The Earth's Atmosphere
The earth's surface is covered by a layer of atmosphere consisting
of a mixture of gases and other solid and liquid particles. The
gaseous materials extend to several hundred kilometers in altitude,
though there is no well defined boundary for the upper limit of
the atmosphere. The first 80 km of the atmosphere contains more
than 99% of the total mass of the earth's atmosphere.
Vertical Structure of the Atmosphere
The vertical profile of the atmosphere is divided into four layers:
troposphere, stratosphere, mesosphere and thermosphere. The tops of
these layers are known as the tropopause, stratopause, mesopause
and thermopause, respectively.
- Troposphere: This layer is characterized by a decrease
in temperature with respect to height, at a rate of about
6.5ºC per kilometer, up to a height of about 10 km.
All the weather activities (water vapour, clouds, precipitation)
are confined to this layer. A layer of aerosol particles normally
exists near to the earth surface. The aerosol concentration
decreases nearly exponentially with height, with a characteristic
height of about 2 km.
- Stratosphere: The temperature at the lower 20 km of the
stratosphere is approximately constant, after which the temperature
increases with height, up to an altitude of about 50 km. Ozone
exists mainly at the stratopause. The troposphere and the stratosphere
together account for more than 99% of the total mass of the atmosphere.
- Mesosphere: The temperature decreases in this layer from
an altitude of about 50 km to 85 km.
- Thermosphere: This layer extends from about 85 km upward
to several hundred kilometers. The temperature may range from 500 K
to 2000 K. The gases exist mainly in the form of thin plasma, i.e.
they are ionized due to bombardment by solar ultraviolet radiation and energetic cosmic rays.
The term upper atmosphere usually refers to the region of the atmosphere above the troposphere.
Many remote sensing satellites follow the near polar
sun-synchronous orbits at a height
around 800 km, which is well above the thermopause.
The atmosphere consists of the following components:
- Permanent Gases:
They are gases present in nearly constant concentration, with
little spatial variation. About 78% by volume of the atmosphere is nitrogen while the life-sustaining oxygen occupies 21%. The remaining one percent consists of the inert gases, carbon dioxide and other gases.
- Gases with Variable Concentration: The concentration of these gases may vary greatly over space and
time. They consist of water vapour, ozone, nitrogeneous and sulphurous compounds.
- Solid and liquid particulates: Other than the gases, the atmosphere also contains solid and
liquid particles such as aerosols, water droplets
and ice crystals. These particles may congregate to form
clouds and haze.
Click here to see the gaseous composition of the atmosphere
The Planet Earth
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