area of wind divergence in upper levels of the troposphere, within a region of low-level warm air advection and rich low-level moisture.
  They travel quickly in the direction of movement of their associated storms, similar to an outflow boundary (gust front), except that the wind is sustained and increases in strength behind the front, generally exceeding hurricane-force.
  
  A warm-weather phenomenon, derechos occur mostly in summer, especially during June, July, and August in the Northern Hemisphere, within areas of moderately strong instability and moderately strong vertical wind shear.
  They may occur at any time of the year and occur as frequently at night as during the daylight hours.
  
  For more information, see Derecho .
• Downburst From Wikipedia:
  A downburst is a strong ground-level wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the point of contact at ground level.
  Often producing damaging winds, it may be confused with a tornado, where high-velocity winds circle a central area, and air moves inward and upward; by contrast, in a downburst, winds are directed downward and then outward from the surface landing point.
  
  Downbursts are created by an area of significantly rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds.
  Dry downbursts are associated with thunderstorms with very little rain, while wet downbursts are created by thunderstorms with high amounts of rainfall.
  Microbursts and macrobursts are downbursts at very small and larger scales respectively.
  Another variety, the heat burst, is created by vertical currents on the backside of old outflow boundaries and squall lines where rainfall is lacking.
  Heat bursts generate significantly higher temperatures due to the lack of rain-cooled air in their formation.
  Downbursts create vertical wind shear or microbursts, which is dangerous to aviation.
  
  For more information, see Downburst .
• Drizzle From Wikipedia:
  Drizzle is a light liquid precipitation consisting of liquid water drops smaller than those of rain – generally smaller than 0.5 mm (0.02 in) in diameter.
  Drizzle is normally produced by low stratiform clouds and stratocumulus clouds.
  Precipitation rates from drizzle are on the order of a millimetre per day or less at the ground.
  Owing to the small size of drizzle drops, under many circumstances drizzle largely evaporates before reaching the surface and so may be undetected by observers on the ground.
  The METAR code for drizzle is DZ and for freezing drizzle is FZDZ.
  
  For more information, see Drizzle .
• Dry season From Wikipedia:
  The dry season is a yearly period of low rainfall, especially in the tropics.
  The weather in the tropics is dominated by the tropical rain belt, which moves from the northern to the southern tropics and back over the course of the year.
  The tropical rain belt lies in the southern hemisphere roughly from October to March; during that time the northern tropics have a dry season with sparser precipitation, and days are typically sunny throughout.
  From April to September, the rain belt lies in the northern hemisphere, and the southern tropics have their dry season.
  Under the Köppen climate classification, for tropical climates, a dry season month is defined as a month when average precipitation is below 60 millimetres (2.4 in).
  
  For more information, see Dry season .
• Dry thunderstorm From Wikipedia:
  A dry thunderstorm or heat storm is a thunderstorm that produces thunder and lightning, but most or all of its precipitation evaporates before reaching the ground.
  Dry lightning refers to lightning strikes occurring in this situation.
  Both are so common in the American West that they are sometimes used interchangeably.
  The latter term is a technical misnomer since lightning itself is neither wet nor dry.
  
  Dry thunderstorms occur essentially in dry conditions, and their lightning is a major cause of wildfires.
  Because of that, the National Weather Service, and other agencies around the world, issue forecasts for its likelihood over large areas.
  
  For more information, see Dry thunderstorm .
• Dust devil From Wikipedia:
  A dust devil is a strong, well-formed, and relatively long-lived whirlwind, ranging from small (half a metre wide and a few metres tall) to large (more than 10 metres wide and more than 1000 metres tall).
  The primary vertical motion is upward. Dust devils are usually harmless, but can on rare occasions grow large enough to pose a threat to both people and property.
  
  They are comparable to tornadoes in that both are a weather phenomenon involving a vertically oriented rotating column of wind.
  Most tornadoes are associated with a larger parent circulation, the mesocyclone on the back of a supercell thunderstorm.
  Dust devils form as a swirling updraft under sunny conditions during fair weather, rarely coming close to the intensity of a tornado.
  
  For more information, see Dust devil .
• Dust storm From Wikipedia:
  A dust storm is a meteorological phenomenon common in arid and semi-arid regions.
  Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface.
  Fine particles are transported by saltation and suspension, a process that moves soil from one place and deposits it in another.
  
  Drylands around North Africa and the Arabian peninsula are the main terrestrial sources of airborne dust.
  It has been argued that poor management of the Earth's drylands, such as neglecting the fallow system, are increasing dust storms size and frequency from desert margins and changing both the local and global climate, and also impacting local economies.
  
  The term sandstorm is used most often in the context of desert sandstorms, especially in the Sahara Desert, or places where sand is a more prevalent soil type than dirt or rock, when, in addition to fine particles obscuring visibility, a considerable amount of larger sand particles are blown closer to the surface.
  The term dust storm is more likely to be used when finer particles are blown long distances, especially when the dust storm affects urban areas.
  
  For more information, see Dust storm .
• European windstorm From Wikipedia:
  European windstorms are the strongest extratropical cyclones which occur across the continent of Europe.
  They form as cyclonic windstorms associated with areas of low atmospheric pressure.
  They are most common in the autumn and winter months.
  On average, the month when most windstorms form is January.
  The seasonal average is 4.6 windstorms.
  Deep low pressure areas are relatively common over the North Atlantic, sometimes starting as nor'easters off the New England coast, and frequently track across the North Atlantic Ocean towards western Europe, past the north coast of Great Britain and Ireland and into the Norwegian Sea.
  However, when they track further south, they can affect almost any country in Europe.
  Commonly affected countries include the United Kingdom, Republic of Ireland, the Netherlands, Norway, the Faroe Islands and Iceland, but any country in Central Europe, Northern Europe and especially Western Europe is occasionally struck by such a storm system.
  
  The strong wind phenomena intrinsic to European windstorms, that give rise to "damage footprints" at the surface, can be placed into three categories, namely the "warm jet", the "cold jet", and the "sting jet".
  These phenomena vary in terms of physical mechanisms, atmospheric structure, spatial extent, duration, severity level, predictability, and location relative to cyclone and fronts.
  
  For more information, see European windstorm .
• Extratropical cyclone From Wikipedia:
  Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth.
  Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes.
  These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth.
  In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.
  
  For more information, see Extratropical cyclone .
• Firestorm From Wikipedia:
  A firestorm is a conflagration which attains such intensity that it creates and sustains its own wind system.
  It is most commonly a natural phenomenon, created during some of the largest bushfires and wildfires.
  Although the word has been used to describe certain large fires, the phenomenon's determining characteristic is a fire with its own storm-force winds from every point of the compass.
  The Black Saturday bushfires and the Great Peshtigo Fire are possible examples of forest fires with some portion of combustion due to a firestorm, as is the Great Hinckley Fire.
  Firestorms have also occurred in cities, usually as a deliberate effect of targeted explosives, such as occurred as a result of the aerial firebombings of Hamburg, Dresden, and the atomic bombing of Hiroshima.
  
  For more information, see Firestorm .
• Fire whirl From Wikipedia:
  A fire whirl – also commonly known as a fire devil, or, (in many cases erroneously), as a fire tornado, firenado, fire swirl, or fire twister – is a whirlwind induced by a fire and often made up of flame or ash.
  They usually start with a whirl of wind or smoke.
  Fire whirls may occur when intense rising heat and turbulent wind conditions combine to form whirling eddies of air.
  These eddies can contract into a tornado-like vortex that sucks in burning debris and combustible gases.
  
  Fire whirls are sometimes colloquially called fire tornadoes, but are not usually classifiable as tornadoes as the vortex in most cases does not extend from the surface to cloud base.
  Also, even in such cases, even those fire whirls are not classic tornadoes, in that their vorticity derives from surface winds and heat-induced lifting, rather than a tornadic mesocyclone aloft.
  
  For more information, see Fire whirl .
• Flash flood From Wikipedia:
  A flash flood is a rapid flooding of geomorphic low-lying areas: washes, rivers, dry lakes and basins.
  It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields.
  Flash floods may occur after the collapse of a natural ice or debris dam, or a human structure such as a man-made dam, as occurred before the Johnstown Flood of 1889.
  Flash floods are distinguished from regular floods by having a timescale of less than six hours.
  The water that is temporarily available is often used by plants with rapid germination and short growth cycles and by specially adapted animal life.
  
  For more information, see Flash flood .
• Flood From Wikipedia:
  A flood is an overflow of water that submerges land that is usually dry.
  The European Union (EU) Floods Directive defines a flood as a covering by water of land not normally covered by water.
  In the sense of "flowing water", the word may also be applied to the inflow of the tide.
  Floods are an area of study of the discipline hydrology and are of significant concern in agriculture, civil engineering and public health.
  
  Flooding may occur as an overflow of water from water bodies, such as a river, lake, or ocean, in which the water overtops or breaks levees, resulting in some of that water escaping its usual boundaries, or it may occur due to an accumulation of rainwater on saturated ground in an areal flood.
  While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, these changes in size are unlikely to be considered significant unless they flood property or drown domestic animals.
  
  Floods can also occur in rivers when the flow rate exceeds the capacity of the river channel, particularly at bends or meanders in the waterway.
  Floods often cause damage to homes and businesses if they are in the natural flood plains of rivers.
  While riverine flood damage can be eliminated by moving away from rivers and other bodies of water, people have traditionally lived and worked by rivers because the land is usually flat and fertile and because rivers provide easy travel and access to commerce and industry.
  Some floods develop slowly, while others such as flash floods, can develop in just a few minutes and without visible signs of rain.
  Additionally, floods can be local, impacting a neighborhood or community, or very large, affecting entire river basins.
  
  For more information, see Flood .
• Foehn wind From Wikipedia:
  A föhn or foehn is a type of dry, warm, down-slope wind that occurs in the lee (downwind side) of a mountain range.
  
  It is a rain shadow wind that results from the subsequent adiabatic warming of air that has dropped most of its moisture on windward slopes (see orographic lift).
  As a consequence of the different adiabatic lapse rates of moist and dry air, the air on the leeward slopes becomes warmer than equivalent elevations on the windward slopes.
  Föhn winds can raise temperatures by as much as 14 °C (25 °F) in just a matter of minutes.
  Central Europe enjoys a warmer climate due to the Föhn, as moist winds off the Mediterranean Sea blow over the Alps.
  
  In some regions, föhn winds are associated with causing "circulatory problems", headaches, or similar ailments.
  Researchers have found, however, the foehn wind's warm temperature to be beneficial to humans in most situations, and have theorised that the reported negative effects may be a result of secondary factors, such as changes in the electrical field or in the ion state of the atmosphere, the wind's relatively low humidity, or the generally unpleasant sensation of being in an environment with strong and gusty winds.
  
  For more information, see Foehn wind .
• Fog From Wikipedia:
  Fog is a visible aerosol consisting of minute water droplets or ice crystals suspended in the air at or near the Earth's surface. Fog can be considered a type of low-lying cloud and is heavily influenced by nearby bodies of water, topography, and wind conditions. In turn, fog has affected many human activities, such as shipping, travel, and warfare. ... ... The term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated locally (such as from a nearby body of water, like a lake or the ocean, or from nearby moist ground or marshes).[2] By definition, fog reduces visibility to less than 1 kilometre (0.62 mi), whereas mist causes lesser impairment of visibility.
  
  For more information, see Fog .
• Freezing rain From Wikipedia:
  Freezing rain is the name given to rain precipitation maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces.
  Unlike sleet, a mixture of rain and snow, ice pellets, or hail, freezing rain is made entirely of liquid droplets.
  The raindrops become supercooled while passing through a sub-freezing layer of air hundreds of meters above the ground, and then freeze upon impact with any surface they encounter, including the ground, trees, electrical wires, aircraft, and automobiles.
  The resulting ice, called glaze ice, can accumulate to a thickness of several centimeters and cover all exposed surfaces.
  The METAR code for freezing rain is FZRA.
  
  A storm that produces a significant thickness of glaze ice from freezing rain is often referred to as an ice storm.
  Although these storms are not particularly violent, freezing rain is notorious for causing travel problems on roadways, breaking tree limbs, and downing power lines from the weight of accumulating ice.
  Downed power lines cause power outages in affected areas while accumulated ice can also pose significant overhead hazards.
  It is also known for being extremely dangerous to aircraft since the ice can effectively 'remould' the shape of the airfoil and flight control surfaces. (See atmospheric icing.)
  
  For more information, see Freezing rain.
• Frost From Wikipedia:
  Frost is the coating or deposit of ice that may form in humid air in cold conditions, usually overnight.
  In temperate climates, it most commonly appears as fragile white crystals or frozen dew drops near the ground, but in cold climates, it occurs in a greater variety of forms.
  Frost is composed of delicate, branched patterns of ice crystals that formed as the result of fractal process development.
  
  The formation of frost is an indication that the air temperature has fallen below the freezing point of water, and plants that have evolved in warmer climates are known to suffer damage when the temperature falls low enough to freeze the water in the cells that make up the plant tissue.
  The tissue damage resulting from this process is known as "frost damage".
  Farmers in those regions where frost damage is known to affect their crops often invest in substantial means to protect their crops from such damage.
  
  For more information, see Frost .
• Glaze (ice) From Wikipedia:
  Glazen or glaze ice, also called glazed frost, is a smooth, transparent and homogeneous ice coating occurring when freezing rain or drizzle hits a surface.
  It is similar in appearance to clear ice, which forms from supercooled water droplets.
  It is a relatively common occurrence in temperate climates in the winter when precipitation forms in warm air aloft and falls into below-freezing temperature at the surface.
  
  For more information, see Glaze (ice) .
• Graupel From Wikipedia:
  Graupel, also called soft hail or snow pellets, is precipitation that forms when supercooled water droplets are collected and freeze on falling snowflakes, forming 2–5 mm (0.08–0.20 in) balls of rime.
  The term graupel comes from the German language.
  
  Graupel is distinct from hail, small hail and ice pellets: the World Meteorological Organization defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail.
  The METAR code for graupel is GS.
  
  For more information, see Graupel .
• Ground frost From Wikipedia:
  Ground frost refers to the various coverings of ice produced by the direct deposition of water vapor on objects and trees, whose surfaces have a temperature below the freezing point of water (0 °C, 32 °F). ... ... The three main types of ground frost are radiation frost (hoar frost), advection frost (advection hoar frost) and evaporation frost.
  The latter is a rare type which occurs when surface moisture evaporates into drier air causing its temperature at the surface to fall at or under the freezing point of water.
  Rime (both soft and hard) is technically not a type of ground frost.
  
  For more information, see Ground frost .
• Gustnado From Wikipedia:
  A gustnado is a short-lived, shallow surface-based vortex which forms within the downburst emanating from a thunderstorm.
  The name is a portmanteau of "gust front tornado", as gustnadoes form due to non-tornadic straight-line wind features in the downdraft (outflow), specifically within the gust front of strong thunderstorms.
  Gustnadoes tend to be noticed when the vortices loft sufficient debris or form condensation cloud to be visible although it is the wind that makes the gustnado, similarly to tornadoes.
  As these eddies very rarely connect from the surface to the cloud base, they are very rarely considered as tornadoes.
  The gustnado has little in common with tornadoes structurally or dynamically in regard to vertical development, intensity, longevity, or formative process --as classic tornadoes are associated with mesocyclones within the inflow (updraft) of the storm, not the outflow.
  
  For more information, see Gustnado .
• Haboob From Wikipedia:
  A haboob (Arabic: translit. habub, lit. 'blasting/drifting') is a type of intense dust storm carried on an atmospheric gravity current, also known as a weather front.
  Haboobs occur regularly in dry land area regions throughout the world. ...
  
  ... During thunderstorm formation, winds move in a direction opposite to the storm's travel, and they move from all directions into the thunderstorm.
  When the storm collapses, and begins to release precipitation, wind directions reverse, gusting outward from the storm and generally gusting the strongest in the direction of the storm's travel.
  
  When this downdraft of cold air, or downburst, reaches the ground, it blows dry, loose silt and clay (collectively, dust) up from the desert, creating a wall of sediment that precedes the storm cloud.
  This wall of dust can be up to 100 km (62 mi) wide and several kilometers in elevation.
  At their strongest, haboob winds often travel at 35–100 km/h (22–62 mph), and they may approach with little or no warning.
  Often rain does not appear at ground level as it evaporates in the hot, dry air (a phenomenon known as virga).
  The evaporation cools the rushing air even further and accelerates it.
  Occasionally, when the rain does persist, it can contain a considerable quantity of dust.
  Severe cases are called mud storms.
  Eye and respiratory system protection is advisable for anyone who must be outside during a haboob.
  Moving to shelter is highly advised during a strong event.
  
  For more information, see Haboob .
• Hail From Wikipedia:
  Hail is a form of solid precipitation.
  It is distinct from ice pellets (American sleet), though the two are often confused.
  It consists of balls or irregular lumps of ice, each of which is called a hailstone.
  Ice pellets (American sleet) fall generally in cold weather while hail growth is greatly inhibited during cold surface temperatures.
  
  Unlike other forms of water ice such as graupel, which is made of rime, and ice pellets, which are smaller and translucent, hailstones usually measure between 5 millimetres (0.2 in) and 15 centimetres (6 in) in diameter.
  The METAR reporting code for hail 5 mm (0.20 in) or greater is GR, while smaller hailstones and graupel are coded GS.
  
  Hail is possible within most thunderstorms as it is produced by cumulonimbus, and within 2 nautical miles (3.7 km) of the parent storm.
  Hail formation requires environments of strong, upward motion of air with the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level.
  In the mid-latitudes, hail forms near the interiors of continents, while in the tropics, it tends to be confined to high elevations.
  
  For more information, see Hail .
• Hard rime From Wikipedia:
  Hard rime is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects.
  It is often seen on trees atop mountains and ridges in winter, when low-hanging clouds cause freezing fog.
  This fog freezes to the windward (wind-facing) side of tree branches, buildings, or any other solid objects, usually with high wind velocities and air temperatures between -2 and -8 °C (28.4 and 17.6 °F).
  
  For more information, see Hard rime .
• Haze From Wikipedia:
  Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates obscure the clarity of the sky.
  The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic ash, dust, sand, and snow.
  Sources for haze particles include farming (ploughing in dry weather), traffic, industry, and wildfires.
  
  For more information, see Haze .
• Heat burst From Wikipedia:
  In meteorology, a heat burst is a rare atmospheric phenomenon characterized by gusty winds along with a rapid increase in temperature and decrease in dew point (moisture).
  Heat bursts typically occur during night-time and are associated with decaying thunderstorms.
  
