The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the

planet Earth and is retained by Earth's gravity. The atmosphere of Earth protects life on Earth by

creating pressure allowing for liquid water to exist on the Earth's surface, absorbing ultraviolet solar
radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature
extremes between day and night (the diurnal temperature variation).
By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide,
and small amounts of other gases.[8] Air also contains a variable amount of water vapor, on average
around 1% at sea level, and 0.4% over the entire atmosphere. Air composition, temperature,
and atmospheric pressure vary with altitude, and air suitable for use in photosynthesis by terrestrial
plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial
atmospheres.
The atmosphere has a mass of about 5.15×1018 kg,[9] three quarters of which is within about 11 km
(6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing
altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at
100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and
outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an
altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on
characteristics such as temperature and composition.
The study of Earth's atmosphere and its processes is called atmospheric science (aerology). Early
pioneers in the field include Léon Teisserenc de Bort and Richard Assmann.[10]

Contents

 1Composition

 2Stratification

o 2.1Exosphere

o 2.2Thermosphere

o 2.3Mesosphere

o 2.4Stratosphere

o 2.5Troposphere

o 2.6Other layers

 3Physical properties

o 3.1Pressure and thickness

o 3.2Temperature and speed of sound

o 3.3Density and mass

 4Optical properties

o 4.1Scattering

o 4.2Absorption

o 4.3Emission

o 4.4Refractive index

 5Circulation

 6Evolution of Earth's atmosphere

o 6.1Earliest atmosphere

o 6.2Second atmosphere

o 6.3Third atmosphere

o 6.4Air pollution

 7Images from space

 8See also

 9References

 10External links

Composition
Main article: Atmospheric chemistry

Mean atmospheric water vapor

The three major constituents of Earth's atmosphere are nitrogen, oxygen, and argon. Water vapor
accounts for roughly 0.25% of the atmosphere by mass. The concentration of water vapor (a
greenhouse gas) varies significantly from around 10 ppm by volume in the coldest portions of the
atmosphere to as much as 5% by volume in hot, humid air masses, and concentrations of other
atmospheric gases are typically quoted in terms of dry air (without water vapor).[11] The remaining
gases are often referred to as trace gases,[12] among which are the greenhouse gases, principally
carbon dioxide, methane, nitrous oxide, and ozone. Besides argon, already mentioned, other noble
gases, neon, helium, krypton, and xenon are also present. Filtered air includes trace amounts of
many other chemical compounds. Many substances of natural origin may be present in locally and
seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral
and organic composition, pollen and spores, sea spray, and volcanic ash. Various
industrial pollutants also may be present as gases or aerosols, such as chlorine (elemental or in
compounds), fluorine compounds and elemental mercury vapor. Sulfur compounds such
as hydrogen sulfide and sulfur dioxide (SO2) may be derived from natural sources or from industrial
air pollution.

Major constituents of dry air, by volume[8]

Gas Volume(A)

Name Formula in ppmv(B) in %

Nitrogen N2 780,840 78.084

Oxygen O2 209,460 20.946

Argon Ar 9,340 0.9340

Carbon dioxide (April,
2019) CO 413.32 0.041332
2

Neon Ne 18.18 0.001818

Helium He 5.24 0.000524

Methane CH4 1.87 0.000187

Krypton Kr 1.14 0.000114

Not included in above dry atmosphere:

Water vapor(C) H2 O 0–30,000(D) 0–3%(D)

 notes:
(A)
 volume fraction is equal to mole fraction for ideal gas only,
    also see volume (thermodynamics)
(B)
 ppmv: parts per million by volume
(C)
 Water vapor is about 0.25% by mass over full atmosphere
(D)
 Water vapor strongly varies locally[11]

The average molecular weight of dry air, which can be used to calculate densities or to convert
between mole fraction and mass fraction, is about 28.946[13] or 28.96[14] g/mol. This is decreased
when the air is humid.
The relative concentration of gases remains constant until about 10,000 m (33,000 ft).[15]

The volume fraction of the main constituents of the Earth's atmosphere as a function of height according to the
MSIS-E-90 atmospheric model.

