Atmosphere of Earth

The atmosphere of Earth is a 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 given to the atmospheric gases
used in breathing and photosynthesis is air . By
volume, dry air contains 78.09% nitrogen, 20.95%
oxygen , 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. Although air content and
atmospheric pressure vary at different layers, air
suitable for the survival of terrestrial plants and
terrestrial animals currently is only known to be
found 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 or aerology . Early
pioneers in the field include Léon Teisserenc de
Bort and Richard Assmann .
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. The remaining gases are often referred to
as trace gases, 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, 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 (SO 2 ) may be derived from natural sources
or from industrial air pollution .
Major constituents of dry air, by volume
Gas
Volume(A)
Name
Formula
in ppmv(B)
in %
Nitrogen
N 2
780,840
78.084
Oxygen
O 2
209,460
20.946
Argon
Ar
9,340
0.9340
Carbon dioxide
CO 2
397
0.0397
Neon
Ne
18.18
0.001818
Helium
He
5.24
0.000524
Methane
CH 4
1.79
0.000179
Not included in above dry atmosphere:
Water vapor (C)
H 2O
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.
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) [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 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.
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 still feel extremely
cold to a human in direct contact because the total
energy of its relatively few number of molecules is
incapable of transferring an adequate amount of
energy to the skin of a human. 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 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 jet-powered aircraft, and too low to
support satellites and orbital or sub-orbital
spacecraft. The mesosphere is mainly accessed by
rocket-powered aircraft and unmanned 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.
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 ,
with some variation due to weather. The
troposphere is bounded above by the tropopause ,
a boundary marked by stable temperatures.
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. 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.
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.
Other layers
Within the five principal layers 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 surfaced-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), 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.
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 meters on clear, calm nights to 3000
m 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.
[19][20][21]
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.
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 5.1480×10 18 kg
(1.135×10 19 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 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 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 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

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