  Although this phenomenon is not fully understood, it is theorized that the event is caused when rain evaporates (virga) into a parcel of cold, dry air high in the atmosphere- making the air denser than its surroundings.
  The parcel descends rapidly, warming due to compression, overshoots its equilibrium level and reaches the surface, similar to a downburst.
  
  Recorded temperatures during heat bursts have reached well above 38 °C (100 °F), sometimes rising by 11 °C (20 °F) or more within only a few minutes.
  More extreme events have also been documented, where temperatures have been reported to exceed 120 °F (49 °C).
  However, such extreme events have never been officially verified.
  Heat bursts are also characterized by extremely dry air and are sometimes associated with very strong, even damaging, winds.
  
  For more information, see Heat burst .
• Heat lightning From Wikipedia:
  Heat lightning, sometimes known as silent lightning or dry lightning (mainly used in the American south; not to be confused with dry thunderstorms, which are also often called dry lightning), is a misnomer used for the faint flashes of lightning on the horizon or other clouds from distant thunderstorms that do not appear to have accompanying sounds of thunder.
  
  The actual phenomenon commonly called heat lightning is simply cloud-to-ground lightning that occurs very far away, with thunder that dissipates before it reaches the observer.
  At night, it is possible to see the flashes of lightning from very far distances, up to 100 miles (160 kilometres), but the sound does not carry that far.
  In Florida, this type of lightning is often seen over the water at night, the remnants of storms that formed during the day along a sea breeze front coming in from the opposite coast.
  
  Heat lightning is not to be confused with electrically-induced luminosity actually generated at mesospheric altitudes above thunderstorm systems (and likewise visible at exceedingly great ranges), a phenomenon known as "sprites".
  
  For more information, see Heat lightning .
• Hoar frost From Wikipedia:
  Hoar frost (also hoarfrost, radiation frost, or pruina) refers to white ice crystals deposited on the ground or loosely attached to exposed objects, such as wires or leaves.
  They form on cold, clear nights when conditions are such that heat radiates out to the open air faster than it can be replaced from nearby sources, such as wind or warm objects.
  Under suitable circumstances, objects cool to below the frost point of the surrounding air, well below the freezing point of water.
  Such freezing may be promoted by effects such as flood frost or frost pocket.
  These occur when ground-level radiation loses cool air until it flows downhill and accumulates in pockets of very cold air in valleys and hollows.
  Hoar frost may freeze in such low-lying cold air even when the air temperature a few feet above ground is well above freezing.
  
  For more information, see Hoar frost .
• Humidity From Wikipedia:
  While humidity itself is a climate variable, it also interacts strongly with other climate variables.
  The humidity is affected by winds and by rainfall.
  
  The most humid cities on earth are generally located closer to the equator, near coastal regions.
  Cities in South and Southeast Asia are among the most humid.
  Kuala Lumpur, Jakarta, and Singapore have very high humidity all year round because of their proximity to water bodies and the equator and often overcast weather.
  Some places experience extreme humidity during their rainy seasons combined with warmth giving the feel of a lukewarm sauna, such as Kolkata, Chennai and Cochin in India, and Lahore in Pakistan.
  Sukkur city located on the Indus River in Pakistan has some of the highest and most uncomfortable dew points in the country, frequently exceeding 30 °C (86 °F) in the Monsoon season.
  High temperatures combine with the high dew point to create heat index in excess of 65 °C (149 °F).
  Darwin, Australia experiences an extremely humid wet season from December to April.
  Shanghai and Hong Kong also have an extreme humid period in their summer months.
  During the South-west and North-east Monsoon seasons (respectively, late May to September and November to March), expect heavy rains and a relatively high humidity post-rainfall.
  Outside the monsoon seasons, humidity is high (in comparison to countries North of the Equator), but completely sunny days abound.
  In cooler places such as Northern Tasmania, Australia, high humidity is experienced all year due to the ocean between mainland Australia and Tasmania.
  In the summer the hot dry air is absorbed by this ocean and the temperature rarely climbs above 35 °C (95 °F).
  
  For more information, see Humidity .
• Ice From Wikipedia:
  Ice is water frozen into a solid state.
  Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaque bluish-white color.
  
  In the Solar System, ice is abundant and occurs naturally from as close to the Sun as Mercury to as far away as the Oort cloud objects.
  Beyond the Solar System, it occurs as interstellar ice.
  It is abundant on Earth's surface – particularly in the polar regions and above the snow line – and, as a common form of precipitation and deposition, plays a key role in Earth's water cycle and climate.
  It falls as snowflakes and hail or occurs as frost, icicles or ice spikes.
  
  For more information, see Ice .
• Ice pellets From Wikipedia:
  Ice pellets are a form of precipitation consisting of small, translucent balls of ice.
  Ice pellets are smaller than hailstones which form in thunderstorms rather than in winter, and are different from graupel ("soft hail") which is made of frosty white rime, and from a mixture of rain and snow which is a slushy liquid or semisolid.
  Ice pellets often bounce when they hit the ground or other solid objects, and make a higher-pitched "tap" when striking objects like jackets, windshields, and dried leaves, compared to the dull splat of liquid raindrops.
  Pellets generally do not freeze into a solid mass unless mixed with freezing rain.
  The METAR code for ice pellets is PL (PE before November 1998).
  
  For more information, see Ice pellets .
• Ice storm From Wikipedia:
  An ice storm is a type of winter storm characterized by freezing rain, also known as a glaze event or, in some parts of the United States, as a silver thaw.
  The U.S. National Weather Service defines an ice storm as a storm which results in the accumulation of at least 0.25-inch (6.4 mm) of ice on exposed surfaces.
  From 1982 to 1994, ice storms were more common than blizzards in the U.S., averaging 16 per year.
  They are generally not violent storms but instead are commonly perceived as gentle rains occurring at temperatures just below freezing.
  
  For more information, see Ice storm.
• Katabatic wind From Wikipedia:
  A katabatic wind (named from the Greek word katabasis, meaning "descending") is the technical name for a drainage wind, a wind that carries high-density air from a higher elevation down a slope under the force of gravity.
  Such winds are sometimes also called fall winds; the spelling catabatic winds also occurs.
  Katabatic winds can rush down elevated slopes at hurricane speeds, but most are not as intense as that, and many are of the order of 10 knots (18 km/h) or less.
  
  Not all downslope winds are katabatic.
  For instance, winds such as the föhn, chinook, and bergwind are rain shadow winds where air driven upslope on the windward side of a mountain range drops its moisture and descends leeward drier and warmer.
  Examples of true katabatic winds include the bora (or bura) in the Adriatic, the Bohemian Wind or Böhmwind in the Ore Mountains, the Santa Ana in southern California, the piteraq winds of Greenland, and the oroshi in Japan.
  Another example is "the Barber", an enhanced katabatic wind that blows over the town of Greymouth in New Zealand when there is a southeast flow over the South Island.
  "The Barber" has a local reputation for its coldness.
  
  For more information, see Katabatic wind .
• Lake-effect snow From Wikipedia:
  Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water, warming the lower layer of air which picks up water vapor from the lake, rises up through the colder air above, freezes and is deposited on the leeward (downwind) shores.
  
  The same effect also occurs over bodies of salt water, when it is termed ocean-effect or bay-effect snow.
  The effect is enhanced when the moving air mass is uplifted by the orographic influence of higher elevations on the downwind shores.
  This uplifting can produce narrow but very intense bands of precipitation, which deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall.
  
  For more information, see Lake-effect snow .
• Landspout From Wikipedia:
  A landspout is a term coined by meteorologist Howard B. Bluestein in 1985 for a kind of tornado not associated with a mesocyclone.
  The Glossary of Meteorology defines a landspout as
  
  "Colloquial expression describing tornadoes occurring with a parent cloud in its growth stage and with its vorticity originating in the boundary layer.
  The parent cloud does not contain a preexisting mid-level mesocyclone.
  The landspout was so named because it looks like "a weak Florida Keys waterspout over land."
  
  Landspouts are a type of tornado which forms during the growth stage of a cumulus congestus cloud by stretching boundary layer vorticity upward and into the cumulus congestus's updraft.
  They generally are smaller and weaker than supercell tornadoes and do not form from a mesocyclone or pre-existing rotation in the cloud.
  Because of this, landspouts are rarely detected by Doppler weather radar.
  
  For more information, see Landspout .
• Laze (geology) From Wikipedia:
  Laze is acid rain and air pollution arising from steam explosions and large plume clouds containing extremely acidic condensate (mainly hydrochloric acid), which occur when molten lava flows enter cold oceans.
  The term laze is a portmanteau of lava and haze.
  
  Laze, created by the interaction of lava and cold seawater, differs from vog, originating from volcanic vents.
  
  For more information, see Laze (geology) .
• Lee wave From Wikipedia:
  In meteorology, lee waves are atmospheric stationary waves.
  The most common form is mountain waves, which are atmospheric internal gravity waves.
  These were discovered in 1933 by two German glider pilots, Hans Deutschmann and Wolf Hirth, above the Krkonoše.
  They are periodic changes of atmospheric pressure, temperature and orthometric height in a current of air caused by vertical displacement, for example orographic lift when the wind blows over a mountain or mountain range.
  They can also be caused by the surface wind blowing over an escarpment or plateau, or even by upper winds deflected over a thermal updraft or cloud street.
  
  The vertical motion forces periodic changes in speed and direction of the air within this air current.
  They always occur in groups on the lee side of the terrain that triggers them.
  Sometimes, mountain waves can help to enhance precipitation amounts downwind of mountain ranges.
  Usually a turbulent vortex, with its axis of rotation parallel to the mountain range, is generated around the first trough; this is called a rotor.
  The strongest lee waves are produced when the lapse rate shows a stable layer above the obstruction, with an unstable layer above and below.
  
  For more information, see Lee wave .
• Lightning From Wikipedia:
  Lightning is a sudden electrostatic discharge that occurs typically during a thunderstorm.
  This discharge occurs between electrically charged regions of a cloud (called intra-cloud lightning or IC), between two clouds (CC lightning), or between a cloud and the ground (CG lightning).
  
  The charged regions in the atmosphere temporarily equalize themselves through this discharge referred to as a flash.
  A lightning flash can also be a strike if it involves an object on the ground.
  Lightning creates light in the form of black body radiation from the very hot plasma created by the electron flow, and sound in the form of thunder.
  Lightning may be seen and not heard when it occurs at a distance too great for the sound to carry as far as the light from the strike or flash.
  
  For more information, see Lightning .
• List of cloud types From Wikipedia:
  The list of cloud types classifies the tropospheric genera as high (cirrus, cirrocumulus, cirrostratus), middle (altocumulus, altostratus), multi-level (nimbostratus, cumulus, cumulonimbus), and low (stratocumulus, stratus) according to the altitude level or levels at which each is normally found.
  Small cumulus are commonly grouped with the low clouds because they do not show significant vertical extent.
  Of the multi-level genus-types, those with the greatest convective activity are often grouped separately as towering vertical.
  The genus types all have Latin names.
  
  For more information, see List of cloud types .
• Mediterranean tropical-like cyclone From Wikipedia:
  A Mediterranean tropical-like cyclones, sometimes referred to as Mediterranean hurricanes or Medicanes, are rare meteorological phenomena observed in the Mediterranean Sea.
  Due to the dry nature of the Mediterranean region, the formation of tropical and subtropical cyclones is infrequent, with only 100 recorded tropical-like storms between 1947 and 2011.
  Most systems remain at or below tropical storm intensity, but on a few rare occasions, some storms have been observed reaching the strength of a Category 1 hurricane.
  No agency, however, is officially responsible for monitoring the formation and development of Medicanes.
  Tropical cyclogenesis typically occurs within two separate regions of the sea.
  The first region, encompassing areas of the western Mediterranean, is more conducive for development than the other, the Ionian Sea to the east.
  However, on very rare occasions, similar tropical-like storms may also develop in the Black Sea.
  
  For more information, see Mediterranean tropical-like cyclone.
• Mesocyclone From Wikipedia:
  A mesocyclone is a vortex of air within a convective storm.
  It is air that rises and rotates around a vertical axis, usually in the same direction as low pressure systems in a given hemisphere.
  They are most often cyclonic, that is, associated with a localized low-pressure region within a severe thunderstorm.
  Such thunderstorms can feature strong surface winds and severe hail.
  Mesocyclones often occur together with updrafts in supercells, within which tornadoes may form at the interchange with certain downdrafts.
  
  Mesocyclones are localized, approximately 2 km (1.2 mi) to 10 km (6.2 mi) in diameter within strong thunderstorms.
  Thunderstorms containing persistent mesocyclones are supercell thunderstorms.
  Mesocyclones occur on the "mesoscale" from a few kilometers to hundreds of kilometers.
  Doppler radar is used to identify mesocyclones.
  A mesovortex is a similar but typically smaller and weaker rotational feature associated with squall lines.
  
  For more information, see Mesocyclone.
• Mesoscale convective system From Wikipedia:
  A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more.
  A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and Mesoscale Convective Complexes (MCCs), and generally form near weather fronts.
  The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.
  
  For more information, see Mesoscale convective system .
• Mesovortices From Wikipedia:
  Mesovortices are small scale rotational features found in convective storms, such as those found in bow echos, supercell thunderstorms, and the eyewall of tropical cyclones.
  They range in size from tens of miles in diameter to a mile or less, and can be immensely intense.
  
  For more information, see Mesovortices.
• Meteotsunami From Wikipedia:
  A meteotsunami or meteorological tsunami is a tsunami-like wave of meteorological origin.
  Meteotsunamis are generated when rapid changes in barometric pressure cause the displacement of a body of water.
  In contrast to "ordinary" impulse-type tsunami sources, a traveling atmospheric disturbance normally interacts with the ocean over a limited period of time (from several minutes to several hours).
  Tsunamis and meteotsunamis are otherwise similar enough that it can be difficult to distinguish one from the other, as in cases where there is a tsunami wave but there are no seismic records of an earthquake.
  
  For more information, see Meteotsunami.
• Microburst From Wikipedia:
  A microburst is an intense small-scale downdraft produced by a thunderstorm or rain shower.
  There are two types of microbursts: wet microbursts and dry microbursts.
  They go through three stages in their cycle, the downburst, outburst, and cushion stages also called "Suriano's Stroke".
  A microburst can be particularly dangerous to aircraft, especially during landing, due to the wind shear caused by its gust front.
  Several fatal and historic crashes have been attributed to the phenomenon over the past several decades, and flight crew training goes to great lengths on how to properly recover from a microburst/wind shear event.
  
  A microburst often has high winds that can knock over fully grown trees.
  They usually last for seconds to minutes.
  
  For more information, see Microburst .
• Mist From Wikipedia:
  Mist is a phenomenon caused by small droplets of water suspended in air.
  Physically, it is an example of a dispersion.
  It is most commonly seen where warm, moist air meets sudden cooling, such as in exhaled air in the winter, or when throwing water onto the hot stove of a sauna.
  It can be created artificially with aerosol canisters if the humidity and temperature conditions are right.
  It can also occur as part of natural weather, when humid air cools rapidly, for example when the air comes into contact with surfaces that are much cooler than the air.
  
  The formation of mist, as of other suspensions, is greatly aided by the presence of nucleation sites on which the suspended water phase can congeal.
  Thus even such unusual sources as small particulates from volcanic eruptions, releases of strongly polar gases, and even the magnetospheric ions associated with polar lights can in right conditions trigger the formation of mist and can make mirrors appear foggy.
  
  For more information, see Mist .
• Monsoon From Wikipedia:
  Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea.
  Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase.
  The term is sometimes incorrectly used for locally heavy but short-term rains, although these rains meet the dictionary definition of monsoon.
  
  The major monsoon systems of the world consist of the West African and Asia-Australian monsoons.
  The inclusion of the North and South American monsoons with incomplete wind reversal has been debated.
  
  For more information, see Monsoon .
• Mountain breeze and valley breeze From Wikipedia:
  A mountain breeze and a valley breeze are two related, localized winds that occur one after the other on a daily cycle.
  They are not the same as katabatic winds, which are larger and stronger.
  
  These winds are opposite from each other.
  While valley breezes occur when the warm air rises up the sides, warm air in a mountain breeze will rise up the middle.
  
  Mountain and valley breezes form through a process similar to sea and land breezes.
  During the day, the sun heats up mountain air rapidly while valley remains relatively cooler.
  Convection causes it to rise, causing a valley breeze.
  At night, the process is reversed.
  During the night the slopes get cooled and the dense air descends into the valley as the mountain wind.
  These breezes occur mostly during calm and clear weather.
  Mountain and valley breezes are other examples of local winds caused by an area’s geography.
  Campers in mountainous areas may feel a warm afternoon quickly change into a cold night soon after the sun sets.
  During the day, the sun warms the air along the mountain slopes.
  This warm air rises up the mountain slopes, creating a valley breeze.
  At nightfall, the air along the mountain slopes cools.
  This cool air moves down the slopes into the valley, producing a mountain breeze.
  
  For more information, see Mountain breeze and valley breeze .
• Multicellular thunderstorm From Wikipedia:
  A multicellular thunderstorm cluster is a thunderstorm that is composed of multiple cells, each being at a different stage in the life cycle of a thunderstorm.
  It appears as several anvils clustered together.
  A cell is an updraft/downdraft couplet.
  These different cells will dissipate as new cells form and continue the life of the multicellular thunderstorm cluster with each cell taking a turn as the dominant cell in the group.
  
  For more information, see Multicellular thunderstorm .
• Multiple-vortex tornado From Wikipedia:
  A multiple-vortex tornado is a tornado that contains several vortices (called subvortices or suction vortices) rotating around, inside of, and as part of the main vortex.
  The only times multiple vortices may be visible are when the tornado is first forming or when condensation and debris is balanced enough so that subvortices are apparent without being obscured.
  They can add over 100 mph to the ground-relative wind in a tornado circulation, and are responsible for most (if not all) cases where narrow arcs of extreme destruction lie right next to weak damage within tornado paths.
  
  For more information, see Multiple-vortex tornado .
• North American High From Wikipedia:
  The North American High (also Canadian High/Anticyclone, sometimes in Europe Greenland High/Anticyclone) is an impermanent high-pressure area or anticyclone created by anticyclogenesis (anticyclone + the Greek word genesis, meaning "birth, origin"), a formative process that occurs when cool or cold dry air settles over North America.
  In summer it is replaced with an Arctic Low, or if it moves to continental land, a North American Low.
  
