The atmosphere of
is the layer of gases, commonly known as air,
that surrounds the planet
and is retained by Earth's gravity.
The atmosphere of
protects life on
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. Air
also contains a variable amount of water vapor, on average around 1%
at sea level, and 0.4% over the entire atmosphere. Air content and
atmospheric pressure vary at different layers, and air suitable for
use in photosynthesis by terrestrial plants and breathing of
terrestrial animals is found only in Earth's troposphere and in
The atmosphere has a mass of about 5.15×1018 kg, 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
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.
2 Structure of the atmosphere
2.1 Principal layers
2.2 Other layers
3 Physical properties
Pressure and thickness
Temperature and speed of sound
3.3 Density and mass
4 Optical properties
4.4 Refractive index
6 Evolution of Earth's atmosphere
6.1 Earliest atmosphere
6.2 Second atmosphere
6.3 Third atmosphere
6.4 Air pollution
7 Images from space
8 See also
10 External links
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). The
remaining gases are often referred to as trace gases, among which
are the greenhouse gases, principally carbon dioxide, methane, nitrous
oxide, and ozone. 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
Not included in above dry atmosphere:
(A) volume fraction is equal to mole fraction for ideal gas only,
also see volume (thermodynamics)
(B) ppmv: parts per million by volume
Water vapor is about 0.25% by mass over full atmosphere
Water vapor strongly varies locally
The relative concentration of gasses remains constant until about
10,000 m (33,000 ft).
Structure of the atmosphere
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
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 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. 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)
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)
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.
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.
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. 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
International Space Station orbits in this layer, between 350 and
420 km (220 and 260 mi).
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
Temperatures drop with increasing altitude to the mesopause that marks
the top of this middle layer of the atmosphere. It is the coldest
Earth and has an average temperature around −85 °C
(−120 °F; 190 K).
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.
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
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.
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.
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,
although this altitude actually varies from about 9 km
(30,000 ft) at the poles to 17 km (56,000 ft) at the
equator, 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
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. 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 (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
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 below. The orange
layer is the troposphere, which gives way to the whitish stratosphere
and then the blue mesosphere.
Within the five principal layers that are largely determined by
temperature, several secondary layers may be distinguished by other
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. 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) or 15 °C (59 °F;
288 K), depending on the reference.
Comparison of the 1962
US Standard Atmosphere
US Standard Atmosphere graph of geometric
altitude against air density, pressure, the speed of sound and
temperature with approximate altitudes of various objects.
Pressure and thickness
Main article: Atmospheric pressure
The average atmospheric pressure at sea level is defined by the
International Standard Atmosphere
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), 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
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 from sea
level, 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:
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 km (33,000 ft) and 13 km (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
International Space Station
International Space Station and
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
Temperature and speed of sound
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
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
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
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See also: Sunlight
Solar radiation (or sunlight) is the energy
Earth receives from the
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.
Main article: 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.
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.
Main article: Absorption (electromagnetic radiation)
Rough plot of Earth's atmospheric transmittance (or opacity) to
various wavelengths of electromagnetic radiation, including visible
Different molecules absorb different wavelengths of radiation. For
example, O2 and O3 absorb almost all wavelengths shorter than 300
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.
Main article: 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 CO2 and H2O.
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, giving rise to
refraction effects when the temperature gradient is large. An example
of such effects is the mirage.
Main article: Atmospheric circulation
An idealised view of three 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
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
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. 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.
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
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
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 2,150 and
2,080 million years ago provide a record of Earth's dynamic
oxygenation evolution. These fluctuations in oxygenation were likely
driven by the Lomagundi carbon isotope excursion.
Oxygen content of the atmosphere over the last billion years
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
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. 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 use carbon dioxide from the
atmosphere, releasing oxygen. 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.
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. Stratospheric ozone depletion is caused
by air pollution, chiefly from chlorofluorocarbons and other
The scientific consensus is that the anthropogenic greenhouse gases
currently accumulating in the atmosphere are the main cause of global
Animation shows the buildup of tropospheric CO2 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 CO2 due to photosynthesis is apparent,
particularly over the boreal forests.
Images from space
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
Deep Space Climate Observatory (DSCOVR) and
Earth as it rotates during a day.
Blue light is scattered more than other wavelengths by the gases in
the atmosphere, giving
Earth a blue halo when seen from space.
The geomagnetic storms cause beautiful displays of aurora across the
Limb view, of Earth's atmosphere. Colors roughly denote the layers of
This image shows the Moon at the centre, with the limb of
the bottom transitioning into the orange-colored troposphere. The
troposphere ends abruptly at the tropopause, which appears in the
image as the sharp boundary between the orange- and blue-colored
atmosphere. The silvery-blue noctilucent clouds extend far above
Earth's atmosphere backlit by the
Sun in an eclipse observed from deep
Apollo 12 in 1969.
Air (classical element)
Atmosphere (on atmospheres in general)
Atmospheric dispersion modeling
Climate Research Facility (ARM) (in
Carbon dioxide in Earth's atmosphere
COSPAR international reference atmosphere (CIRA)
Environmental impact of aviation
Historical temperature record
Standard Dry Air
U.S. Standard Atmosphere
Water vapor in Earth's atmosphere
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