Stratification
Earth's atmosphere Lower 4 layers of the atmosphere in 3 dimensions as seen diagonally from above the
exobase. Layers drawn to scale, objects within the layers are not to scale. Aurorae shown here at the bottom of
the thermosphere can actually form at any altitude in this atmospheric layer.

In general, air pressure and density decrease with altitude in the atmosphere. However, temperature
has a more complicated profile with altitude, and may remain relatively constant or even increase
with altitude in some regions (see the temperature section, below). Because the general pattern of
the temperature/altitude profile, or lapse rate, is constant and measurable by means of
instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish
atmospheric layers. In this way, Earth's atmosphere can be divided (called atmospheric stratification)
into five main layers. Excluding the exosphere, the atmosphere has four primary layers, which are
the troposphere, stratosphere, mesosphere, and thermosphere.[16] From highest to lowest, the five
main layers are:

 Exosphere: 700 to 10,000 km (440 to 6,200 miles)

 Thermosphere: 80 to 700 km (50 to 440 miles)[17]
 Mesosphere: 50 to 80 km (31 to 50 miles)
 Stratosphere: 12 to 50 km (7 to 31 miles)
 Troposphere: 0 to 12 km (0 to 7 miles)[18]
Exosphere
Main article: Exosphere

The exosphere is the outermost layer of Earth's atmosphere (i.e. the upper limit of the atmosphere).
It extends from the exobase, which is located at the top of the thermosphere at an altitude of about
700 km above sea level, to about 10,000 km (6,200 mi; 33,000,000 ft) where it merges into the solar
wind.
This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier
molecules including nitrogen, oxygen and carbon dioxide closer to the exobase. The atoms and
molecules are so far apart that they can travel hundreds of kilometers without colliding with one
another. Thus, the exosphere no longer behaves like a gas, and the particles constantly escape into
space. These free-moving particles follow ballistic trajectories and may migrate in and out of
the magnetosphere or the solar wind.
The exosphere is located too far above Earth for any meteorological phenomena to be possible.
However, the aurora borealis and aurora australis sometimes occur in the lower part of the
exosphere, where they overlap into the thermosphere. The exosphere contains most of the satellites
orbiting Earth.

Thermosphere
Main article: Thermosphere

The thermosphere is the second-highest layer of Earth's atmosphere. It extends from the
mesopause (which separates it from the mesosphere) at an altitude of about 80 km (50 mi;
260,000 ft) up to the thermopause at an altitude range of 500–1000 km (310–620 mi; 1,600,000–
3,300,000 ft). The height of the thermopause varies considerably due to changes in solar activity.
[17]
 Because the thermopause lies at the lower boundary of the exosphere, it is also referred to as
the exobase. The lower part of the thermosphere, from 80 to 550 kilometres (50 to 342 mi) above
Earth's surface, contains the ionosphere.
The temperature of the thermosphere gradually increases with height. Unlike the stratosphere
beneath it, wherein a temperature inversion is due to the absorption of radiation by ozone, the
inversion in the thermosphere occurs due to the extremely low density of its molecules. The
temperature of this layer can rise as high as 1500 °C (2700 °F), though the gas molecules are so far
apart that its temperature in the usual sense is not very meaningful. The air is so rarefied that an
individual molecule (of oxygen, for example) travels an average of 1 kilometre (0.62 mi; 3300 ft)
between collisions with other molecules.[19] Although the thermosphere has a high proportion of
molecules with high energy, it would not feel hot to a human in direct contact, because its density is
too low to conduct a significant amount of energy to or from the skin.
This layer is completely cloudless and free of water vapor. However, non-hydrometeorological
phenomena such as the aurora borealis and aurora australis are occasionally seen in the
thermosphere. The International Space Station orbits in this layer, between 350 and 420 km (220
and 260 mi).