  North American Highs move eastwards across the continent, often in the company of one or more low-pressure cells or cyclones.
  Its cold, dense air does not extend usually above 3 km (1.9 mi), lower than the Canadian Rockies.
  Sometimes, in winter it breaks free and passes over the Rockies and brings a cold front into Southwestern United States and Mexico, freezing crops and bringing snow into Mexico's mountains as far south as Jalisco.
  The high’s usual location east of the Rockies shelters it from the relatively warm Pacific Ocean and helps it maintain its strength.
  The average January sea level pressure at its centre is about 1,020 millibars (30.12 inches of mercury).
  The Canadian high often moves southeastward until it eventually reaches the Atlantic Ocean, where it merges with the Azores high.
  In the summer the Canadian high circulates cool, dry air to the United States east of the Rockies and parts of southern Canada.
  
  For more information, see North American High .
• Orographic precipitation From Wikipedia:
  Orographic precipitation, also known as relief precipitation, is precipitation generated by a forced upward movement of air upon encountering a physiographic upland (see anabatic wind).
  This lifting can be caused by two mechanisms:
  
  - The upward deflection of large scale horizontal flow by the orography.
  - The anabatic or upward vertical propagation of moist air up an orographic slope caused by daytime heating of the mountain barrier surface.
  
  Upon ascent, the air that is being lifted will expand and cool.
  This adiabatic cooling of a rising moist air parcel may lower its temperature to its dew point, thus allowing for condensation of the water vapor contained within it, and hence the formation of a cloud.
  If enough water vapor condenses into cloud droplets, these droplets may become large enough to fall to the ground as precipitation.
  In parts of the world subjected to relatively consistent winds (for example the trade winds), a wetter climate prevails on the windward side of a mountain than on the leeward (downwind) side as moisture is removed by orographic precipitation.
  Drier air (see katabatic wind) is left on the descending, generally warming, leeward side where a rain shadow is formed.
  
  For more information, see Orographic precipitation .
• Pacific hurricane From Wikipedia:
  A Pacific hurricane is a mature tropical cyclone that develops within the eastern and central Pacific Ocean to the east of 180°W, north of the equator.
  For tropical cyclone warning purposes, the northern Pacific is divided into three regions: the eastern (North America to 140°W), central (140°W to 180°), and western (180° to 100°E), while the southern Pacific is divided into 2 sections, the Australian region (90E to 160°E) and the southern Pacific basin between 160°E and 120°W.
  Identical phenomena in the western north Pacific are called typhoons.
  This separation between the two basins has a practical convenience, however, as tropical cyclones rarely form in the central north Pacific due to high vertical wind shear, and few cross the dateline.
  
  For more information, see Pacific hurricane .
• Polar easterlies From Wikipedia:
  The polar easterlies (also Polar Hadley cells) are the dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the North and South Poles towards low-pressure areas within the Westerlies at high latitudes.
  Cold air subsides at the poles creating the high pressure, forcing an equatorward outflow of air; that outflow is then deflected westward by the Coriolis effect.
  Unlike the westerlies in the middle latitudes, the polar easterlies are often weak and irregular.
  These prevailing winds blow from the polar easterlies are one of the five primary wind zones, known as wind belts, that make up our atmosphere's circulatory system.
  This particular belt of wind begins at approximately 60 degrees north and south latitude and reaches to the poles.
  When air moves near the poles, cold temperatures shrink the air.
  This promotes air from warmer latitudes to flow into the area, causing a polar high-pressure zone.
  Air from this high-pressure zone then rushes toward the low-pressure zone surrounding the sub-polar region.
  The flow of air is altered by the Earth's rotation and deflected west, hence the name, between high- and low-pressure zones.
  Temperature and air pressure differentials and the earth's rotation combine to form global currents.
  
  For more information, see Polar easterlies .
• Precipitation From Wikipedia:
  In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravity.
  The main forms of precipitation include drizzle, rain, sleet, snow, graupel and hail.
  Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates".
  Thus, fog and mist are not precipitation but suspensions, because the water vapor does not condense sufficiently to precipitate.
  Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air.
  Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud.
  Short, intense periods of rain in scattered locations are called "showers."
  
  For more information, see Precipitation .
• Precipitation shaft From Wikipedia:
  A precipitation shaft is a weather phenomenon, visible from the ground at large distances from the storm system, as a dark vertical shaft of heavy rain, hail, or snow, generally localized over a relatively small area.
  
  This is different from a virga, which is a shaft of precipitation that evaporates before reaching the ground.
  
  For more information, see Precipitation shaft .
• Precipitation types From Wikipedia:
  In meteorology, "precipitation types" can include the character or phase of the precipitation which is falling to ground level.
  There are three distinct ways that precipitation can occur.
  Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation.
  Orographic precipitation occurs when moist air is forced upwards over rising terrain, such as a mountain.
  
  Precipitation can also fall in either liquid or solid phases, or transition between them.
  Liquid forms of precipitation include rain and drizzle.
  Rain or drizzle which freezes on contact within a subfreezing air mass gains the preceding adjective "freezing", becoming known as freezing rain or freezing drizzle.
  Frozen forms of precipitation include snow, ice needles, sleet, hail, and graupel.
  Intensity is classified either by rate of fall, or by visibility restriction. ...
  
  ... Precipitation falls in many forms, or phases. They can be subdivided into:
  
  Liquid precipitation:
  - Drizzle (DZ)
  - Rain (RA)
  - Fog condensation on vegetation foliage, dripping on forest soil.
  
  Freezing precipitation:
  - Freezing drizzle (FZDZ)
  - Freezing rain (FZRA)
  - Rain and snow mixed / Slush (RASN)
  - Sun Shower
  
  Frozen precipitation:
  - Snow (SN)
  - Snow grains (SG)
  - Ice pellets / Sleet (PL)
  - Hail (GR)
  - Snow pellets / Graupel (GS)
  - Ice crystals (IC)
  
  For more information, see Precipitation types.
• Prevailing winds From Wikipedia:
  Prevailing winds are winds that blow predominantly from a single general direction over a particular point on the Earth's surface.
  The dominant winds are the trends in direction of wind with the highest speed over a particular point on the Earth's surface.
  A region's prevailing and dominant winds are enacted by global patterns of movement in the Earth's atmosphere.
  In general, easterly flow occurs at low and medium latitudes globally.
  In the mid-latitudes, westerly winds are the rule and their strength is largely determined by the polar cyclone.
  In areas where winds tend to be light, the sea breeze/land breeze cycle is the most important to the prevailing wind; in areas which have variable terrain, mountain and valley breezes dominate the wind pattern.
  Highly elevated surfaces can induce a thermal low, which then augments the environmental wind flow.
  
  For more information, see Prevailing winds.
• Rain From Wikipedia:
  Rain is liquid water in the form of droplets that have condensed from atmospheric water vapor and then becomes heavy enough to fall under gravity.
  Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth.
  It provides suitable conditions for many types of ecosystems, as well as water for hydroelectric power plants and crop irrigation.
  
  The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts.
  If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can organize into narrow rainbands.
  In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains.
  On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass.
  The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes.
  
  For more information, see Rain .
• Rain and snow mixed From Wikipedia:
  Rain and snow mixed is precipitation composed of rain and partially melted snow.
  Unlike ice pellets, which are hard, and freezing rain, which is fluid until striking an object, this precipitation is soft and translucent, but it contains some traces of ice crystals, from partially fused snowflakes.
  In any one location, it usually occurs briefly as a transition phase from rain to snow or vice versa. Its METAR code is RASN.
  
  For more information, see Rain and snow mixed .
• Rainband From Wikipedia:
  A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated.
  Rainbands can be stratiform or convective, and are generated by differences in temperature.
  When noted on weather radar imagery, this precipitation elongation is referred to as banded structure.
  Rainbands within tropical cyclones are curved in orientation.
  Tropical cyclone rainbands contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm.
  The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.
  
  Rainbands spawned near and ahead of cold fronts can be squall lines which are able to produce tornadoes.
  Rainbands associated with cold fronts can be warped by mountain barriers perpendicular to the front's orientation due to the formation of a low-level barrier jet.
  Bands of thunderstorms can form with sea breeze and land breeze boundaries, if enough moisture is present.
  If sea breeze rainbands become active enough just ahead of a cold front, they can mask the location of the cold front itself.
  Banding within the comma head precipitation pattern of an extratropical cyclone can yield significant amounts of rain or snow.
  Behind extratropical cyclones, rainbands can form downwind of relative warm bodies of water such as the Great Lakes.
  If the atmosphere is cold enough, these rainbands can yield heavy snow.
  
  For more information, see Rainband .
• Rain shadow From Wikipedia:
  A rain shadow is a dry area on the leeward side of a mountainous area (away from the wind).
  The mountains block the passage of rain-producing weather systems and cast a "shadow" of dryness behind them.
  Wind and moist air is drawn by the prevailing winds towards the top of the mountains, where it condenses and precipitates before it crosses the top.
  The air, without much moisture left, advances across the mountains creating a drier side called the "rain shadow".
  
  For more information, see Rain shadow .
• Rear flank downdraft From Wikipedia:
  The rear flank downdraft or RFD is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm.
  These areas of descending air are thought to be essential in the production of many supercellular tornadoes.
  Large hail within the rear flank downdraft often shows up brightly as a hook on weather radar images, producing the characteristic hook echo, which often indicates the presence of a tornado.
  
  For more information, see Rear flank downdraft .
• Satellite tornado From Wikipedia:
  A satellite tornado is a tornado that rotates around a larger, primary tornado and interacts with the same mesocyclone.
  Satellite tornadoes occur apart from the primary tornado and are not considered subvortices; the primary tornado and satellite tornadoes are considered to be separate tornadoes.
  The cause of satellite tornadoes is not known.
  Such tornadoes are more often anticyclonic than are typical tornadoes and these pairs may be referred to as tornado couplets.
  Satellite tornadoes most commonly form in association with very large and intense tornadoes.
  
  Satellite tornadoes are relatively uncommon.
  When a satellite tornado does occur there is often more than one orbiting satellite spawned during the life cycle of the tornado or with successive primary tornadoes spawned by the parent supercell (a process known as cyclic tornadogenesis and leading to a tornado family).
  On tornado outbreak days, if satellite tornadoes occur with one supercell, there is an elevated probability of their occurrence with other supercells.
  
  For more information, see Satellite tornado .
• Sea breeze From Wikipedia:
  A sea breeze or onshore breeze is any wind that blows from a large body of water toward or onto a landmass; it develops due to differences in air pressure created by the differing heat capacities of water and dry land.
  As such, sea breezes are more localised than prevailing winds.
  Because land absorbs solar radiation far more quickly than water, a sea breeze is a common occurrence along coasts after sunrise.
  By contrast, a land breeze or offshore breeze is the reverse effect: dry land also cools more quickly than water and, after sunset, a sea breeze dissipates and the wind instead flows from the land towards the sea.
  Sea breezes and land breezes are both important factors in coastal regions' prevailing winds.
  The term offshore wind may refer to any wind over open water.
  
  For more information, see Sea breeze .
• Sea ice From Wikipedia:
  Sea ice arises as seawater freezes.
  Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density).
  Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans.
  Much of the world's sea ice is enclosed within the polar ice packs in the Earth's polar regions: the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean.
  Polar packs undergo a significant yearly cycling in surface extent, a natural process upon which depends the Arctic ecology, including the ocean's ecosystems.
  Due to the action of winds, currents and temperature fluctuations, sea ice is very dynamic, leading to a wide variety of ice types and features.
  Sea ice may be contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into the ocean.
  Depending on location, sea ice expanses may also incorporate icebergs.
  
  For more information, see Sea ice .
• Sea smoke From Wikipedia:
  Sea smoke, frost smoke, or steam fog, is fog which is formed when very cold air moves over warmer water.
  Arctic sea smoke is sea smoke forming over small patches of open water in sea ice.
  
  It forms when a light wind of very cold air mixes with a shallow layer of saturated warm air immediately above the warmer water.
  The warmer air is cooled beyond the dew point and can no longer hold as much water vapor, so the excess condenses out.
  The effect is similar to the "steam" produced over a hot bath or a hot drink, or even an exercising person.
  
  Sea smoke has a turbulent appearance and may form spiralling columns.
  It is usually not very high and lookouts on ships can usually see over it (but small boats may have very poor visibility) because the fog is confined to the layer of warm air above the sea.
  However, sea smoke columns 20–30 metres high have been observed.
  Because this type of fog requires very low air temperatures, it is uncommon in temperate climates, but is common in the Arctic and Antarctic.
  
  For more information, see Sea smoke .
• Shelf ice From Wikipedia:
  Shelf ice is ice that forms when a portion of a lake surface freezes.
  It is often then washed upon the shore.
  The phenomenon is common within the Great Lakes. ...
  
  ... Shelf ice forms from float ice. Float ice is like drift ice, but seldom becomes large enough to support humans or large animals.
  The cold temperatures freeze small areas of the open lake, forming float ice.
  Similar ice will form along the shoreline when the lake is calm.
  Over time, the ice will build up from continued freezing on the open lake and from snow accumulation on the surface.
  The surface of the Great Lake seldom remains calm enough for the entire surface to freeze.
  Note: The smaller lakes, Lake St. Clair, Lake Erie, and Lake Ontario will freeze over more often than the larger upper lakes.
  The wind pushes the float ice and creates waves which also move the ice.
  Over time, the ice will accumulate on the leeward shores of the lake.
  Like sand, which is moved by the wind and the waves, the ice begins to accumulate along the east and southern shore of Lake Michigan.
  When pushed onto the beach, it remains and over time, the ice accumulates on top of the beach and the ice already pushed ashore.
  
  On days with a strong off-shore wind, an ice sheet may reach out into the lakes as far as the eye can see.
  But across the lake, there is likely to be open water several miles wide.
  
  For more information, see Shelf ice .
• Siberian High From Wikipedia:
  The Siberian High (also Siberian Anticyclone) is a massive collection of cold dry air that accumulates in the northeastern part of Eurasia from September until April.
  It is usually centered on Lake Baikal.
  It reaches its greatest size and strength in the winter when the air temperature near the center of the high-pressure area is often lower than -40 °C (-40 °F).
  The atmospheric pressure is often above 1,040 millibars (31 inHg).
  The Siberian High is the strongest semi-permanent high in the northern hemisphere and is responsible for both the lowest temperature in the Northern Hemisphere, of -67.8 °C (-90.0 °F) on 15 January 1885 at Verkhoyansk, and the highest pressure, 1083.8 mbar (108.38 kPa, 32.01 inHg) at Agata, Krasnoyarsk Krai on 31 December 1968, ever recorded.
  The Siberian High is responsible both for severe winter cold and attendant dry conditions with little snow and few or no glaciers across Siberia, Mongolia, and China.
  During the summer, the Siberian High is largely replaced by the Asiatic low.
  
  For more information, see Siberian High .
• Snow From Wikipedia:
  Snow refers to forms of ice crystals that precipitate from the atmosphere (usually from clouds) and undergo changes on the Earth's surface.
  It pertains to frozen crystalline water throughout its life cycle, starting when, under suitable conditions, the ice crystals form in the atmosphere, increase to millimeter size, precipitate and accumulate on surfaces, then metamorphose in place, and ultimately melt, slide or sublimate away.
  Snowstorms organize and develop by feeding on sources of atmospheric moisture and cold air.
  Snowflakes nucleate around particles in the atmosphere by attracting supercooled water droplets, which freeze in hexagonal-shaped crystals.
  Snowflakes take on a variety of shapes, basic among these are platelets, needles, columns and rime.
  As snow accumulates into a snowpack, it may blow into drifts.
  Over time, accumulated snow metamorphoses, by sintering, sublimation and freeze-thaw.
  Where the climate is cold enough for year-to-year accumulation, a glacier may form.
  Otherwise, snow typically melts seasonally, causing runoff into streams and rivers and recharging groundwater.
  
  For more information, see Snow .
• Snowpack From Wikipedia:
  Snowpack forms from layers of snow that accumulate in geographic regions and high altitudes where the climate includes cold weather for extended periods during the year.
  Snowpacks are an important water resource that feed streams and rivers as they melt.
  Therefore, snowpacks are both the drinking water source for many communities and a potential source of flooding (in case of sudden melting).
  Snowpacks also contribute mass to glaciers in their accumulation zone.
  
  Assessing the formation and stability of snowpacks is important in the study and prediction of avalanches.
  Scientists study the physical properties of snow under different conditions and their evolution, and more specifically snow metamorphism, snow hydrology (that is, the contribution of snow melt to catchment hydrology), the evolution of snow cover with climate change and its effect on the ice-albedo feedback and hydrology, both on the ground and by using remote sensing.
  Snow is also studied in a more global context of impact on animal habitats and plant succession.
  An important effort is put into snow classification, both as an hydrometeor and on the ground.
  
  For more information, see Snowpack .
• Soft rime From Wikipedia:
  Soft rime is a white ice deposition that forms when the water droplets in light freezing fog or mist freeze to the outer surfaces of objects, with calm or light wind.
  The fog freezes usually to the windward side of tree branches, wires, or any other solid objects.
  
  Soft rime is similar in appearance to hoar frost; but whereas rime is formed by vapour first condensing to liquid droplets (of fog, mist or cloud) and then attaching to a surface, hoar frost is formed by direct deposition from water vapour to solid ice.
  A heavy coating of hoar frost, called white frost, is very similar in appearance to soft rime, but the formation process is different: it happens when there is no fog, but very high levels of air relative humidity (above 90%) and temperatures below -8 °C (17.6 °F).
  
  Soft rime formations have the appearance of white ice needles and scales; they are fragile and can be easily shaken off objects.
  Factors that favour soft rime are small drop size, slow accretion of liquid water, high degree of supercooling, and fast dissipation of latent heat of fusion. The opposite conditions favour ice with higher densities, such as hard rime or clear ice.
  
  For more information, see Soft rime .
• South Atlantic tropical cyclone From Wikipedia:
  South Atlantic tropical cyclones are unusual weather events that occur in the Southern Hemisphere.
  Strong wind shear, which disrupts the formation of cyclones, as well as a lack of weather disturbances favorable for development in the South Atlantic Ocean make any strong tropical system extremely rare, and Catarina in 2004 is the only recorded South Atlantic hurricane in history.
  South Atlantic storms have developed year-round, with activity peaking during the months from November through May in this basin.
  Since 2011, the Brazilian Navy Hydrographic Center has started to assign names to tropical and subtropical systems in western side of this basin near Brazil, when they have sustained wind speeds of at least 65 km/h (40 mph), the generally accepted minimum sustained wind velocity for a disturbance to be designated as a tropical storm in the North Atlantic basin.
  Below is a list of notable South Atlantic tropical and subtropical cyclones.
  
  For more information, see South Atlantic tropical cyclone .
• South Pacific tropical cyclone From Wikipedia:
  A South Pacific tropical cyclone is a non frontal, low pressure system that has developed, within an environment of warm sea surface temperatures and little vertical wind shear aloft in the South Pacific Ocean.
  Within the Southern Hemisphere there are officially three areas where tropical cyclones develop on a regular basis, these areas are the South-West Indian Ocean between Africa and 90°E, the Australian region between 90°E and 160°E and the South Pacific basin between 160°E and 120°W.
  
