Atmosphere of Earth
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)
·
Mesosphere:
50 to 80 km (31 to 50 miles)
·
Stratosphere:
12 to 50 km (7 to 31 miles)
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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
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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 magnetosphereor 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 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 itstemperature 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 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.[14] 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-associatedcloud 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
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