Mesosphere
Main article: Mesosphere

The mesosphere is the third highest layer of Earth's atmosphere, occupying the region above the
stratosphere and below the thermosphere. It extends from the stratopause at an altitude of about
50 km (31 mi; 160,000 ft) to the mesopause at 80–85 km (50–53 mi; 260,000–280,000 ft) above sea
level.
Temperatures drop with increasing altitude to the mesopause that marks the top of this middle layer
of the atmosphere. It is the coldest place on Earth and has an average temperature around
−85 °C (−120 °F; 190 K).[20][21]
Just below the mesopause, the air is so cold that even the very scarce water vapor at this altitude
can be sublimated into polar-mesospheric noctilucent clouds. These are the highest clouds in the
atmosphere and may be visible to the naked eye if sunlight reflects off them about an hour or two
after sunset or a similar length of time before sunrise. They are most readily visible when the Sun is
around 4 to 16 degrees below the horizon. Lightning-induced discharges known as transient
luminous events (TLEs) occasionally form in the mesosphere above tropospheric thunderclouds.
The mesosphere is also the layer where most meteors burn up upon atmospheric entrance. It is too
high above Earth to be accessible to jet-powered aircraft and balloons, and too low to permit orbital
spacecraft. The mesosphere is mainly accessed by sounding rockets and rocket-powered aircraft.

Stratosphere
Main article: Stratosphere

The stratosphere is the second-lowest layer of Earth's atmosphere. It lies above the troposphere and
is separated from it by the tropopause. This layer extends from the top of the troposphere at roughly
12 km (7.5 mi; 39,000 ft) above Earth's surface to the stratopause at an altitude of about 50 to 55 km
(31 to 34 mi; 164,000 to 180,000 ft).
The atmospheric pressure at the top of the stratosphere is roughly 1/1000 the pressure at sea level.
It contains the ozone layer, which is the part of Earth's atmosphere that contains relatively high
concentrations of that gas. The stratosphere defines a layer in which temperatures rise with
increasing altitude. This rise in temperature is caused by the absorption of ultraviolet radiation (UV)
radiation from the Sun by the ozone layer, which restricts turbulence and mixing. Although the
temperature may be −60 °C (−76 °F; 210 K) at the tropopause, the top of the stratosphere is much
warmer, and may be near 0 °C.[22]
The stratospheric temperature profile creates very stable atmospheric conditions, so the
stratosphere lacks the weather-producing air turbulence that is so prevalent in the troposphere.
Consequently, the stratosphere is almost completely free of clouds and other forms of weather.
However, polar stratospheric or nacreous clouds are occasionally seen in the lower part of this layer
of the atmosphere where the air is coldest. The stratosphere is the highest layer that can be
accessed by jet-powered aircraft.

Troposphere
Main article: Troposphere

The troposphere is the lowest layer of Earth's atmosphere. It extends from Earth's surface to an
average height of about 12 km (7.5 mi; 39,000 ft), although this altitude varies from about 9 km
(5.6 mi; 30,000 ft) at the geographic poles to 17 km (11 mi; 56,000 ft) at the Equator,[18] with some
variation due to weather. The troposphere is bounded above by the tropopause, a boundary marked
in most places by a temperature inversion (i.e. a layer of relatively warm air above a colder one), and
in others by a zone which is isothermal with height.[23][24]
Although variations do occur, the temperature usually declines with increasing altitude in the
troposphere because the troposphere is mostly heated through energy transfer from the surface.
Thus, the lowest part of the troposphere (i.e. Earth's surface) is typically the warmest section of the
troposphere. This promotes vertical mixing (hence, the origin of its name in the Greek word
τρόπος, tropos, meaning "turn"). The troposphere contains roughly 80% of the mass of Earth's
atmosphere.[25] The troposphere is denser than all its overlying atmospheric layers because a larger
atmospheric weight sits on top of the troposphere and causes it to be most severely compressed.
Fifty percent of the total mass of the atmosphere is located in the lower 5.6 km (3.5 mi; 18,000 ft) of
the troposphere.
Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where
most of Earth's weather takes place. It has basically all the weather-associated cloud genus types
generated by active wind circulation, although very tall cumulonimbus thunder clouds can penetrate
the tropopause from below and rise into the lower part of the stratosphere. Most
conventional aviation activity takes place in the troposphere, and it is the only layer that can be
accessed by propeller-driven aircraft.
Space Shuttle Endeavour orbiting in the thermosphere. Because of the angle of the photo, it appears to
straddle the stratosphere and mesosphere that actually lie more than 250 km (160 mi) below. The orange layer
is the troposphere, which gives way to the whitish stratosphere and then the blue mesosphere.[26]