  For more information, see South Pacific tropical cyclone .
• Squall From Wikipedia:
  A squall is a sudden, sharp increase in wind speed that is usually associated with active weather, such as rain showers, thunderstorms, or heavy snow.
  Squalls refer to an increase in the sustained winds over a short time interval, as there may be higher gusts during a squall event.
  They usually occur in a region of strong mid-level height falls, or mid-level tropospheric cooling, which force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.
  
  For more information, see Squall .
• Squall line From Wikipedia:
  A squall line (also known as a quasi-linear convective system or QLCS) is a line of thunderstorms forming along or ahead of a cold front.
  In the early 20th century, the term was used as a synonym for cold front.
  It contains heavy precipitation, hail, frequent lightning, strong straight-line winds, and possibly tornadoes and waterspouts.
  Strong straight-line winds can occur where the squall line is in the shape of a bow echo.
  Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present.
  Some bow echoes which develop within the summer season are known as derechos, and they move quite fast through large sections of territory.
  On the back edge of the rain shield associated with mature squall lines, a wake low can be present, sometimes associated with a heat burst.
  
  For more information, see Squall line .
• Steam devil From Wikipedia:
  A steam devil is a small, weak whirlwind over water (or sometimes wet land) that has drawn fog into the vortex, thus rendering it visible.
  
  They form over large lakes and oceans during cold air outbreaks while the water is still relatively warm, and can be an important mechanism in vertically transporting moisture.
  They are a component of sea smoke.
  
  Smaller steam devils and steam whirls can form over geyser basins even in warm weather because of the very high water temperatures.
  Although observations of steam devils are generally quite rare, hot springs in Yellowstone Park produce them on a daily basis.
  
  Steam devils have only been reported and studied since the 1970s.
  They are weaker than waterspouts and distinct from them.
  The latter are more akin to weak tornadoes over water.
  
  For more information, see Steam devil .
• Storm From Wikipedia:
  A storm is any disturbed state of an environment or in an astronomical body's atmosphere especially affecting its surface, and strongly implying severe weather.
  It may be marked by significant disruptions to normal conditions such as strong wind, tornadoes, hail, thunder and lightning (a thunderstorm), heavy precipitation (snowstorm, rainstorm), heavy freezing rain (ice storm), strong winds (tropical cyclone, windstorm), or wind transporting some substance through the atmosphere as in a dust storm, blizzard, sandstorm, etc.
  Storms have the potential to harm lives and property via storm surge, heavy rain or snow causing flooding or road impassibility, lightning, wildfires, and vertical wind shear.
  However, systems with significant rainfall and duration help alleviate drought in places they move through.
  Heavy snowfall can allow special recreational activities to take place which would not be possible otherwise, such as skiing and snowmobiling.
  
  For more information, see Storm .
• Storm surge From Wikipedia:
  A storm surge, storm flood or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low pressure weather systems (such as tropical cyclones and strong extratropical cyclones), the severity of which is affected by the shallowness and orientation of the water body relative to storm path, as well as the timing of tides.
  Most casualties during tropical cyclones occur as the result of storm surges.
  It is a measure of the rise of water beyond what would be expected by the normal movement related to tides.
  
  The two main meteorological factors contributing to a storm surge are a long fetch of winds spiraling inward toward the storm, and a low-pressure-induced dome of water drawn up under and trailing the storm's center.
  
  For more information, see Storm surge .
• Subtropical cyclone From Wikipedia:
  A subtropical cyclone is a weather system that has some characteristics of a tropical and an extratropical cyclone.
  
  As early as the 1950s, meteorologists were uncertain whether they should be characterized as tropical or extratropical cyclones.
  They were officially recognized by the National Hurricane Center in 1972.
  Subtropical cyclones have begun to receive names from the official tropical cyclone lists in the North Atlantic, South-west Indian Ocean, and South Atlantic basins.
  
  There are two definitions currently used for subtropical cyclones.
  Across the north Atlantic and southwest Indian Ocean, they require central convection fairly near the center and a warming core in the mid-levels of the troposphere.
  Across the eastern half of the northern Pacific, they require a mid-tropospheric cyclone to cut off from the main belt of the westerlies and only a weak surface circulation.
  Subtropical cyclones have broad wind patterns with maximum sustained winds located farther from the center than typical tropical cyclones, and have no weather fronts linked into their center.
  
  For more information, see Subtropical cyclone .
• Sunshower From Wikipedia:
  A sunshower or sun shower is a meteorological phenomenon in which rain falls while the sun is shining.
  A sunshower is usually the result of accompanying winds associated with a rain storm sometimes miles away, blowing the airborne raindrops into an area where there are no clouds, therefore causing a sunshower.
  Sometimes a sunshower is created when a single rain cloud passes overhead, and the Sun's angle keeps the sunlight from being obstructed by overhead clouds.
  
  Sunshower conditions often lead to the appearance of a rainbow, if the sun is at a low enough angle.
  Although used in the United States, Canada, Australia, New Zealand, Ireland and the UK, the term "sunshower" is rarely found in dictionaries.
  Additionally, the phenomenon has a wide range of sometimes remarkably similar folkloric names in cultures around the world.
  A common theme is that of clever animals and tricksters getting married or related to the devil, although many variations of this theme are in existence.
  
  For more information, see Sunshower .
• Supercell From Wikipedia:
  A supercell is a thunderstorm characterized by the presence of a mesocyclone: a deep, persistently rotating updraft.
  For this reason, these storms are sometimes referred to as rotating thunderstorms.
  Of the four classifications of thunderstorms (supercell, squall line, multi-cell, and single-cell), supercells are the overall least common and have the potential to be the most severe.
  Supercells are often isolated from other thunderstorms, and can dominate the local weather up to 32 kilometres (20 mi) away.
  They tend to last 2-4 hours.
  
  Supercells are often put into three classification types: Classic, Low-precipitation (LP), and High-precipitation (HP).
  LP supercells are usually found in climates that are more arid, such as the high plains of the United States, and HP supercells are most often found in moist climates.
  Supercells can occur anywhere in the world under the right pre-existing weather conditions, but they are most common in the Great Plains of the United States in an area known as Tornado Alley and in the Tornado Corridor of Argentina, Uruguay and southern Brazil.
  
  For more information, see Supercell .
• Thermal From Wikipedia:
  A thermal column (or thermal) is a column of rising air in the lower altitudes of Earth's atmosphere, a form of atmospheric updraft.
  Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example of convection, specifically atmospheric convection.
  The Sun warms the ground, which in turn warms the air directly above it. ... ... The warmer air nearer to the surface expands, becoming less dense than the surrounding air.
  The lighter air rises and cools due to its expansion in the lower pressure at higher altitudes.
  It stops rising when it has cooled to the same temperature as the surrounding air.
  Dark earth, urban areas, and roadways are good examples of sources of thermals.
  
  Associated with a thermal is a downward flow surrounding the thermal column.
  The downward-moving exterior is caused by colder air being displaced at the top of the thermal.
  
  The size and strength of thermals are influenced by the properties of the lower atmosphere (the troposphere).
  Generally, when the air is cold, bubbles of warm air are formed by the ground heating the air above it and can rise like a hot air balloon.
  The air is then referred to as unstable.
  If there is a warm layer of air higher up, an inversion can prevent thermals from rising high and the air is said to be stable.
  
  For more information, see Thermal .
• Thundersnow From Wikipedia:
  Thundersnow, also known as a winter thunderstorm or a thundersnowstorm, is an unusual kind of thunderstorm with snow falling as the primary precipitation instead of rain.
  It typically falls in regions of strong upward motion within the cold sector of an extratropical cyclone. Thermodynamically, it is not different from any other type of thunderstorm, but the top of the cumulonimbus cloud is usually quite low.
  In addition to snow, graupel, or hail may fall.
  
  For more information, see Thundersnow .
• Thunderstorm From Wikipedia:
  A thunderstorm, also known as an electrical storm, lightning storm, or thundershower, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder.
  Thunderstorms occur in a type of cloud known as a cumulonimbus.
  They are usually accompanied by strong winds, heavy rain, and sometimes snow, sleet, hail, or, in contrast, no precipitation at all.
  Thunderstorms may line up in a series or become a rainband, known as a squall line.
  Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes.
  Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones.
  While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear s ometimes causes a deviation in their course at a right angle to the wind shear direction.
  
  Thunderstorms result from the rapid upward movement of warm, moist air, sometimes along a front.
  As the warm, moist air moves upward, it cools, condenses, and forms a cumulonimbus cloud that can reach heights of over 20 kilometres (12 mi).
  As the rising air reaches its dew point temperature, water vapor condenses into water droplets or ice, reducing pressure locally within the thunderstorm cell.
  Any precipitation falls the long distance through the clouds towards the Earth's surface.
  As the droplets fall, they collide with other droplets and become larger.
  The falling droplets create a downdraft as it pulls cold air with it, and this cold air spreads out at the Earth's surface, occasionally causing strong winds that are commonly associated with thunderstorms.
  
  For more information, see Thunderstorm .
• Tornado From Wikipedia:
  A tornado is a rapidly rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud.
  The windstorm is often referred to as a twister, whirlwind or cyclone, although the word cyclone is used in meteorology to name a weather system with a low-pressure area in the center around which winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern.
  Tornadoes come in many shapes and sizes, and they are often visible in the form of a condensation funnel originating from the base of a cumulonimbus cloud, with a cloud of rotating debris and dust beneath it.
  Most tornadoes have wind speeds less than 110 miles per hour (180 km/h), are about 250 feet (80 m) across, and travel a few miles (several kilometers) before dissipating.
  The most extreme tornadoes can attain wind speeds of more than 300 miles per hour (480 km/h), are more than two miles (3 km) in diameter, and stay on the ground for dozens of miles (more than 100 km).
  
  For more information, see Tornado .
• Tornado family From Wikipedia:
  A tornado family is a series of tornadoes spawned by the same supercell thunderstorm.
  These families form a line of successive or parallel tornado paths and can cover a short span or a vast distance.
  Tornado families are sometimes mistaken as a single continuous tornado, especially prior to the 1970s.
  Sometimes the tornado tracks can overlap and expert analysis is necessary to determine whether or not damage was created by a family or a single tornado.
  In some cases, such as the Hesston-Goessel, Kansas tornadoes of March 1990, different tornadoes of a tornado family merge, making discerning whether an event was continuous or not more difficult.
  
  For more information, see Tornado family .
• Tornado outbreak From Wikipedia:
  A tornado outbreak is the occurrence of multiple tornadoes spawned by the same synoptic scale weather system.
  The number of tornadoes required to qualify as an outbreak typically are at least six to ten.
  
  The tornadoes usually occur within the same day, or continue into the early morning hours of the succeeding day, and within the same region.
  Most definitions allow for a break in tornado activity (time elapsed from the end of last tornado to the beginning of next tornado) of six hours.
  If tornado activity indeed resumes after such a lull, many definitions consider the event to be a new outbreak.
  A series of continuous or nearly continuous tornado outbreak days is a tornado outbreak sequence.
  Tornado outbreaks usually occur from March through June in the Great Plains of the United States and Canada, the Midwestern United States, and the Southeastern United States in an area colloquially referred to as Tornado Alley.
  Tornado outbreaks do occur during other times of the year and in other parts of the world, however.
  A secondary less active and annually inconsistent tornado "season" in the U.S. occurs in late autumn.
  
  For more information, see Tornado outbreak .
• Tornado outbreak sequence From Wikipedia:
  A tornado outbreak sequence, or tornado outbreak day sequence, sometimes referred to as an extended tornado outbreak, is a period of continuous or nearly continuous high tornado activity consisting of a series of tornado outbreaks over multiple days with no or very few days lacking tornado outbreaks.
  
  Major tornado outbreak sequences occurred in the United States in May 1917, 1930, 1949, 1965, 2003, and 2011.
  Another exceptional outbreak sequence apparently occurred during mid to late May 1896.
  Although some days lacked tornado outbreaks, the period from mid to late April 2011 also was a period of especially high tornado activity.
  
  Tornado outbreak sequences tend to dominate the tornado statistics for a year and often cause a spike in tornado numbers for the entire year.
  Not all periods of active tornado occurrences are outbreak sequences, there must be no break in the activity to satisfy the definition.
  Active periods happen from every year to every several years whereas continuously active periods are less common and can be rare depending on the parameters applied to define a sequence.
  
  For more information, see Tornado outbreak sequence .
• Trade winds From Wikipedia:
  The trade winds are the prevailing pattern of easterly surface winds found in the tropics, within the lower portion of the Earth's atmosphere, in the lower section of the troposphere near the Earth's equator.
  The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, strengthening during the winter and when the Arctic oscillation is in its warm phase.
  Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries, and enabled colonial expansion into the Americas and trade routes to become established across the Atlantic and Pacific oceans.
  
  In meteorology, the trade winds act as the steering flow for tropical storms that form over the Atlantic, Pacific, and southern Indian Oceans and make landfall in North America, Southeast Asia, and Madagascar and eastern Africa, respectively.
  Trade winds also transport African dust westward across the Atlantic Ocean into the Caribbean Sea, as well as portions of southeastern North America. Shallow cumulus clouds are seen within trade wind regimes, and are capped from becoming taller by a trade wind inversion, which is caused by descending air aloft from within the subtropical ridge.
  The weaker the trade winds become, the more rainfall can be expected in the neighboring landmasses.
  
  For more information, see Trade winds .
• Training (meteorology) From Wikipedia:
  In meteorology, training denotes repeated areas of rain, typically associated with thunderstorms, that move over the same region in a relatively short period of time.
  Training thunderstorms are capable of producing excessive rainfall totals, often causing flash flooding.
  The name training is derived from how a train and its cars travel along a track (moving along a single path), without the track moving. ...
  
  ... Showers and thunderstorms along thunderstorm trains usually develop in one area of stationary instability, and are advanced along a single path by prevailing winds.
  Additional showers and storms can also develop when the gust front from a storm collides with warmer air outside of the storm.
  The same process repeats in the new storms, until overall conditions in the surrounding atmosphere become too stable for support of thunderstorm activity.
  Showers and storms can also develop along stationary fronts, and winds move them down the front.
  The showers that often accompany thunderstorms are usually thunderstorms that are not completely developed.
  
  For more information, see Training (meteorology) .
• Tropical cyclone From Wikipedia:
  A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain.
  Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone.
  A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; while in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".
  
  For more information, see Tropical cyclone .
• Tropical cyclone basins From Wikipedia:
  Traditionally, areas of tropical cyclone formation are divided into seven basins.
  These include the north Atlantic Ocean, the eastern and western parts of the northern Pacific Ocean, the southwestern Pacific, the southwestern and southeastern Indian Oceans, and the northern Indian Ocean (Arabian Sea and Bay of Bengal).
  The western Pacific is the most active and the north Indian the least active.
  An average of 86 tropical cyclones of tropical storm intensity form annually worldwide, with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones (at least of Category 3 intensity).
  
  For more information, see Tropical cyclone basins .
• Typhoon From Wikipedia:
  A typhoon is a mature tropical cyclone that develops between 180° and 100°E in the Northern Hemisphere.
  This region is referred to as the Northwestern Pacific Basin, and is the most active tropical cyclone basin on Earth, accounting for almost one-third of the world's annual tropical cyclones.
  For organizational purposes, the northern Pacific Ocean is divided into three regions: the eastern (North America to 140°W), central (140° to 180°W), and western (180° to 100°E).
  The Regional Specialized Meteorological Center (RSMC) for tropical cyclone forecasts is in Japan, with other tropical cyclone warning centers for the northwest Pacific in Hawaii (the Joint Typhoon Warning Center), the Philippines and Hong Kong.
  While the RSMC names each system, the main name list itself is coordinated among 18 countries that have territories threatened by typhoons each year.
  Only the Philippines use their own naming list for systems approaching the country.
  
  A typhoon differs from a cyclone or hurricane only on the basis of location.
  A hurricane is a storm that occurs in the Atlantic Ocean or northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a tropical cyclone occurs in the south Pacific or Indian Ocean.
  
  For more information, see Typhoon .
• Valley exit jet From Wikipedia:
  A valley exit jet is a strong, down-valley, elevated air current that emerges above the intersection of the valley and its adjacent plain.
  These winds frequently reach a maximum of 20 m/s (45 mph) at a height of 40–200 m (130–660 ft) above the ground.
  Surface winds below the jet may sway vegetation but are significantly weaker.
  
  The presence of these strong nighttime down-valley air flows has been documented at the mouth of many Alpine valleys that merge with basins, such as the Inn Valley of Austria, where the jet is strong enough to be heard at the ground.
  In the United States, exit jet signatures have been observed at the North Fork Gunnison River at Paonia, Colorado, the exit of South Boulder Creek south of Boulder, Colorado and the mouth of Spanish Fork Canyon in Utah.
  
  For more information, see Valley exit jet .
• Vertical draft From Wikipedia:
  An updraft is a small-scale current of rising air, often within a cloud. ... ... Localized regions of warm or cool air will exhibit vertical movement.
  A mass of warm air will typically be less dense than the surrounding region, and so will rise until it reaches air that is either warmer or less dense than itself.
  The converse will occur for a mass of cool air, and is known as subsidence.
  This movement of large volumes of air, especially when regions of hot, wet air rise, can create large clouds, and is the central source of thunderstorms.
  Drafts can also be conceived by low or high pressure regions.
  A low pressure region will attract air from the surrounding area, which will move towards the center and then rise, creating an updraft.
  A high pressure region will then attract air from the surrounding area, which will move towards the center and sink, spawning a downdraft.
  
  For more information, see Vertical draft .
• Virga From Wikipedia:
  In meteorology, a virga is an observable streak or shaft of precipitation falling from a cloud that evaporates or sublimates before reaching the ground.
  A shaft of precipitation that doesn't evaporate before reaching the ground is a precipitation shaft.
  At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is often due to compressional heating, because the air pressure increases closer to the ground.
  It is very common in deserts and temperate climates.
  In North America, it is commonly seen in the Western United States and the Canadian Prairies.
  It is also very common in the Middle East, Australia, and North Africa.
  
  Virgae can cause varying weather effects, because as rain is changed from liquid to vapor form, it removes significant amounts of heat from the air due to water's high heat of vaporization.
  Precipitation falling into these cooling down drafts may eventually reach the ground.
  In some instances these pockets of colder air can descend rapidly, creating a wet or dry microburst which can be extremely hazardous to aviation.
  Conversely, precipitation evaporating at high altitude can compressionally heat as it falls, and result in a gusty downburst which may substantially and rapidly warm the surface temperature.
  This fairly rare phenomenon, a heat burst, also tends to be of exceedingly dry air.
  Virgae also have a role in seeding storm cells.
  This is because small particles from one cloud are blown into neighboring supersaturated air and act as nucleation particles for the next thunderhead cloud to begin forming.
  
