ATMOSPHERE
Atmosphere
is a thick gaseous envelope that surrounds the earth and extends thousands of
kilometers above the earth's surface. Much of the life on the earth exists
because of the atmosphere otherwise the earth would have been barren. Nitrogen
and Oxygen comprise 99% of the total volume of the atmosphere.
Structure
of the Atmosphere
The
atmosphere consists of almost concentric layers of air with varying density and
temperature.
a)
Troposphere:
• Lowest layer of the atmosphere.
• The height of troposphere is 16 km
thick over the equator and 10 km thick at the poles.
• All weather phenomena are confined to
troposphere (e.g. fog, cloud, frost, rainfall, storms, etc.)
• Temperature decreases with height in
this layer roughly at the rate of 6.5° per 1000 metres, which is called normal
lapse rate.
• Upper limit of the troposphere is
called tropopause which is about 1.5 km.
b)
Stratosphere:
• The stratosphere is more or less
devoid of major weather phenomenon but there is circulation of feeble winds and
cirrus cloud in the lower stratosphere.
• Jet aircrafts fly through the
lower stratosphere because it provides perfect flying conditions.
• Ozone layer lies within the
stratosphere mostly at the altitude of 15 to 35 km above earth's surface.
• Ozone layer acts as a
protective cover as it absorbs ultra-voilet rays of solar radiation.
• Depletion of ozone may result
in rise of temperature of ground surface and lower atmosphere.
• Temperature rises from -60°C at the
base of the stratosphere to its upper boundary as it absorbs ultra-voilet rays.
• Upper limit of the Stratosphere
is called stratopause.
c) Mesosphere
• Mesosphere extends to the height
of 50-90 km.
• Temperature decreases with
height. It reaches a minimum of -80°C at an altitude of 80-90 km
• The upper limit is called mesopause.
d)
Thermosphere
•
It lies at 80 km to 640 km above the earth's surface.
•
It is also known as ionosphere.
•
Temperature increases rapidly with increasing height.
•
It is an electrically charged layer. This layer is produced due to interaction
of solar radiation and the chemicals present, thus disappears with the sunset.
•
There are a number of layers in thermosphere e.g. D-layer, E-layer, F-layer and
G-layer.
•
Radio waves transmitted from earth are reflected back to the earth by these
layers.
e)
Exosphere
•
This is the uppermost layer of the atmosphere extending beyond the ionosphere.
•
The density is very low and temperature becomes 5568°C.
•
This layer merges with the outer space.
About
Ionosphere
At
heights of 80 km (50 miles), the gas is so thin that free electrons can exist
for short periods of time before they are captured by a nearby positive ion.
This portion of the atmosphere is ionized and contains plasma which is referred
to as the ionosphere. The Ultraviolet (UV), X-Ray and shorter wavelengths of
solar radiation ionizes the atmosphere. The ionosphere is broken down into the
D, E and F regions
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