Other layers
Within the five principal layers above, that are largely determined by temperature, several secondary
layers may be distinguished by other properties:

 The ozone layer is contained within the stratosphere. In this layer ozone concentrations are

about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very
small compared to the main components of the atmosphere. It is mainly located in the lower
portion of the stratosphere from about 15–35 km (9.3–21.7 mi; 49,000–115,000 ft), though the
thickness varies seasonally and geographically. About 90% of the ozone in Earth's atmosphere
is contained in the stratosphere.
 The ionosphere is a region of the atmosphere that is ionized by solar radiation. It is
responsible for auroras. During daytime hours, it stretches from 50 to 1,000 km (31 to 621 mi;
160,000 to 3,280,000 ft) and includes the mesosphere, thermosphere, and parts of the
exosphere. However, ionization in the mesosphere largely ceases during the night, so auroras
are normally seen only in the thermosphere and lower exosphere. The ionosphere forms the
inner edge of the magnetosphere. It has practical importance because it influences, for example,
radio propagation on Earth.
 The homosphere and heterosphere are defined by whether the atmospheric gases are well
mixed. The surface-based homosphere includes the troposphere, stratosphere, mesosphere,
and the lowest part of the thermosphere, where the chemical composition of the atmosphere
does not depend on molecular weight because the gases are mixed by turbulence.[27] This
relatively homogeneous layer ends at the turbopause found at about 100 km (62 mi; 330,000 ft),
the very edge of space itself as accepted by the FAI, which places it about 20 km (12 mi;
66,000 ft) above the mesopause.
Above this altitude lies the heterosphere, which includes the exosphere and most of the
thermosphere. Here, the chemical composition varies with altitude. This is because
the distance that particles can move without colliding with one another is large compared
with the size of motions that cause mixing. This allows the gases to stratify by molecular
weight, with the heavier ones, such as oxygen and nitrogen, present only near the bottom of
the heterosphere. The upper part of the heterosphere is composed almost completely of
hydrogen, the lightest element.[clarification needed]

 The planetary boundary layer is the part of the troposphere that is closest to Earth's surface
and is directly affected by it, mainly through turbulent diffusion. During the day the planetary
boundary layer usually is well-mixed, whereas at night it becomes stably stratified with weak
or intermittent mixing. The depth of the planetary boundary layer ranges from as little as
 about 100 metres (330 ft) on clear, calm nights to 3,000 m (9,800 ft) or more during the
afternoon in dry regions.
The average temperature of the atmosphere at Earth's surface is 14 °C (57 °F; 287 K)[28] or 15 °C
(59 °F; 288 K),[29] depending on the reference.[30][31][32]

Physical properties

Comparison of the 1962 US Standard Atmosphere graph of geometric altitude against air

density, pressure, the speed of sound and temperature with approximate altitudes of various objects.[33]

Pressure and thickness

Main article: Atmospheric pressure

The average atmospheric pressure at sea level is defined by the International Standard