  For more information, see Virga .
• Vog From Wikipedia:
  Vog is a form of air pollution that results when sulfur dioxide and other gases and particles emitted by an erupting volcano react with oxygen and moisture in the presence of sunlight.
  The word is a portmanteau of the words "volcanic", "smog", and "fog".
  The term is in common use in the Hawaiian islands, where the Kilauea volcano, on the Island of Hawaii (aka "The Big Island"), has been erupting continuously since January 3, 1983.
  Based on June 2008 measurements, Kilauea emits 2,000–4,000 tons of sulfur dioxide (SO₂) every day.
  
  For more information, see Vog .
• Waterspout From Wikipedia:
  A waterspout is an intense columnar vortex (usually appearing as a funnel-shaped cloud) that occurs over a body of water.
  Some are connected to a cumulus congestus cloud, some to a cumuliform cloud and some to a cumulonimbus cloud.
  In the common form, it is a non-supercell tornado over water.
  
  While it is often weaker than most of its land counterparts, stronger versions spawned by mesocyclones do occur.
  Most waterspouts do not suck up water; they are small and weak rotating columns of air over water.
  
  While waterspouts form mostly in the tropics and subtropical areas, other areas also report waterspouts, including Europe, Australia, New Zealand, the Great Lakes, Antarctica and on rare occasions, the Great Salt Lake.
  Although rare, waterspouts have been observed in connection with lake-effect snow precipitation bands.
  
  Waterspouts have a five-part life cycle: formation of a dark spot on the water surface, spiral pattern on the water surface, formation of a spray ring, development of the visible condensation funnel, and ultimately decay.
  
  For more information, see Waterspout .
• Westerlies From Wikipedia:
  The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude.
  They originate from the high-pressure areas in the horse latitudes and trend towards the poles and steer extratropical cyclones in this general manner.
  Tropical cyclones which cross the subtropical ridge axis into the westerlies recurve due to the increased westerly flow.
  The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.
  
  The westerlies are strongest in the winter hemisphere and times when the pressure is lower over the poles, while they are weakest in the summer hemisphere and when pressures are higher over the poles.
  The westerlies are particularly strong, especially in the Southern Hemisphere, in areas where land is absent, because land amplifies the flow pattern, making the current more north-south oriented, slowing the westerlies.
  The strongest westerly winds in the middle latitudes can come in the roaring forties, between 40 and 50 degrees latitude.
  The westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse.
  For more information, see Westerlies .
• Wet season From Wikipedia:
  The monsoon season, is the time of year when most of a region's average annual rainfall occurs.
  Generally the season lasts at least a month.
  The term "green season" is also sometimes used as a euphemism by tourist authorities.
  Areas with wet seasons are dispersed across portions of the tropics and subtropics.
  
  Under the Köppen climate classification, for tropical climates, a wet season month is defined as a month where average precipitation is 60 millimetres (2.4 in) or more.
  In contrast to areas with savanna climates and monsoon regimes, Mediterranean climates have wet winters and dry summers.
  Dry and rainy months are characteristic of tropical seasonal forests: in contrast to tropical rainforests, which do not have dry or wet seasons, since their rainfall is equally distributed throughout the year.[5] Some areas with pronounced rainy seasons will see a break in rainfall mid-season, when the intertropical convergence zone or monsoon trough moves to higher latitudes in the middle of the warm season.
  
  For more information, see Wet season .
• Whirlwind From Wikipedia:
  A whirlwind is a weather phenomenon in which a vortex of wind (a vertically oriented rotating column of air) forms due to instabilities and turbulence created by heating and flow (current) gradients.
  Whirlwinds occur all over the world and in any season. ...
  
  ... Whirlwinds are subdivided into two main types, the great (or major) whirlwinds and the lesser (or minor) whirlwinds.
  The first category includes tornadoes, waterspouts, and landspouts.
  The range of atmospheric vortices constitute a continuum and are difficult to categorize definitively.
  Some lesser whirlwinds may sometimes form in a similar manner to greater whirlwinds with related increase in intensity.
  These intermediate types include the gustnado and the fire whirl.
  Other lesser whirlwinds include dust devils, as well as steam devils, snow devils, debris devils, leaf devils, and shear eddies such as the mountainado and eddy whirlwinds.
  
  For more information, see Whirlwind .
• Whiteout (weather) From Wikipedia:
  Whiteout is a weather condition in which visibility and contrast are severely reduced by snow or sand.
  The horizon disappears completely and there are no reference points, leaving the individual with a distorted orientation.
  Whiteout has been defined as: "A condition of diffuse light when no shadows are cast, due to a continuous white cloud layer appearing to merge with the white snow surface.
  No surface irregularities of the snow are visible, but a dark object may be clearly seen.
  There is no visible horizon."
  
  A whiteout may be due simply to extremely heavy snowfall rates as seen in lake effect conditions, or to other factors such as diffuse lighting from overcast clouds, mist or fog, or a background of snow.
  People can be lost in their own front yards during a true whiteout, when the door is only 10 feet (3 meters) away, and they would have to feel their way back.
  Motorists have to stop their cars where they are, as the road is impossible to see.
  Normal snowfalls and blizzards, where snow is falling at 1 or 2 inches per hour (2.5 or 5.1 cm/h), or where the relief visibility is not clear yet having a clear field of view for over 30 feet (9 meters), are often incorrectly called whiteouts.
  
  For more information, see Whiteout (weather) .
• White squall From Wikipedia:
  A white squall is a sudden and violent windstorm at sea which is not accompanied by the black clouds generally characteristic of a squall.
  The name refers to the white-capped waves and broken water, its meager warning to any unlucky seaman caught in its path.
  White squalls are rare at sea, but common on the Great Lakes of North America.
  
  A white squall is the culprit of many sea stories and blamed for quite a few tragedies.
  It is described as a sudden increase in wind velocity in tropical and sub-tropical waters, and lacks the usual dark, ominous squall clouds.
  The white squall may be a microburst.
  
  For more information, see White squall .
• Wind From Wikipedia:
  Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. ... ... In meteorology, winds are often referred to according to their strength, and the direction from which the wind is blowing.
  Short bursts of high-speed wind are termed gusts.
  Strong winds of intermediate duration (around one minute) are termed squalls.
  Long-duration winds have various names associated with their average strength, such as breeze, gale, storm, and hurricane.
  Wind occurs on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption of solar energy between the climate zones on Earth.
  The two main causes of large-scale atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis effect).
  Within the tropics, thermal low circulations over terrain and high plateaus can drive monsoon circulations.
  In coastal areas the sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can dominate local winds. ...
  
  ... Wind is caused by differences in the atmospheric pressure.
  When a difference in atmospheric pressure exists, air moves from the higher to the lower pressure area, resulting in winds of various speeds.
  On a rotating planet, air will also be deflected by the Coriolis effect, except exactly on the equator.
  Globally, the two major driving factors of large-scale wind patterns (the atmospheric circulation) are the differential heating between the equator and the poles (difference in absorption of solar energy leading to buoyancy forces) and the rotation of the planet.
  Outside the tropics and aloft from frictional effects of the surface, the large-scale winds tend to approach geostrophic balance.
  Near the Earth's surface, friction causes the wind to be slower than it would be otherwise.
  Surface friction also causes winds to blow more inward into low-pressure areas.
  
  For more information, see Wind .
• Winter storm From Wikipedia:
  A winter storm is an event in which varieties of precipitation are formed that only occur at low temperatures, such as snow or sleet, or a rainstorm where ground temperatures are low enough to allow ice to form (i.e. freezing rain).
  In temperate continental climates, these storms are not necessarily restricted to the winter season, but may occur in the late autumn and early spring as well.
  Very rarely, they may form in summer, though it would have to be an abnormally cold summer, such as the summer of 1816 in the Northeastern United States.
  
  For more information, see Winter storm .

Climate Zones/Climate Types:

• Geographical zone From Wikipedia:
  The five main latitude regions of the Earth's surface comprise geographical zones, divided by the major circles of latitude.
  The differences between them relate to climate.
  They are as follows:
  
  - The North frigid zone, between the Arctic Circle 66.5° N and the North Pole 90° N. Covers 4.12% of Earth's surface.
  - The North temperate zone, between the Tropic of Cancer 23.5° N and the Arctic Circle 66.5° N. Covers 25.99% of Earth's surface.
  - The Torrid zone, between the Tropic of Cancer 23.5° N and the Tropic of Capricorn 23.5° S. Covers 39.78% of Earth's surface.
  - The South temperate zone, between the Tropic of Capricorn 23.5° S and the Antarctic Circle 66.5° S. Covers 25.99% of Earth's surface.
  - The South frigid zone, from Antarctic Circle 66.5° S and the South Pole 90° S. Covers 4.12% of Earth's surface.
  
  Earth's climatic zones
  - Ice cap
  - Tundra
  - Boreal
  - Warm temperate
  - Subtropical
  - Tropical
  
  On the basis of latitudinal extent, the globe is divided into three broad heat zones.
  
  For more information, see Geographical zone .
• Middle latitudes From Wikipedia:
  The middle latitudes (also called the mid-latitudes, sometimes midlatitudes, or moderate latitudes) of Earth lie between 23°26'22" and 66°33'39" north, and between 23°26'22" and 66°33'39" south.
  They include Earth's subtropical and temperate zones, which lie between the tropics and the polar circles (between the Tropic of Cancer and the Arctic Circle, and between the Tropic of Capricorn and the Antarctic Circle).
  Weather fronts and extratropical cyclones are usually found in this area, as well as occasional tropical cyclones, which have traveled from their areas of formation closer to the Equator.
  
  The prevailing winds in the middle latitudes are often very strong.
  These parts of the world also see a wide variety of fast-changing weather as cold air masses from the poles and warm air masses from the tropics push up and down over them, sometimes alternating within hours of each other, especially in the roaring forties (latitudes between 40° and 50° in both hemispheres).
  
  There are four types of mid-latitude climates: mediterranean, humid subtropical, marine west coast, and humid continental.
  
  For more information, see Middle latitudes .
• Polar regions of Earth From Wikipedia:
  The polar regions, also called the frigid zones, of Earth are the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles.
  These high latitudes are dominated by Earth's polar ice caps: the northern resting on the Arctic Ocean and the southern on the continent of Antarctica.
  
  For more information, see Polar regions of Earth .
• Subtropics From Wikipedia:
  The subtropics are geographic and climate zones located roughly between the tropics at latitude 23.5° (the Tropic of Cancer and Tropic of Capricorn) and temperate zones (normally referring to latitudes 35–66.5°) north and south of the Equator.
  
  Subtropical climates are often characterized by warm to hot summers and cool to mild winters with infrequent frost.
  Most subtropical climates fall into two basic types - humid subtropical, where rainfall is often concentrated in the warmest months (for example Brisbane, Queensland or Jacksonville, Florida) and dry summer (or Mediterranean), where seasonal rainfall is concentrated in the cooler months (for example Naples, Italy or Los Angeles, California).
  
  Subtropical climates can occur at high elevations within the tropics, such as in the southern end of the Mexican Plateau and in Vietnam and Taiwan.
  Six climate classifications use the term to help define the various temperature and precipitation regimes for the planet Earth.
  
  For more information, see Subtropics .
• Tropics From Wikipedia:
  The tropics are a region of the Earth surrounding the Equator.
  They are delimited in latitude by the Tropic of Cancer in the Northern Hemisphere at 23°26'12.7" (or 23.43687°) N and the Tropic of Capricorn in the Southern Hemisphere at 23°26'12.7" (or 23.43687°) S; these latitudes correspond to the axial tilt of the Earth.
  The tropics are also referred to as the tropical zone and the torrid zone (see geographical zone).
  The tropics include all the areas on the Earth where the Sun contacts the zenith, a point directly overhead, at least once during the solar year (which is a subsolar point).
  
  The tropics are distinguished from the other climatic and biomatic regions of Earth, which are the middle latitudes and the polar regions on either side of the equatorial zone.
  
  The tropics comprise 40% of the Earth's surface area and contain 36% of the Earth's landmass.
  As of 2014, the region is home to 40% of the world population, and this figure is projected to reach 50% by the late 2030s.
  
  For more information, see Tropics .


• Alpine climate From Wikipedia:
  Alpine climate is the average weather (climate) for the regions above the tree line.
  This climate is also referred to as a mountain climate or highland climate. ...
  
  ... There are multiple definitions of alpine climate.
  
  One simple definition is the climate which causes trees to fail to grow due to cold.
  According to the Holdridge life zone system, alpine climate occurs when the mean biotemperature of a location is between 1.5 and 3 °C (34.7 and 37.4 °F), which prevents tree growth.
  Biotemperature is defined as the temperature, except all temperatures below 0 °C (32 °F) are treated as 0 °C (32 °F), because plants are dormant below freezing.
  
  In the Köppen climate classification, the alpine climate is part of "Group E", along with the polar climate, where no month has a mean temperature higher than 10 °C (50 °F).
  
  For more information, see Alpine climate .
• Climate of Antarctica From Wikipedia:
  Antarctica has the coldest climate on the Earth.
  Antarctica's lowest air temperature record was set on 21 July 1983, with -89.2 °C (-128.6 °F) at Vostok Station.
  Satellite measurements have identified even lower ground temperatures, down to -93.2 °C (-135.8 °F) at the cloud free East Antarctic Plateau on 10 August 2010.
  It is also extremely dry (technically a desert), averaging 166 mm (6.5 in) of precipitation per year.
  On most parts of the continent the snow rarely melts and is eventually compressed to become the glacier ice that makes up the ice sheet.
  Weather fronts rarely penetrate far into the continent, because of the katabatic winds.
  Most of Antarctica has an ice cap climate (Köppen EF) with very cold, generally extremely dry weather.
  
  For more information, see Climate of Antarctica .
• Climate of the Arctic From Wikipedia:
  The climate of the Arctic is characterized by long, cold winters and short, cool summers.
  There is a large amount of variability in climate across the Arctic, but all regions experience extremes of solar radiation in both summer and winter.
  Some parts of the Arctic are covered by ice (sea ice, glacial ice, or snow) year-round, and nearly all parts of the Arctic experience long periods with some form of ice on the surface.
  Average January temperatures range from about -34 °C to 0 °C (-29 to +32 °F), and winter temperatures can drop below -50 °C (-58 °F) over large parts of the Arctic.
  Average July temperatures range from about -10 to +10 °C (14 to 50 °F), with some land areas occasionally exceeding 30 °C (86 °F) in summer.
  
  The Arctic consists of ocean that is largely surrounded by land.
  As such, the climate of much of the Arctic is moderated by the ocean water, which can never have a temperature below -2 °C (28 °F).
  In winter, this relatively warm water, even though covered by the polar ice pack, keeps the North Pole from being the coldest place in the Northern Hemisphere, and it is also part of the reason that Antarctica is so much colder than the Arctic.
  In summer, the presence of the nearby water keeps coastal areas from warming as much as they might otherwise.
  
  For more information, see Climate of the Arctic .
• Continental climate From Wikipedia:
  Continental climates are defined in the Köppen climate classification as having the coldest month with the temperature never rising above 0.0° C (32°F) all month long.
  
  Continental climates often have a significant annual variation in temperature (hot summers and cold winters).
  They tend to occur in the middle latitudes (40 to 55 north), where prevailing winds blow overland, and temperatures are not moderated by bodies of water such as oceans or seas.
  Continental climates occur mostly in the Northern Hemisphere, which has the kind of large landmasses required for this type of climate to develop.
  Most of northern and northeastern China, eastern and southeastern Europe, central and southeastern Canada, and the central and upper eastern United States have this type of climate.
  
  In Continental climates, precipitation tends to be moderate in amount, concentrated mostly in the warmer months.
  Only a few areas, in the mountains of the Pacific Northwest of North America and in Iran, northern Iraq, adjacent Turkey, Afghanistan, Pakistan, and Central Asia show a winter maximum in precipitation.
  A portion of the annual precipitation falls as snowfall, and snow often remains on the ground for more than a month.
  Summers in continental climates can feature thunderstorms and frequent cool temperatures, however summer weather is more stable than winter weather.
  
  For more information, see Continental climate .
• Desert climate From Wikipedia:
  The desert climate (in the Köppen climate classification BWh and BWk, sometimes also BWn), also known as an arid climate, is a climate in which precipitation is too low to sustain any vegetation at all, or at most a very scanty shrub, and does not meet the criteria to be classified as a polar climate.
  
  An area that features this climate usually experiences from 25 to 200 mm (7.87 inches) per year of precipitation and in some years may experience no precipitation at all.
  Averages may be even less such as in Arica, Chile, where precipitation normals annually stand at around 1 mm per year.
  In some instances, an area may experience more than 200 mm of precipitation annually, but is considered a desert climate because the region loses more water via evapotranspiration than falls as precipitation (Tucson, Arizona, and Alice Springs, Northern Territory, are examples of this).
  
  For more information, see Desert climate .
• Humid continental climate From Wikipedia:
  A humid continental climate (Köppen prefix D and a third letter of a or b) is a climatic region defined by Russo-German climatologist Wladimir Köppen in 1900, typified by large seasonal temperature differences, with warm to hot (and often humid) summers and cold (sometimes severely cold in the northern areas) winters.
  Precipitation is usually distributed throughout the year.
  The definition of this climate regarding temperature is as follows: the mean temperature of the coldest month must be below -3 °C (26.6 °F) (or 0 °C (32.0 °F)) and there must be at least four months whose mean temperatures are at or above 10 °C (50 °F).
  In addition, the location in question must not be semi-arid or arid.
  The Dfb, Dwb and Dsb subtypes are also known as hemiboreal.
  
  For more information, see Humid continental climate .
• Humid subtropical climate From Wikipedia:
  A humid subtropical climate is a zone of climate characterized by hot and humid summers, and mild to chilly winters.
  These climates normally lie on the southeast side of all continents, generally between latitudes 25° and 35° and are located poleward from adjacent tropical climates.
  While many subtropical climates tend to be located at or near coastal locations, in some cases they extend inland, most notably in China and the United States.
  
  For more information, see Humid subtropical climate .
• Ice cap climate From Wikipedia:
  An ice cap climate is a polar climate where the temperature never exceeds 0 °C (32 °F).
  The climate covers areas in or near the polar regions, such as Antarctica and Greenland, as well as the highest mountaintops.
  Such areas are covered by a permanent layer of ice and have no vegetation, but they may have animal life, that usually feeds from the oceans.
  Ice cap climates are inhospitable to human life.
  Antarctica, the coldest continent on Earth, sustains no permanent human residents, but has some civil inhabitants in proximity to research stations in coastal settlements that are maritime polar and there are some communities that are situated in a transitional zone between the two climates, but barely qualify as a tundra.
  Some places like Antarctica had a different climate before having an ice cap climate.
  
