Atmosphere of Earth

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  (Redirected from Air)
"Air" redirects here. For other uses, see Air (disambiguation).
"Qualities of air" redirects here. It is not to be confused with Air quality.

Blue light is scattered more than other wavelengths by the gases in the

atmosphere, giving Earth a blue halowhen seen from space onboard ISS at
a height of 402–424 km.
 Composition of Earth's atmosphere by volume. The lower pie represents
the trace gases that together compose about 0.038% of the atmosphere
(0.043% with CO2 at 2014 concentration). The numbers are from a variety
of years (mainly 1987, with CO2and methane from 2009) and do not
represent any single source.
The atmosphere of Earth is the layer of gases surrounding the
planet Earth that is retained by Earth's gravity. The atmosphere protects life
on Earth by absorbing ultraviolet solar radiation, warming the surface
through heat retention (greenhouse effect), and
reducing temperature extremes between day and night (the diurnal
temperature variation).
The common name air (English pronunciation: /ɛər/) is given to the
atmospheric gases used in breathing and photosynthesis. By volume, dry
air contains 78.09% nitrogen, 20.95% oxygen,[1] 0.93% argon,
0.039% 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 andatmospheric
pressure vary at different layers, and air suitable for the survival
of terrestrial plants and terrestrial animals is found only in
Earth's troposphere and artificial atmospheres.
The atmosphere has a mass of about 5.15×10 18 kg,[2] 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.[3]

Contents
  [hide] 

 1Composition
 2Structure of the atmosphere
o 2.1Principal layers
 2.1.1Exosphere
 2.1.2Thermosphere
 2.1.3Mesosphere
 2.1.4Stratosphere
 2.1.5Troposphere
o 2.2Other 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
Air is mainly composed of nitrogen, oxygen, and argon, which together
constitute the major gases of the atmosphere. 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 ppmv 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 provided for dry air without any water vapor. [4] The remaining
gases are often referred to as trace gases,[5] among which are
the greenhouse gases such as 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, pollenand 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[6]

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 CO2 397 0.0397

Neon Ne 18.18 0.001818

Helium He 5.24 0.000524

Methane CH4 1.79 0.000179

Not included in above dry atmosphere:

Water vapor(C) H2O 10–50,000(D) 0.001%–5%(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[4]

Structure of the atmosphere

Principal layers
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 recognizable
through means such as balloon soundings, the temperature behavior
provides a useful metric to distinguish between atmospheric layers. In this
way, Earth's atmosphere can be divided (called atmospheric stratification)
into five main layers. Excluding the exosphere, Earth has four primary
layers, which are the troposphere, stratosphere, mesosphere, and
thermosphere.[7] 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)[8]
 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)[9]
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
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). The exosphere merges with
the emptiness of outer space, where there is no atmosphere.
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 thesolar 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 thethermopause 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.[8] 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.
This atmospheric layer undergoes a gradual increase in temperature with
height. Unlike the stratosphere, 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.[10]Even though the thermosphere
has a very high proportion of molecules with immense amounts of energy,
the thermosphere would not feel hot to a human in direct contact, because
the low density in the thermosphere would not be able to conduct a
significant amount of energy to or from the skin. In other words, a person
would not feel warm because of the thermosphere's extremely low
pressure.
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 320 and 380 km (200 and
240 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).[11]
[12]

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. A type of lightning referred to as
either sprites or ELVES, occasionally form far 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 aircraft and balloons, and too low to permit orbital spacecraft. The
mesosphere is mainly accessed by sounding rockets.
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.[13]
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. This 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, although this altitude
actually varies from about 9 km (30,000 ft) at the poles to 17 km (56,000 ft)
at the equator,[9] 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. [14][15]
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.[16] 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. It is primarily
composed of nitrogen (78%) and oxygen (21%) with only small
concentrations of other trace gases.
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.
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.[35] Stratospheric ozone depletion is caused by air
pollution, chiefly from chlorofluorocarbons and other ozone-depleting
substances.