Atmosphere as 101325 pascals (760.00 Torr; 14.6959 psi; 760.00 mmHg). This is sometimes
referred to as a unit of standard atmospheres (atm). Total atmospheric mass is 5.1480×1018 kg
(1.135×1019 lb),[34] about 2.5% less than would be inferred from the average sea level pressure
and Earth's area of 51007.2 megahectares, this portion being displaced by Earth's mountainous
terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the
pressure is measured. Thus air pressure varies with location and weather.
If the entire mass of the atmosphere had a uniform density equal to sea level density (about
1.2 kg per m3) from sea level upwards, it would terminate abruptly at an altitude of 8.50 km
(27,900 ft). It actually decreases exponentially with altitude, dropping by half every 5.6 km
(18,000 ft) or by a factor of 1/e every 7.64 km (25,100 ft), the average scale height of the
atmosphere below 70 km (43 mi; 230,000 ft). However, the atmosphere is more accurately
modeled with a customized equation for each layer that takes gradients of temperature,
molecular composition, solar radiation and gravity into account.
In summary, the mass of Earth's atmosphere is distributed approximately as follows:[35]

 50% is below 5.6 km (18,000 ft).

 90% is below 16 km (52,000 ft).
 99.99997% is below 100 km (62 mi; 330,000 ft), the Kármán line. By international
convention, this marks the beginning of space where human travelers are
considered astronauts.
By comparison, the summit of Mt. Everest is at 8,848 m (29,029 ft); commercial airliners typically
cruise between 10 and 13 km (33,000 and 43,000 ft) where the thinner air improves fuel
economy; weather balloons reach 30.4 km (100,000 ft) and above; and the highest X-15 flight in
1963 reached 108.0 km (354,300 ft).
Even above the Kármán line, significant atmospheric effects such as auroras still
occur. Meteors begin to glow in this region, though the larger ones may not burn up until they
penetrate more deeply. The various layers of Earth's ionosphere, important to HF
radio propagation, begin below 100 km and extend beyond 500 km. By comparison,
the International Space Station and Space Shuttle typically orbit at 350–400 km, within the F-
layer of the ionosphere where they encounter enough atmospheric drag to require reboosts
every few months. Depending on solar activity, satellites can experience noticeable atmospheric
drag at altitudes as high as 700–800 km.

Temperature and speed of sound

Main articles: Atmospheric temperature and Speed of sound

Temperature trends in two thick layers of the atmosphere as measured between January 1979 and
December 2005 by Microwave Sounding Units and Advanced Microwave Sounding
Units on NOAA weather satellites. The instruments record microwaves emitted from oxygen molecules in
the atmosphere. Source:[36]

The division of the atmosphere into layers mostly by reference to temperature is discussed
above. Temperature decreases with altitude starting at sea level, but variations in this trend
begin above 11 km, where the temperature stabilizes through a large vertical distance through
the rest of the troposphere. In the stratosphere, starting above about 20 km, the temperature
increases with height, due to heating within the ozone layer caused by capture of
significant ultraviolet radiation from the Sun by the dioxygen and ozone gas in this region. Still
another region of increasing temperature with altitude occurs at very high altitudes, in the aptly-
named thermosphere above 90 km.
Because in an ideal gas of constant composition the speed of sound depends only on
temperature and not on the gas pressure or density, the speed of sound in the atmosphere with
altitude takes on the form of the complicated temperature profile (see illustration to the right),
and does not mirror altitudinal changes in density or pressure.
Density and mass

Temperature and mass density against altitude from the NRLMSISE-00 standard atmosphere model (the
eight dotted lines in each "decade" are at the eight cubes 8, 27, 64, ..., 729)

Main article: Density of air

The density of air at sea level is about 1.2 kg/m3 (1.2 g/L, 0.0012 g/cm3). Density is not
measured directly but is calculated from measurements of temperature, pressure and humidity
using the equation of state for air (a form of the ideal gas law). Atmospheric density decreases
as the altitude increases. This variation can be approximately modeled using the barometric
formula. More sophisticated models are used to predict orbital decay of satellites.
The average mass of the atmosphere is about 5 quadrillion (5×1015) tonnes or 1/1,200,000 the
mass of Earth. According to the American National Center for Atmospheric Research, "The total
mean mass of the atmosphere is 5.1480×1018 kg with an annual range due to water vapor of 1.2
or 1.5×1015 kg, depending on whether surface pressure or water vapor data are used; somewhat
smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27×1016 kg
and the dry air mass as 5.1352 ±0.0003×1018 kg."