  For more information, see Ice cap climate .
• Mediterranean climate From Wikipedia:
  A Mediterranean climate or dry summer climate is characterized by rainy winters and dry summers.
  While the climate receives its name from the Mediterranean Basin.
  These are generally located on the western coasts of continents, between roughly 30 and 45 degrees north and south of the equator, typically between oceanic climates towards the poles (where they tend to be wetter), and semi-arid and arid climates towards the equator (where they tend to be drier).
  
  In essence, and due to the seasonal shift of the subtropical high-pressure belts with the apparent movement of the Sun, a Mediterranean climate is an intermediate type between these other climates, with winters somewhat mimicking winters in oceanic climates and summers imitating sunny weather in semi-arid and arid climates.
  But contrary to oceanic climates, there are always a number of clear, sunny days in the winter season.
  
  For more information, see Mediterranean climate .
• Oceanic climate From Wikipedia:
  An oceanic climate, also known as a marine or maritime climate, is the Köppen classification of climate typical of west coasts in higher middle latitudes of continents, and generally features cool summers (relative to their latitude) and cool winters, with a relatively narrow annual temperature range and few extremes of temperature, with the exception for transitional areas to continental, subarctic and highland climates.
  Oceanic climates are defined as having a monthly mean temperature below 22 °C (72 °F) in the warmest month, and above 0 °C (32 °F) (or -3 °C (27 °F)) in the coldest month.
  
  For more information, see Oceanic climate .
• Polar climate From Wikipedia:
  The polar climate regions are characterized by a lack of warm summers.
  Every month in a polar climate has an average temperature of less than 10 °C (50 °F). Regions with polar climate cover more than 20% of the Earth.
  Most of these regions are far from the equator, and in this case, summer days are extremely long and winter days are extremely short.
  A polar climate consists of cool summers and very cold winters, which results in treeless tundra, glaciers, or a permanent or semi-permanent layer of ice.
  
  For more information, see Polar climate .
• Semi-arid climate From Wikipedia:
  A semi-arid climate or steppe climate is the climate of a region that receives precipitation below potential evapotranspiration, but not as low as a desert climate.
  There are different kinds of semi-arid climates, depending on variables such as temperature, and they give rise to different biomes. ...
  
  ... A more precise definition is given by the Köppen climate classification, which treats steppe climates (BSk and BSh) as intermediates between desert climates (BW) and humid climates in ecological characteristics and agricultural potential.
  Semi-arid climates tend to support short or scrubby vegetation and are usually dominated by either grasses or shrubs.
  
  To determine if a location has a semi-arid climate, the precipitation threshold must first be determined.
  Finding the precipitation threshold (in millimeters) involves first multiplying the average annual temperature in °C by 20, then adding 280 if 70% or more of the total precipitation is in the high-sun half of the year (April through September in the Northern temperate zone, or October through March in the Southern), or 140 if 30%–70% of the total precipitation is received during the applicable period, or 0 if less than 30% of the total precipitation is so received.
  If the area's annual precipitation is less than the threshold but more than half the threshold, it is classified as a BS (steppe climate).
  
  Furthermore, to delineate "hot semi-arid climates" from "cold semi-arid climates", there are three widely used isotherms: Either a mean annual temperature of 18°C, or a mean temperature of 0°C or -3°C in the coldest month, so that a location with a "BS" type climate with the appropriate temperature above whichever isotherm is being used is classified as "hot semi-arid" (BSh), and a location with the appropriate temperature below the given isotherm is classified as "cold semi-arid" (BSk).
  
  For more information, see Semi-arid climate .
• Subarctic climate From Wikipedia:
  The subarctic climate (also called subpolar climate, subalpine climate, or boreal climate) is a climate characterised by long, usually very cold winters, and short, cool to mild summers.
  It is found on large landmasses, away from the moderating effects of an ocean, generally at latitudes from 50° to 70°N poleward of the humid continental climates.
  These climates represent Köppen climate classification Dfc, Dwc, Dsc, Dfd, Dwd and Dsd.
  In very small areas at high altitudes around the Mediterranean Basin, Iran, Kyrgyzstan, Tajikistan, Turkey, Alaska and other parts of the northwestern United States (Eastern Washington, Eastern Oregon and Idaho) the climate is classified as Dsc with a dry summer climate, such as in Seneca, Oregon.
  
  For more information, see Subarctic climate .
• Temperate climate From Wikipedia:
  In geography, the temperate or tepid climates of Earth occur in the middle latitudes, which span between the tropics and the polar regions of Earth.
  These zones generally have wider temperature ranges throughout the year and more distinct seasonal changes compared to tropical climates, where such variations are often small.
  
  The Köppen climate classification defines a climate as "temperate" when the coldest month has a mean temperature above -3 °C (26.6 °F) but below 18 °C (64.4 °F). ... ... The north temperate zone extends from (approximately 23.5° north) to the Arctic Circle (approximately 66.5° north latitude).
  The south temperate zone extends from (approximately 23.5° south) to the Antarctic Circle (at approximately 66.5° south).
  
  In some climate classifications, the temperate zone is often divided into several smaller climate zones, based on monthly temperatures, the coldest month, and rainfall.
  These include humid subtropical climate, Mediterranean climate, oceanic, and continental climate.
  
  For more information, see Temperate climate .
• Tropical climate From Wikipedia:
  A tropical climate in the Köppen climate classification is a non-arid climate in which all twelve months have mean temperatures of at least 18 °C (64 °F).
  In tropical climates there are often only two seasons: a wet season and a dry season.
  Tropical climates are frost-free, and changes in the solar angle are small.
  In tropical climates temperature remains relatively constant (hot) throughout the year.
  
  For more information, see Tropical climate .
• Tropical marine climate From Wikipedia:
  A tropical marine climate is a tropical climate that is primarily influenced by the ocean.
  It is usually experienced by islands and coastal areas 10° to 20° north and south of the equator.
  There are two main seasons in a tropical marine climate: the wet season and the dry season.
  The annual rainfall is 1000 to over 1500 mm.
  The temperature ranges from 20 °C to 35 °C.
  The trade winds blow all year round and are moist, as they pass over warm seas.
  These climatic conditions are found, for example, across the Caribbean; the eastern coasts of Brazil, Madagascar and Queensland; and many islands in tropical waters.
  
  For more information, see Tropical marine climate .
• Tropical monsoon climate From Wikipedia:
  A tropical monsoon climate (occasionally known as a tropical wet climate or a tropical monsoon and trade-wind littoral climate) is a type of climate that corresponds to the Köppen climate classification category "Am".
  Tropical monsoon climates have monthly mean temperatures above 18 °C (64.4 °F) in every month of the year and feature wet and dry seasons, as tropical savanna climates do.
  A tropical monsoon climate, however, has its driest month seeing on average less than 60 mm, but more than 4% the total annual precipitation.
  This latter fact is in direct contrast to a tropical savanna climate, whose driest month sees less than 60 mm of precipitation and also less than 100 – [total annual precipitation {mm}/25] of precipitation.
  In essence, a tropical monsoon climate tends to either see more rainfall than a tropical savanna climate or have less pronounced dry seasons.
  Additionally, a tropical monsoon climate tends to see less variance in temperatures during the course of the year than a tropical savanna climate.
  This climate has a driest month which nearly always occurs at or soon after the "winter" solstice for that side of the equator.
  
  For more information, see Tropical monsoon climate .
• Tropical rainforest climate From Wikipedia:
  A tropical rainforest climate, also known as an equatorial climate, is a tropical climate usually (but not always) found along the equator.
  Regions with this climate typically feature tropical rainforests, and it is designated Af by the Köppen climate classification.
  A tropical rainforest climate is typically hot and wet.
  Places with this climate typically lie within 10º of the equator. ...
  
  ... Tropical rainforests have a type of tropical climate in which there is no dry season—all months have an average precipitation value of at least 60 mm (2.4 in).
  Tropical rainforests have no summer or winter; it is typically hot and wet throughout the year and rainfall is both heavy and frequent.
  One day in an equatorial climate can be very similar to the next, while the change in temperature between day and night may be larger than the average change in temperature during the year.
  
  For more information, see Tropical rainforest climate .
• Tropical savanna climate From Wikipedia:
  Tropical savanna climate or tropical wet and dry climate is a type of climate that corresponds to the Köppen climate classification categories "Aw" and "As".
  Tropical savanna climates have monthly mean temperatures above 18 °C (64 °F) in every month of the year and typically a pronounced dry season, with the driest month having less than 60 mm (2.36 inches) of precipitation and also less than 100 – [total annual precipitation {mm}/25] of precipitation.
  This latter fact is in direct contrast to a tropical monsoon climate, whose driest month sees less than 60 mm of precipitation but has more than 100 – [total annual precipitation {mm}/25] of precipitation.
  In essence, a tropical savanna climate tends to either see less rainfall than a tropical monsoon climate or have more pronounced dry season(s). ...
  
  ... There are generally four types of tropical savanna climates:
  
  - Distinct wet and dry seasons of relatively equal duration.
  Most of the region's annual rainfall is experienced during the wet season and very little precipitation falls during the dry season.
  
  - A lengthy dry season and a relatively short wet season.
  This version features seven or more dry season months and five or fewer wet season months.
  There are variations within this version: - - On one extreme, the region receives just enough precipitation during the short wet season to preclude it from a semi-arid climate classification. This drier variation of the tropical savanna climate is typically found adjacent to regions with semi-arid climates.
  - - On the other extreme, the climate features a lengthy dry season followed by a short but extremely rainy wet season. However, regions with this variation of the climate do not experience enough rainfall during the wet season to qualify as a tropical monsoon climate.
  
  - A lengthy wet season and a relatively short dry season.
  This version features seven or more wet season months and five or fewer dry season months.
  This version's precipitation pattern is similar to precipitation patterns observed in some tropical monsoon climates, but does not experience enough rainfall during the wet season to be classified as such.
  
  - A dry season with a noticeable amount of rainfall followed by a rainy wet season.
  In essence, this version mimics the precipitation patterns more commonly found in a tropical monsoon climate, but do not receive enough precipitation during either the dry season or the year to be classified as such.
  
  For more information, see Tropical savanna climate .
• Tundra climate From Wikipedia:
  Tundra climates ordinarily fit the Köppen climate classification ET, signifying a local climate in which at least one month has an average temperature high enough to melt snow (0 °C (32 °F)), but no month with an average temperature in excess of 10 °C (50 °F).
  The cold limit generally meets the EF climates of permanent ice and snows; the warm-summer limit generally corresponds with the poleward or altitudinal limit of trees, where they grade into the subarctic climates designated Dfd, Dwd and Dsd (extreme winters as in parts of Siberia), Dfc typical in Alaska, Canada, parts of Scandinavia, European Russia, and Western Siberia (cold winters with months of freezing), or even Cfc (no month colder than -3 °C (27 °F) as in parts of Iceland and southernmost South America).
  Tundra climates as a rule are hostile to woody vegetation even where the winters are comparatively mild by polar standards, as in Iceland.
  
  For more information, see Tundra climate .


• Köppen climate classification From Wikipedia:
  The Köppen climate classification is one of the most widely used climate classification systems.
  It was first published by the climatologist Wladimir Köppen (1846-1940) in 1884, with several later modifications by Köppen, notably in 1918 and 1936.
  Later, the climatologist Rudolf Geiger (1954, 1961) introduced some changes to the classification system, which is thus sometimes called the Köppen–Geiger climate classification system.
  
  The Köppen climate classification divides climates into five main climate groups, with each group being divided based on seasonal precipitation and temperature patterns.
  The five main groups are A (tropical), B (dry), C (temperate), D (continental), and E (polar).
  Each group and subgroup is represented by a letter.
  All climates are assigned a main group (the first letter).
  All climates except for those in the E group are assigned a seasonal precipitation subgroup (the second letter).
  For example, Af indicates a tropical rainforest climate.
  The system assigns a temperature subgroup for all groups other than those in the A group, indicated by the third letter for climates in B, C, and D, and the second letter for climates in E.
  For example, Cfb indicates an oceanic climate with warm summers as indicated by the ending b.
  Climates are classified based on specific criteria unique to each climate type.
  
  For more information, see Köppen climate classification .
• Trewartha climate classification From Wikipedia:
  The Trewartha climate classification is a climate classification system published by American geographer Glenn Thomas Trewartha in 1966, and updated in 1980.
  It is a modified version of the 1899 Köppen system, created to answer some of the deficiencies of the Köppen system.
  The Trewartha system attempts to redefine the middle latitudes to be closer to vegetation zoning and genetic climate systems.
  It was considered a more true or "real world" reflection of the global climate.[2]
  
  The changes were seen as most effective on the large landmasses in Asia and North America, where many areas fall into a single group (C) in the Köppen system.
  For example, under the standard Köppen system, Washington and Oregon are classed into the same climate zone as Southern California, even though the two regions have strikingly different weather and vegetation.
  Another example was classifying cities like London in the same climate group as Brisbane or New Orleans, despite great differences in seasonal temperatures and native plant life.
  
  For more information, see Trewartha climate classification .

The Earth's Changing climate:

• Abrupt climate change From Wikipedia:
  An abrupt climate change occurs when the climate system is forced to transition to a new climate state at a rate that is determined by the climate system energy-balance, and which is more rapid than the rate of change of the external forcing.
  Past events include the end of the Carboniferous Rainforest Collapse, Younger Dryas, Dansgaard-Oeschger events, Heinrich events and possibly also the Paleocene–Eocene Thermal Maximum.
  The term is also used within the context of global warming to describe sudden climate change that is detectable over the time-scale of a human lifetime.
  One proposed cause of such events is feedback loops within the climate system both enhance small perturbations and cause a variety of stable states. ...
  
  ... Timescales of events described as 'abrupt' may vary dramatically.
  Changes recorded in the climate of Greenland at the end of the Younger Dryas, as measured by ice-cores, imply a sudden warming of +10 °C (+18 °F) within a timescale of a few years.
  Other abrupt changes are the +4 °C (+7.2 °F) on Greenland 11,270 years ago[7] or the abrupt +6 °C (11 °F) warming 22,000 years ago on Antarctica.
  By contrast, the Paleocene-Eocene thermal maximum may have initiated anywhere between a few decades and several thousand years.
  Finally, Earth Systems models project that under ongoing greenhouse gas emissions as early as 2047, the Earth's near surface temperature could depart from the range of variability in the last 150 years, affecting over 3 billion people and most places of great species diversity on Earth.
  
  For more information, see Abrupt climate change .
• Climate change From Wikipedia:
  Climate change is a change in the statistical distribution of weather patterns when that change lasts for an extended period of time (i.e., decades to millions of years).
  Climate change may refer to a change in average weather conditions, or in the time variation of weather within the context of longer-term average conditions.
  Climate change is caused by factors such as biotic processes, variations in solar radiation received by Earth, plate tectonics, and volcanic eruptions.
  Certain human activities have been identified as primary causes of ongoing climate change, often referred to as global warming.
  There is no general agreement in scientific, media or policy documents as to the precise term to be used to refer to anthropogenic forced change; either "global warming" or "climate change" may be used.
  
  For more information, see Climate change .
• Climate change feedback From Wikipedia:
  Climate change feedback is important in the understanding of global warming because feedback processes may amplify or diminish the effect of each climate forcing, and so play an important part in determining the climate sensitivity and future climate state.
  Feedback in general is the process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first.
  Positive feedback amplifies the change in the first quantity while negative feedback reduces it.
  The term "forcing" means a change which may "push" the climate system in the direction of warming or cooling.
  An example of a climate forcing is increased atmospheric concentrations of greenhouse gases.
  By definition, forcings are external to the climate system while feedbacks are internal; in essence, feedbacks represent the internal processes of the system.
  Some feedbacks may act in relative isolation to the rest of the climate system; others may be tightly coupled; hence it may be difficult to tell just how much a particular process contributes.
  Forcings, feedbacks and the dynamics of the climate system determine how much and how fast the climate changes.
  The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, which in turn leads to further warming.
  The main negative feedback comes from the Stefan–Boltzmann law, the amount of heat radiated from the Earth into space changes with the fourth power of the temperature of Earth's surface and atmosphere.
  
  For more information, see Climate change feedback .
• Global dimming From Wikipedia:
  Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s.
  The effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 1960–1990.
  However, after discounting an anomaly caused by the eruption of Mount Pinatubo in 1991, a very slight reversal in the overall trend has been observed.
  
  Global dimming is thought to have been caused by an increase in particulates such as sulfate aerosols in the atmosphere due to human action.
  
  It has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas.
  Global dimming also creates a cooling effect that may have partially counteracted the effect of greenhouse gases on global warming.
  
  For more information, see Global dimming .
• Global warming From Wikipedia:
  Global warming, also referred to as climate change, is the observed century-scale rise in the average temperature of the Earth's climate system and its related effects.
  Multiple lines of scientific evidence show that the climate system is warming.
  Many of the observed changes since the 1950s are unprecedented in the instrumental temperature record, which extends back to the mid-19th century, and in paleoclimate proxy records of climate change over thousands of years.
  The phenomenon is sometimes called "anthropogenic global warming" or "anthropogenic climate change" in view of the dominant role of human activity as its cause.
  
  For more information, see Global warming .
• Greenhouse and icehouse Earth From Wikipedia:
  Throughout the Phanerozoic history of the Earth, the planet's climate has been fluctuating between two dominant climate states: the greenhouse Earth and the icehouse Earth.
  These two climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur only during an icehouse period and tend to last less than 1 million years.
  There are five known great glaciations in Earth's climate history; the main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide, changes in the Earth's orbit, and oceanic and orogenic changes due to tectonic plate dynamics.
  Greenhouse and icehouse periods have profoundly shaped the evolution of life on Earth. ...
  
  ... A "greenhouse Earth" or "hothouse Earth" is a period in which there are no continental glaciers whatsoever on the planet, the levels of carbon dioxide and other greenhouse gases (such as water vapor and methane) are high, and sea surface temperatures (SSTs) range from 28 °C (82.4 °F) in the tropics to 0 °C (32 °F) in the polar regions. ...
  
  ... Earth is more commonly placed in a greenhouse state throughout the epochs, and the Earth has been in this state for approximately 80% of the past 500 million years, which makes understanding the direct causes somewhat difficult.
  