Optical properties
See also: Sunlight

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Solar radiation (or sunlight) is the energy Earth receives from the Sun. Earth also emits radiation
back into space, but at longer wavelengths that we cannot see. Part of the incoming and emitted
radiation is absorbed or reflected by the atmosphere. In May 2017, glints of light, seen as
twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice
crystals in the atmosphere.[37][38]

Scattering
Main article: Atmospheric scattering

When light passes through Earth's atmosphere, photons interact with it through scattering. If the

light does not interact with the atmosphere, it is called direct radiation and is what you see if you
were to look directly at the Sun. Indirect radiation is light that has been scattered in the
atmosphere. For example, on an overcast day when you cannot see your shadow there is no
direct radiation reaching you, it has all been scattered. As another example, due to a
phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than
longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light.
This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass
through more atmosphere than normal to reach your eye. Much of the blue light has been
scattered out, leaving the red light in a sunset.

Absorption
Main article: Absorption (electromagnetic radiation)

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic

radiation, including visible light.

Different molecules absorb different wavelengths of radiation. For example, O2 and O3 absorb
almost all wavelengths shorter than 300 nanometers. Water (H2O) absorbs many wavelengths
above 700 nm. When a molecule absorbs a photon, it increases the energy of the molecule.
This heats the atmosphere, but the atmosphere also cools by emitting radiation, as discussed
below.
The combined absorption spectra of the gases in the atmosphere leave "windows" of
low opacity, allowing the transmission of only certain bands of light. The optical window runs
from around 300 nm (ultraviolet-C) up into the range humans can see, the visible
spectrum (commonly called light), at roughly 400–700 nm and continues to the infrared to
around 1100 nm. There are also infrared and radio windows that transmit some infrared
and radio waves at longer wavelengths. For example, the radio window runs from about one
centimeter to about eleven-meter waves.

Emission
Further information: Emission (electromagnetic radiation)

Emission is the opposite of absorption, it is when an object emits radiation. Objects tend to emit
amounts and wavelengths of radiation depending on their "black body" emission curves,
therefore hotter objects tend to emit more radiation, with shorter wavelengths. Colder objects
emit less radiation, with longer wavelengths. For example, the Sun is approximately
6,000 K (5,730 °C; 10,340 °F), its radiation peaks near 500 nm, and is visible to the human eye.
Earth is approximately 290 K (17 °C; 62 °F), so its radiation peaks near 10,000 nm, and is much
too long to be visible to humans.
Because of its temperature, the atmosphere emits infrared radiation. For example, on clear
nights Earth's surface cools down faster than on cloudy nights. This is because clouds (H2O) are
strong absorbers and emitters of infrared radiation. This is also why it becomes colder at night at
higher elevations.
The greenhouse effect is directly related to this absorption and emission effect. Some gases in
the atmosphere absorb and emit infrared radiation, but do not interact with sunlight in the visible
spectrum. Common examples of these are CO
2 and H2O.

Refractive index
Distortive effect of atmospheric refraction upon the shape of the sun at the horizon.

Main article: Atmospheric refraction

See also: Scintillation (astronomy)

The refractive index of air is close to, but just greater than 1. Systematic variations in refractive
index can lead to the bending of light rays over long optical paths. One example is that, under
some circumstances, observers onboard ships can see other vessels just over
the horizon because light is refracted in the same direction as the curvature of Earth's surface.
The refractive index of air depends on temperature,[39] giving rise to refraction effects when the
temperature gradient is large. An example of such effects is the mirage.

Circulation
Main article: Atmospheric circulation

An idealised view of three pairs of large circulation cells.

Atmospheric circulation is the large-scale movement of air through the troposphere, and the
means (with ocean circulation) by which heat is distributed around Earth. The large-scale
structure of the atmospheric circulation varies from year to year, but the basic structure remains
fairly constant because it is determined by Earth's rotation rate and the difference in solar
radiation between the equator and poles.

Evolution of Earth's atmosphere

See also: History of Earth and Paleoclimatology
Earliest atmosphere
The first atmosphere consisted of gases in the solar nebula, primarily hydrogen. There were
probably simple hydrides such as those now found in the gas giants (Jupiter and Saturn),
notably water vapor, methane and ammonia.[40]

Second atmosphere
Outgassing from volcanism, supplemented by gases produced during the late heavy
bombardment of Earth by huge asteroids, produced the next atmosphere, consisting largely
of nitrogen plus carbon dioxide and inert gases.[40] A major part of carbon-dioxide emissions
dissolved in water and reacted with metals such as calcium and magnesium during weathering
of crustal rocks to form carbonates that were deposited as sediments. Water-related sediments
have been found that date from as early as 3.8 billion years ago.[41]
About 3.4 billion years ago, nitrogen formed the major part of the then stable "second
atmosphere". The influence of life has to be taken into account rather soon in the history of the
atmosphere, because hints of early life-forms appear as early as 3.5 billion years ago.[42] How
Earth at that time maintained a climate warm enough for liquid water and life, if the early Sun put
out 30% lower solar radiance than today, is a puzzle known as the "faint young Sun paradox".
The geological record however shows a continuous relatively warm surface during the complete
early temperature record of Earth – with the exception of one cold glacial phase about 2.4 billion
years ago. In the late Archean Eon an oxygen-containing atmosphere began to develop,
apparently produced by photosynthesizing cyanobacteria (see Great Oxygenation Event), which
have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon
isotopy (isotope ratio proportions) strongly suggests conditions similar to the current, and that
the fundamental features of the carbon cycle became established as early as 4 billion years ago.
Ancient sediments in the Gabon dating from between about 2.15 and 2.08 billion years ago
provide a record of Earth's dynamic oxygenation evolution. These fluctuations in oxygenation
were likely driven by the Lomagundi carbon isotope excursion.[43]

Third atmosphere

Oxygen content of the atmosphere over the last billion years[44][45]

The constant re-arrangement 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.
Before this time, 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 removed oxygen. This point signifies 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.[46] The following time span from 541 million years ago to the
present day is the Phanerozoic Eon, during the earliest period of which, the Cambrian, oxygen-
requiring 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 about 30% around 280 million years ago, significantly higher than today's 21%. Two
main processes govern changes in the atmosphere: Plants using carbon dioxide from the
atmosphere and releasing oxygen, and then plants using some oxygen at night by the process
of photorespiration with the remainder of the oxygen being used to breakdown adjacent organic
material. 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 great enough for this rapid development of animals.
[47]

Air pollution
Main article: Air pollution

Air pollution is the introduction into the atmosphere of chemicals, particulate matter or biological

materials that cause harm or discomfort to organisms.[48] Stratospheric ozone depletion is caused
by air pollution, chiefly from chlorofluorocarbons and other ozone-depleting substances.
The scientific consensus is that the anthropogenic greenhouse gases currently accumulating in
the atmosphere are the main cause of global warming.[49]

Animation shows the buildup of tropospheric CO

2 in the Northern Hemisphere with a maximum around May. The maximum in the vegetation cycle follows
in the late summer. Following the peak in vegetation, the drawdown of atmospheric CO
2 due to photosynthesis is apparent, particularly over the boreal forests.

Images from space

Main article: Weather satellite

On October 19, 2015, NASA started a website containing daily images of the full sunlit side of
Earth on http://epic.gsfc.nasa.gov/. The images are taken from the Deep Space Climate
Observatory (DSCOVR) and show Earth as it rotates during a day.[50]