  ... An "icehouse Earth" is the earth as it experiences an ice age.
  Unlike a greenhouse Earth, an icehouse Earth has ice sheets present, and these sheets wax and wane throughout times known as glacial periods and interglacial periods.
  During an icehouse Earth, greenhouse gases tend to be less abundant, and temperatures tend to be cooler globally.
  The Earth is currently in an icehouse stage, as ice sheets are present on both poles and glacial periods have occurred at regular intervals over the past million years. ...
  
  For more information, see Greenhouse and icehouse Earth .
• List of periods and events in climate history From Wikipedia:
  The list of periods and events in climate history includes some notable climate events known to paleoclimatology.
  Knowledge of precise climatic events decreases as the record goes further back in time.
  The timeline of glaciation covers ice ages specifically, which tend to have their own names for phases, often with different names used for different parts of the world.
  The names for earlier periods and events come from geology and paleontology.
  The marine isotope stages (MIS) are often used to express dating within the Quaternary.
  
  For more information, see List of periods and events in climate history .
• Milankovitch cycles From Wikipedia:
  Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years.
  The term is named for Serbian geophysicist and astronomer Milutin Milankovic.
  In the 1920s, he hypothesized that variations in eccentricity, axial tilt, and precession of the Earth's orbit resulted in cyclical variation in the solar radiation reaching the Earth, and that this orbital forcing strongly influenced climatic patterns on Earth.
  
  For more information, see Milankovitch cycles .
• Plate tectonics and climate change From Wikipedia:
  Over the course of millions of years, the motion of tectonic plates reconfigures global land and ocean areas and generates topography.
  This can affect both global and local patterns of climate and atmosphere-ocean circulation.
  
  The position of the continents determines the geometry of the oceans and therefore influences patterns of ocean circulation.
  The locations of the seas are important in controlling the transfer of heat and moisture across the globe, and therefore, in determining global climate.
  A recent example of tectonic control on ocean circulation is the formation of the Isthmus of Panama about 5 million years ago, which shut off direct mixing between the Atlantic and Pacific Oceans.
  This strongly affected the ocean dynamics of what is now the Gulf Stream and may have led to Northern Hemisphere ice cover.
  During the Carboniferous period, about 300 to 360 million years ago, plate tectonics may have triggered large-scale storage of carbon and increased glaciation.
  Geologic evidence points to a "megamonsoonal" circulation pattern during the time of the supercontinent Pangaea, and climate modeling suggests that the existence of the supercontinent was conducive to the establishment of monsoons.
  
  The size of continents is also important.
  Because of the stabilizing effect of the oceans on temperature, yearly temperature variations are generally lower in coastal areas than they are inland.
  A larger supercontinent will therefore have more area in which climate is strongly seasonal than will several smaller continents or islands.
  
  For more information, see Plate tectonics and climate change .
• Radiative forcing From Wikipedia:
  Radiative forcing or climate forcing is the difference between insolation (sunlight) absorbed by the Earth and energy radiated back to space.
  The influences that cause changes to the Earth’s climate system altering Earth’s radiative equilibrium, forcing temperatures to rise or fall, are called climate forcings.
  Positive radiative forcing means Earth receives more incoming energy from sunlight than it radiates to space.
  This net gain of energy will cause warming. Conversely, negative radiative forcing means that Earth loses more energy to space than it receives from the sun, which produces cooling.
  Typically, radiative forcing is quantified at the tropopause or at the top of the atmosphere (often accounting for rapid adjustments in temperature) in units of watts per square meter of the Earth's surface.
  Positive forcing (incoming energy exceeding outgoing energy) warms the system, while negative forcing (outgoing energy exceeding incoming energy) cools it.
  Causes of radiative forcing include changes in insolation and the concentrations of radiatively active gases, commonly known as greenhouse gases, and aerosols.
  
  For more information, see Radiative forcing .
• Snowball Earth From Wikipedia:
  The Snowball Earth hypothesis proposes that Earth surface's became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago).
  Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record.
  Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean and emphasize the difficulty of escaping an all-frozen condition.
  A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water.
  
  For more information, see Snowball Earth .
• Solar cycle From Wikipedia:
  The solar cycle or solar magnetic activity cycle is the nearly periodic 11-year change in the Sun's activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (changes in the number and size of sunspots, flares, and other manifestations).
  They have been observed (by changes in the sun's appearance and by changes seen on Earth, such as auroras) for centuries.
  The changes on the Sun cause effects in space, in the atmosphere, and on Earth's surface.
  While it is the dominant variable in solar activity, aperiodic fluctuations also occur.
  
  For more information, see Solar cycle .
• Volcanism and climate change From Wikipedia:
  The eruptions considered to be large enough to affect the Earth's climate on a scale of more than 1 year are the ones that inject over 100,000 tons of SO₂ into the stratosphere.
  This is due to the optical properties of SO₂ and sulfate aerosols, which strongly absorb or scatter solar radiation, creating a global layer of sulfuric acid haze.
  On average, such eruptions occur several times per century, and cause cooling (by partially blocking the transmission of solar radiation to the Earth's surface) for a period of a few years.
  The eruption of Mount Pinatubo in 1991, the second largest terrestrial eruption of the 20th century, affected the climate substantially, subsequently global temperatures decreased by about 0.5 °C (0.9 °F) for up to three years.
  Thus, the cooling over large parts of the Earth reduced surface temperatures in 1991–93, the equivalent to a reduction in net radiation of 4 watts per square meter.
  The Mount Tambora eruption in 1815 caused the Year Without a Summer.
  Much larger eruptions, known as large igneous provinces, occur only a few times every fifty – one hundred million years – through flood basalt, and caused in Earth past global warming and mass extinctions.
  
  For more information, see Volcanism and climate change .

For more information, see Paleoclimatology,
Climate,
Glossary of meteorology. close

Transition to Coverage with Plant and Animal Lifeforms

The aim in this present module is to outline the timeline of major events in the transition from the time Earth was completely devoid of life (from its formation around 4.6 billion years ago up to when life first appeared, sometime before 3.7 billion years ago) and the present time, when it is home to a great diversity of lifeforms, and much of its surface has been transformed by the constructions of an intelligent species.
The focus here is on the changing appearance of the Earth, and not the nature of lifeforms nor the details of the processes whereby lifeforms evolved. Neither is the focus on the nature of humans, nor the history of the human world, nor the composition of the present human world.
All of these are considered in other areas.
Some of these topics are discribed in more detail elsewhere, for example in the history of life on Earth, and the origin and development of the human world.

• Earth before life The Earth would have been quite hostile to life in te first several hundred million years of its existence.
  From Wikipedia:
  The Hadean is a geologic eon of the Earth predating the Archean.
  It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years ago. ...
  
  ... "Hadean" (from Hades, the Greek god of the underworld) describes the hellish conditions then prevailing on Earth: the planet had just formed and was still very hot owing to its recent accretion, the abundance of short-lived radioactive elements, and frequent collisions with other Solar System bodies.
  
  For more information, see Hadean .
• Early life (in the Archean) From Wikipedia:
  The Archean Eon (also spelled Archaean or Archæan) is one of the four geologic eons of Earth history, occurring 4,000 to 2,500 million years ago (4 to 2.5 billion years ago).
  During the Archean, the Earth's crust had cooled enough to allow the formation of continents and life started to form. ...
  
  ... The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon or early in the Archean Eon.
  
  The earliest evidence for life on Earth are graphite of biogenic origin found in 3.7-billion-year-old metasedimentary rocks discovered in Western Greenland.
  
  The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria.
  The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia.
  Stromatolites are found throughout the Archean and become common late in the Archean.
  Cyanobacteria were instrumental in creating free oxygen in the atmosphere.
  
  Further evidence for early life is found in 3.47-billon-year-old baryte, in the Warrawoona Group of Western Australia.
  This mineral shows sulfur fractionation of as much as 21.1%, which is evidence of sulfate-reducing bacteria that metabolize sulfur-32 more readily than sulfur-34.
  
  For more information, see Early life (in the Archean) .
• The earliest microbial mats From Wikipedia:
  A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea.
  Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in deserts. ...
  
  ... Microbial mats are among the oldest clear signs of life, as microbially induced sedimentary structures (MISS) formed 3,480 million years ago have been found in western Australia.
  At that early stage the mats' structure may already have been similar to that of modern mats that do not include photosynthesizing bacteria.
  It is even possible that non-photosynthesizing mats were present as early as 4,000 million years ago.
  If so, their energy source would have been hydrothermal vents (high-pressure hot springs around submerged volcanoes), and the evolutionary split between bacteria and archea may also have occurred around this time.
  
  The earliest mats were probably small, single-species biofilms of chemotrophs that relied on hydrothermal vents to supply both energy and chemical "food".
  Within a short time (by geological standards) the build-up of dead microorganisms would have created an ecological niche for scavenging heterotrophs, possibly methane-emitting and sulfate-reducing organisms that would have formed new layers in the mats and enriched their supply of biologically useful chemicals.
  
  For more information, see The earliest microbial mats .
• Colonization of land by plants From Wikipedia:
  Land plants evolved from a group of green algae, perhaps as early as 850 mya, but algae-like plants might have evolved as early as 1 billion years ago.
  The closest living relatives of land plants are the charophytes, specifically Charales; assuming that the Charales' habit has changed little since the divergence of lineages, this means that the land plants evolved from a branched, filamentous alga dwelling in shallow fresh water, perhaps at the edge of seasonally desiccating pools.
  However, some recent evidence suggests that land plants might have originated from unicellular terrestrial charophytes similar to extant Klebsormidiophyceae. ...
  
  ... Plants were not the first photosynthesisers on land. Weathering rates suggest that organisms capable of photosynthesis were already living on the land 1,200 million years ago, and microbial fossils have been found in freshwater lake deposits from 1,000 million years ago, but the carbon isotope record suggests that they were too scarce to impact the atmospheric composition until around 850 million years ago.
  These organisms, although phylogenetically diverse,[15] were probably small and simple, forming little more than an algal scum.
  
  However, evidence of the earliest land plants occurs much later at about 470Ma, in lower middle Ordovician rocks from Saudi Arabia and Gondwana in the form of spores with decay-resistant walls.
  
  For more information, see
  Colonization of land by plants .
• Diversification of plant and animal lifeforms in the "Cambrian explosion" From Wikipedia:
  The Cambrian explosion or Cambrian radiation was an event approximately 541 million years ago in the Cambrian period when most major animal phyla appeared in the fossil record.
  It lasted for about 20 –25 million years.
  It resulted in the divergence of most modern metazoan phyla.
  The event was accompanied by major diversification of other organisms.
  
  Before the Cambrian explosion, most organisms were simple, composed of individual cells occasionally organized into colonies.
  Over the following 70 to 80 million years, the rate of diversification accelerated, and the variety of life began to resemble that of today.
  Almost all present animal phyla appeared during this period.
  
  The Cambrian explosion has generated extensive scientific debate.
  
  For more information, see
  Cambrian explosion .
• Diversification of animal lifeforms From Wikipedia:
  Animals are monophyletic, meaning they are derived from a common ancestor. ...
  
  ... 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian.
  25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of the Wnt and TGF-beta signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body's system of axes (in three dimensions), and another 7 are for transcription factors including homeodomain proteins involved in the control of development.
  
  The phylogenetic tree (of major lineages only) indicates approximately how many millions of years ago (mya) the lineages split. [Note: Follow the link below for a better view of this tree diagram (with some pictures).
  Click on the name of any the major animal groups shown to arrive at the Wikiedia page for this group.]
  

  
  

  Apoikozoa

  Choanoflagellata    Animalia   Porifera     Eumetazoa   Ctenophora    ParaHoxozoa     Placozoa      Cnidaria      Bilateria   Xenacoelomorpha    Nephrozoa Deuterostomia   Chordata      Echinodermata and allies      Protostomia Ecdysozoa   Arthropoda and allies      Nematoda and allies    >529 mya Spiralia Gnathifera   Rotifera and allies      Chaetognatha      Platytrochozoa   Platyhelminthes and allies    Lophotrochozoa   Mollusca      Annelida and allies    550 mya 580 mya   610 mya 650 mya Triploblasts 680 mya   760 mya

  950 mya

  
  For more information, see
  Phylogeny (of animals) .
• Evolutionary history of plants From Wikipedia:
  The evolution of plants has resulted in a wide range of complexity, from the earliest algal mats, through multicellular marine and freshwater green algae, terrestrial bryophytes, lycopods and ferns, to the complex gymnosperms and angiosperms of today.
  While many of the earliest groups to appear continue to thrive, as exemplified by red and green algae in marine environments, more recently derived groups have displaced previously ecologically dominant ones, e.g. the ascendance of flowering plants over gymnosperms in terrestrial environments.
  
  There is evidence that cyanobacteria and multicellular photosynthetic eukaryotes lived in freshwater communities on land as early as 1 billion years ago, and that communities of complex, mutilcellular photosynthesizing organisms existed on land in the late Precambrian, around 850 million years ago.
  
  Evidence of the emergence of embryophyte land plants first occurs in the mid-Ordovician (~470 million years ago), and by the middle of the Devonian (~390 million years ago), many of the features recognised in land plants today were present, including roots and leaves.
  Late Devonian (~370 million years ago) free-sporing plants such as Archaeopteris had secondary vascular tissue that produced wood and had formed forests of tall trees.
  Also by late Devonian, Elkinsia, an early seed fern, had evolved seeds.
  Evolutionary innovation continued into the Carboniferous and still continues today.
  Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed.
  This may have set the scene for the appearance of the flowering plants in the Triassic (~200 million years ago), and their later diversification in the Cretaceous and Paleogene.
  The latest major group of plants to evolve were the grasses, which became important in the mid-Paleogene, from around 40 million years ago.
  The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO₂ and warm, dry conditions of the tropics over the last 10 million years.
  
  For more information, see
  Evolutionary history of plants ,
  Phylogenetic plant tree, showing the major clades and traditional groups. .
• Transition of animals out of the sea onto land From Wikipedia:
  Fossil evidence has shown that sea creatures, likely related to arthropods, first began to make forays on to land around 530 million years ago.
  There is little reason to believe, however, that animals first began living reliably on land around this same time period.
  A more likely hypothesis is that these early arthropods' motivation for venturing on to dry land was to mate (as modern horseshoe crabs do) or lay eggs out of the reach of predators.
  As time went on, evidence suggests that by approximately 375 million years ago the bony fish best adapted to life in shallow coastal/swampy waters (such as Tiktaalik roseae), were much more viable as amphibians than were their arthropod predecessors.
  Thanks to relatively strong, muscular limbs (which were likely weight-bearing, thus making them a preferable alternative to traditional fins in extremely shallow water), and lungs which existed in conjunction with gills, Tiktaalik and animals like it were able to establish a strong foothold on land by the end of the Devonian period.
  As such, they are likely the most recent common ancestor of all modern tetrapods.
  
  (Vertebrate land invasion) The aquatic to terrestrial transition of vertebrate organisms occurred in the late Devonian era and was an important step in the evolutionary history of modern land vertebrates.
  The transition allowed animals to escape competitive pressure from the water and explore niche opportunities on land.
  Fossils from this period have allowed scientists to identify some of the species that existed during this transition, such as Tiktaalik and Acanthostega.
  Many of these species were also the first to develop adaptations suited to terrestrial over aquatic life, such as neck mobility and hindlimb locomotion.
  
  The late Devonian vertebrate transition was not the only terrestrial invasion in evolutionary history.
  The vertebrate transition was preceded by the plant and invertebrate terrestrial invasion.
  These invasions allowed for the appropriate niche development that would ultimately facilitate the vertebrate invasion.
  Furthermore, the late Devonian event is only significant in that it was the first land invasion by vertebrate organisms, and that it resulted in an explosion of vertebrate biodiversity due to the many different niches that these species were able to exploit.
  However, aquatic species have continued to develop adaptations suited to terrestrial life (and vice versa) from the late Devonian to the Holocene.
  
  For more information, see
  Terrestrialization (of animals) ,
  Vertebrate land invasion .
• Evolution of Humans From Wikipedia:
  Human evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates – in particular genus Homo – and leading to the emergence of Homo sapiens as a distinct species of the hominid family, the great apes.
  This process involved the gradual development of traits such as human bipedalism and language. ...
  
  ... Within the Hominoidea (apes) superfamily, the Hominidae family diverged from the Hylobatidae (gibbon) family some 15–20 million years ago; African great apes (subfamily Homininae) diverged from orangutans (Ponginae) about 14 million years ago; the Hominini tribe (humans, Australopithecines and other extinct biped genera, and chimpanzee) parted from the Gorillini tribe (gorillas) between 8-9 million years ago; and, in turn, the subtribes Hominina (humans and biped ancestors) and Panina (chimps) separated 4-7.5 million years ago. ...
  
  ... Human evolution from its first separation from the last common ancestor of humans and chimpanzees is characterized by a number of morphological, developmental, physiological, and behavioral changes.
  The most significant of these adaptations are bipedalism, increased brain size, lengthened ontogeny (gestation and infancy), and decreased sexual dimorphism. ...
  
  ... Recent evidence suggests that humans may have left Africa half a million years earlier than previously thought.
  A joint Franco-Indian team has found human artifacts in the Siwalk Hills north of New Delhi dating back at least 2.6 million years.
  This is earlier than the previous earliest finding of genus Homo at Dmanisi, in Georgia, dating to 1.85 million years.
  Although controversial, tools found at a Chinese cave strengthen the case that humans used tools as far back as 2.48 million years ago.
  This suggests that the Asian "Chopper" tool tradition, found in Java and northern China may have left Africa before the appearance of the Acheulian hand axe. ...
  
  ... In contrast the "out of Africa" model proposed that modern H. sapiens speciated in Africa recently (that is, approximately 200,000 years ago) and the subsequent migration through Eurasia resulted in nearly complete replacement of other Homo species. ...
  
  ... Sequencing mtDNA and Y-DNA sampled from a wide range of indigenous populations revealed ancestral information relating to both male and female genetic heritage, and strengthened the Out of Africa theory and weakened the views of Multiregional Evolutionism.
  Aligned in genetic tree differences were interpreted as supportive of a recent single origin.
  Analyses have shown a greater diversity of DNA patterns throughout Africa, consistent with the idea that Africa is the ancestral home of mitochondrial Eve and Y-chromosomal Adam, and that modern human dispersal out of Africa has only occurred over the last 55,000 years. ...
  
  "Out of Africa" has thus gained much support from research using female mitochondrial DNA and the male Y chromosome.
  After analysing genealogy trees constructed using 133 types of mtDNA, researchers concluded that all were descended from a female African progenitor, dubbed Mitochondrial Eve.
  "Out of Africa" is also supported by the fact that mitochondrial genetic diversity is highest among African populations. ...
  
  ... There are still differing theories on whether there was a single exodus from Africa or several.
  A multiple dispersal model involves the Southern Dispersal theory, which has gained support in recent years from genetic, linguistic and archaeological evidence.
  In this theory, there was a coastal dispersal of modern humans from the Horn of Africa crossing the Bab el Mandib to Yemen at a lower sea level around 70,000 years ago.
  This group helped to populate Southeast Asia and Oceania, explaining the discovery of early human sites in these areas much earlier than those in the Levant.
  This group seems to have been dependent upon marine resources for their survival.
  
  For more information, see
  Human evolution .
• The spread of civilization The last 10,000 or so years has seen major changes on the surface of the Earth brought about by humans.
  These changes include:
  - the cutting down of forsts and their replacement with farmland
  - the building of towns and cities and other dwellings for people to live in
  - the spread of road- and rail networks over the Earth's surface
  - the construction of ports and the creation of shipping networks
  - the building of airports and the creation of air transport networks
  - the construction of large numbers of vehicles of many types to transport people, goods and materials across the world's surface
  - changes in the composition of the atmosphere as a result of industry and transportation
  - the release of large quanties of man-made materials into the oceans
  - the construction of large numbers of buildings for work and industry, leisure, health, government and administration, etc., etc.
  
  Note: Here the interest is in only the physical changes caused to the surface of the Earth by the spread of civilization, and not the many other aspects of civilization (some of which are considered elsewhere).
  
  From Wikipedia:
  ... The term "civilization" as it is now most commonly understood, a complex state with centralisation, social stratification and specialization of labour, corresponds to early empires that arise in the Fertile Crescent in the Early Bronze Age, around roughly 3000 BC.
  Gordon Childe defined the emergence of civilization as the result of two successive revolutions: the Neolithic Revolution, triggering the development of settled communities, and the Urban Revolution. ...
  
  ... At first, the Neolithic was associated with shifting subsistence cultivation, where continuous farming led to the depletion of soil fertility resulting in the requirement to cultivate fields further and further removed from the settlement, eventually compelling the settlement itself to move.
  In major semi-arid river valleys, annual flooding renewed soil fertility every year, with the result that population densities could rise significantly.
  This encouraged a secondary products revolution in which people used domesticated animals not just for meat, but also for milk, wool, manure and pulling ploughs and carts—a development that spread through the Eurasian Oecumene.
  
  The earlier neolithic technology and lifestyle was established first in Western Asia (for example at Göbekli Tepe, from about 9,130 BCE), and later in the Yellow River and Yangtze basins in China (for example the Pengtoushan culture from 7,500 BCE), and later spread. Mesopotamia is the site of the earliest developments of the Neolithic Revolution from around 10,000 BCE, with civilizations developing from 6,500 years ago.
  This area has been identified as having "inspired some of the most important developments in human history including the invention of the wheel, the development of cuneiform script, mathematics, astronomy and agriculture."
  Similar pre-civilized "neolithic revolutions" also began independently from 7,000 BCE in northwestern South America (the Norte Chico civilization)[38] and Mesoamerica. ....
  
  ... This climate change shifted the cost-benefit ratio of endemic violence between communities, which saw the abandonment of unwalled village communities and the appearance of walled cities, associated with the first civilizations.
  
  This "urban revolution" marked the beginning of the accumulation of transferrable surpluses, which helped economies and cities develop.
  It was associated with the state monopoly of violence, the appearance of a soldier class and endemic warfare, the rapid development of hierarchies, and the appearance of human sacrifice. ...
  
  ... The Bronze Age collapse was followed by the Iron Age around 1200 BCE, during which a number of new civilizations emerged, culminating in a period from the 8th to the 3rd century BCE which Karl Jaspers termed the Axial Age, presented as a critical transitional phase leading to classical civilization.
  William Hardy McNeill proposed that this period of history was one in which culture contact between previously separate civilizations saw the "closure of the oecumene" and led to accelerated social change from China to the Mediterranean, associated with the spread of coinage, larger empires and new religions. ...
  ... A major technological and cultural transition to modernity began approximately 1500 CE in Western Europe, and from this beginning new approaches to science and law spread rapidly around the world, incorporating earlier cultures into the industrial and technological civilization of the present.
  
  For more information, see
  History of the world .

For more information, see Evolutionary history of life. close

Future History of the Earth and Solar System.

From Wikipedia:
The biological and geological future of Earth can be extrapolated based upon the estimated effects of several long-term influences.
These include the chemistry at Earth's surface, the rate of cooling of the planet's interior, the gravitational interactions with other objects in the Solar System, and a steady increase in the Sun's luminosity.
An uncertain factor in this extrapolation is the ongoing influence of technology introduced by humans, such as climate engineering, which could cause significant changes to the planet.
The current Holocene extinction is being caused by technology and the effects may last for up to five million years.
In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.

• Human influence (on the future of the Earth) From Wikipedia:
  Humans play a key role in the biosphere, with the large human population dominating many of Earth's ecosystems.
  This has resulted in a widespread, ongoing mass extinction of other species during the present geological epoch, now known as the Holocene extinction.
  The large-scale loss of species caused by human influence since the 1950s has been called a biotic crisis, with an estimated 10% of the total species lost as of 2007.
  At current rates, about 30% of species are at risk of extinction in the next hundred years.
  The Holocene extinction event is the result of habitat destruction, the widespread distribution of invasive species, hunting, and climate change.
  In the present day, human activity has had a significant impact on the surface of the planet.
  More than a third of the land surface has been modified by human actions, and humans use about 20% of global primary production.
  The concentration of carbon dioxide in the atmosphere has increased by close to 30% since the start of the Industrial Revolution. ...
  
  ... There are multiple scenarios for known risks that can have a global impact on the planet.
  From the perspective of humanity, these can be subdivided into survivable risks and terminal risks.
  Risks that humanity pose to itself include climate change, the misuse of nanotechnology, a nuclear holocaust, warfare with a programmed superintelligence, a genetically engineered disease, or a disaster caused by a physics experiment.
  Similarly, several natural events may pose a doomsday threat, including a highly virulent disease, the impact of an asteroid or comet, runaway greenhouse effect, and resource depletion.
  There may also be the possibility of an infestation by an extraterrestrial lifeform.
  The actual odds of these scenarios are difficult if not impossible to deduce.
  
  For more information, see Human influence (on the future of the Earth) .
• Random events From Wikipedia:
  As the Sun orbits the Milky Way, wandering stars may approach close enough to have a disruptive influence on the Solar System.
  A close stellar encounter may cause a significant reduction in the perihelion distances of comets in the Oort cloud—a spherical region of icy bodies orbiting within half a light year of the Sun.
  Such an encounter can trigger a 40-fold increase in the number of comets reaching the inner Solar System.
  Impacts from these comets can trigger a mass extinction of life on Earth.
  These disruptive encounters occur at an average of once every 45 million years. ...
  
  ... The energy release from the impact of an asteroid or comet with a diameter of 5–10 km (3.1–6.2 mi) or larger is sufficient to create a global environmental disaster and cause a statistically significant increase in the number of species extinctions.
  Among the deleterious effects resulting from a major impact event is a cloud of fine dust ejecta blanketing the planet, which lowers land temperatures by about 15 °C (27 °F) within a week and halts photosynthesis for several months.
  The mean time between major impacts is estimated to be at least 100 million years.
  During the last 540 million years, simulations demonstrated that such an impact rate is sufficient to cause 5–6 mass extinctions and 20–30 lower severity events.
  This matches the geologic record of significant extinctions during the Phanerozoic Eon.
  Such events can be expected to continue into the future. ...
  
  ... A supernova is a cataclysmic explosion of a star.
  Within the Milky Way galaxy, supernova explosions occur on average once every 40 years.
  During the history of the Earth, multiple such events have likely occurred within a distance of 100 light years.
  Explosions inside this distance can contaminate the planet with radioisotopes and possibly impact the biosphere. ...
  ... A supernova explosion at a distance of 26 light years will reduce the ozone column density by half.
  On average, a supernova explosion occurs within 32 light years once every few hundred million years, resulting in a depletion of the ozone layer lasting several centuries.
  Over the next two billion years, there will be about 20 supernova explosions and one gamma ray burst that will have a significant impact on the planet's biosphere. ...
  
  ... Computer simulations of the Solar System's evolution over the next five billion years suggest that there is a small (less than 1%) chance that a collision could occur between Earth and either Mercury, Venus, or Mars.
  During the same interval, the odds that the Earth will be scattered out of the Solar System by a passing star are on the order of one part in 105.
  In such a scenario, the oceans would freeze solid within several million years, leaving only a few pockets of liquid water about 14 km (8.7 mi) underground.
  There is a remote chance that the Earth will instead be captured by a passing binary star system, allowing the planet's biosphere to remain intact.
  The odds of this happening are about one chance in three million.
  
  For more information, see Random events .
• Glaciation From Wikipedia:
  The Earth is passing through an ice age known as the quaternary glaciation, and is presently in the Holocene interglacial period.
  This period would normally be expected to end in about 25,000 years.
  However, the increased rate of carbon dioxide release into the atmosphere by humans may delay the onset of the next glacial period until at least 50,000–130,000 years from now.
  On the other hand, a global warming period of finite duration (based on the assumption that fossil fuel use will cease by the year 2200) will probably only impact the glacial period for about 5,000 years.
  Thus, a brief period of global warming induced through a few centuries worth of greenhouse gas emission would only have a limited impact in the long term.
  
  For more information, see Glaciation .
• Obliquity From Wikipedia:
  Based on computer models, the presence of the Moon appears to stabilize the obliquity of the Earth, which may help the planet to avoid dramatic climate changes.
  This stability is achieved because the Moon increases the precession rate of the Earth's spin axis, thereby avoiding resonances between the precession of the spin and precession of the planet's orbital plane (that is, the precession motion of the ecliptic).
  However, as the semimajor axis of the Moon's orbit continues to increase, this stabilizing effect will diminish.
  At some point, perturbation effects will probably cause chaotic variations in the obliquity of the Earth, and the axial tilt may change by angles as high as 90° from the plane of the orbit.
  This is expected to occur between 1.5 and 4.5 billion years from now.
  
  A high obliquity would probably result in dramatic changes in the climate and may destroy the planet's habitability.
  When the axial tilt of the Earth exceeds 54°, the yearly insolation at the equator is less than that at the poles.
  The planet could remain at an obliquity of 60° to 90° for periods as long as 10 million years.
  
  For more information, see Obliquity .
• Geodynamics From Wikipedia:
  Tectonics-based events will continue to occur well into the future and the surface will be steadily reshaped by tectonic uplift, extrusions, and erosion.
  Mount Vesuvius can be expected to erupt about 40 times over the next 1,000 years.
  During the same period, about five to seven earthquakes of magnitude 8 or greater should occur along the San Andreas Fault, while about 50 magnitude 9 events may be expected worldwide.
  Mauna Loa should experience about 200 eruptions over the next 1,000 years, and the Old Faithful Geyser will likely cease to operate.
  The Niagara Falls will continue to retreat upstream, reaching Buffalo in about 30,000–50,000 years.
  
  In 10,000 years, the post-glacial rebound of the Baltic Sea will have reduced the depth by about 90 m (300 ft).
  The Hudson Bay will decrease in depth by 100 m over the same period.
  After 100,000 years, the island of Hawaii will have shifted about 9 km (5.6 mi) to the northwest.
  The planet may be entering another glacial period by this time.
  
  For more information, see Geodynamics .
• Continental drift From Wikipedia:
  Christopher Scotese and his colleagues have mapped out the predicted motions several hundred million years into the future as part of the Paleomap Project.
  In their scenario, 50 million years from now the Mediterranean sea may vanish and the collision between Europe and Africa will create a long mountain range extending to the current location of the Persian Gulf.
  Australia will merge with Indonesia, and Baja California will slide northward along the coast.
  New subduction zones may appear off the eastern coast of North and South America, and mountain chains will form along those coastlines.
  To the south, the migration of Antarctica to the north will cause all of its ice sheets to melt.
  This, along with the melting of the Greenland ice sheets, will raise the average ocean level by 90 m (300 ft).
  The inland flooding of the continents will result in climate changes.
  
  As this scenario continues, by 100 million years from the present the continental spreading will have reached its maximum extent and the continents will then begin to coalesce.
  In 250 million years, North America will collide with Africa while South America will wrap around the southern tip of Africa.
  The result will be the formation of a new supercontinent (sometimes called Pangaea Ultima), with the Pacific Ocean stretching across half the planet.
  The continent of Antarctica will reverse direction and return to the South Pole, building up a new ice cap. ...
  
  ... In an extroversion model, the closure of the Pacific Ocean would be complete in about 350 million years.
  This marks the completion of the current supercontinent cycle, wherein the continents split apart and then rejoin each other about every 400–500 million years.
  Once the supercontinent is built, plate tectonics may enter a period of inactivity as the rate of subduction drops by an order of magnitude.
  This period of stability could cause an increase in the mantle temperature at the rate of 30–100 °C (54–180 °F) every 100 million years, which is the minimum lifetime of past supercontinents. As a consequence, volcanic activity may increase.
  
  For more information, see Continental drift .
• Solar evolution From Wikipedia:
  When the Sun first became a main sequence star, it radiated only 70% of the current luminosity.
  The luminosity has increased in a nearly linear fashion to the present, rising by 1% every 110 million years.
  Likewise, in three billion years the Sun is expected to be 33% more luminous.
  The hydrogen fuel at the core will finally be exhausted in five billion years, when the Sun will be 67% more luminous than at present.
  Thereafter the Sun will continue to burn hydrogen in a shell surrounding its core, until the luminosity reaches 121% above the present value.
  This marks the end of the Sun's main sequence lifetime, and thereafter it will pass through the subgiant stage and evolve into a red giant.
  
  By this time, the collision of the Milky Way and Andromeda galaxies should be underway.
  Although this could result in the Solar System being ejected from the newly combined galaxy, it is considered unlikely to have any adverse effect on the Sun or its planets.
  
  For more information, see Solar evolution .
• Climate impact From Wikipedia:
  ... Within the next 600 million years from the present, the concentration of CO₂ will fall below the critical threshold needed to sustain C₃ photosynthesis: about 50 parts per million.
  At this point, trees and forests in their current forms will no longer be able to survive, the last living trees being evergreen conifers.
  However, C₄ carbon fixation can continue at much lower concentrations, down to above 10 parts per million.
  Thus plants using C₄ photosynthesis may be able to survive for at least 0.8 billion years and possibly as long as 1.2 billion years from now, after which rising temperatures will make the biosphere unsustainable. ...
  
  ... As temperatures continue to rise, the last animal life will be driven back toward the poles, and possibly underground.
  They would become primarily active during the polar night, aestivating during the polar day due to the intense heat.
  Much of the surface would become a barren desert and life would primarily be found in the oceans.
  However, due to a decrease of the amount or organic matter coming to the oceans from the land as well as oxygen in the water, life would disappear there too following a similar path to that on Earth's surface.
  This process would start with the loss of freshwater species and conclude with invertebrates, particularly those that do not depend on living plants such as termites or those near hydrothermal vents such as worms of the genus Riftia.
  As a result of these processes, multi-cellular lifeforms may be extinct in about 800 million years, and eukaryotes in 1.3 billion years, leaving only the prokaryotes.
  
  For more information, see Climate impact .
• Loss of oceans From Wikipedia:
  One billion years from now, about 27% of the modern ocean will have been subducted into the mantle.
  If this process were allowed to continue uninterrupted, it would reach an equilibrium state where 65% of the current surface reservoir would remain at the surface.
  Once the solar luminosity is 10% higher than its current value, the average global surface temperature will rise to 320 K (47 °C; 116 °F).
  The atmosphere will become a "moist greenhouse" leading to a runaway evaporation of the oceans. ...
  
  ... The net result would be a loss of the world's sea water by about 1.1 billion years from the present.
  This will be a simple dramatic step in annihilating all life on Earth. ...
  
  .... By 2.8 billion years from now, the surface temperature of the Earth will have reached 422 K (149 °C; 300 °F), even at the poles.
  At this point, any remaining life will be extinguished due to the extreme conditions.
  
  For more information, see Loss of oceans .
• Red giant stage From Wikipedia:
  Once the Sun changes from burning hydrogen at its core to burning hydrogen around its shell, the core will start to contract and the outer envelope will expand.
  The total luminosity will steadily increase over the following billion years until it reaches 2,730 times the Sun's current luminosity at the age of 12.167 billion years.
  Most of Earth's atmosphere will be lost to space and its surface will consist of a lava ocean with floating continents of metals and metal oxides as well as icebergs of refractory materials, with its surface temperature reaching more than 2,400 K (2,130 °C; 3,860 °F).
  The Sun will experience more rapid mass loss, with about 33% of its total mass shed with the solar wind.
  The loss of mass will mean that the orbits of the planets will expand.
  The orbital distance of the Earth will increase to at most 150% of its current value.
  
  The most rapid part of the Sun's expansion into a red giant will occur during the final stages, when the Sun will be about 12 billion years old.
  It is likely to expand to swallow both Mercury and Venus, reaching a maximum radius of 1.2 AU (180,000,000 km).
  The Earth will interact tidally with the Sun's outer atmosphere, which would serve to decrease Earth's orbital radius.
  Drag from the chromosphere of the Sun would also reduce the Earth's orbit.
  These effects will act to counterbalance the effect of mass loss by the Sun, and the Earth will probably be engulfed by the Sun.
  
  For more information, see Red giant stage .
• Post red-giant stage From Wikipedia:
  After fusing helium in its core to carbon, the Sun will begin to collapse again, evolving into a compact white dwarf star after ejecting its outer atmosphere as a planetary nebula.
  In 50 billion years, if the Earth and Moon are not engulfed by the Sun, they will become tidelocked, with each showing only one face to the other.
  Thereafter, the tidal action of the Sun will extract angular momentum from the system, causing the lunar orbit to decay and the Earth's spin to accelerate.
  
  In about 65 billion years, it is estimated that the Moon may end up colliding with the Earth, assuming they are not destroyed by the red giant Sun, due to the remaining energy of the Earth–Moon system being sapped by the remnant Sun, causing the Moon to slowly move inwards toward the Earth.
  Over time intervals of around 30 trillion years, the Sun will undergo a close encounter with another star.
  As a consequence, the orbits of their planets can become disrupted, potentially ejecting them from the system entirely.
  If Earth is not destroyed by the expanding red giant Sun in 7.6 billion years and not ejected from its orbit by a stellar encounter, its ultimate fate will be that it collides with the black dwarf Sun due to the decay of its orbit via gravitational radiation, in 10²⁰ (100 quintillion) years.
  
  For more information, see Post red-giant stage .

For more information, see
Future of Earth ,
Timeline of the far future . close

For more information, see
History of Earth,
Geological history of Earth.