Page 136 - SST Class 07

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Air

The earth’s atmosphere comprises different gases The higher we are above the earth’s surface, the
that cover the planet like a blanket. It’s in many colder it gets. But, this is not true for the whole of
layers. The part of the atmosphere that we are the earth’s atmosphere. There are certain por-
able to see from the land, has the clouds, the tions of the atmosphere where temperature stays
rainbow, birds, airplanes and kites, is actually only the same and other portions where it even rises
a thin layer when compared to the full size of the as we go up.
earth. The earth is huge spherical ball slightly LAYERS OF ATMOSPHERE
tilted on its axis, third in position from the sun, in The earth’s atmosphere has five main layers.
the Solar System. The radius of the earth is
From the lower layer to the higher layers, they
roughly about 6400 kms, but all our airplanes can
are—the troposphere, stratosphere, mesosphere,
go as high as 16 kms only which is hardly any-
thermosphere and exosphere.
thing when compared to the whole of the earth.
As one moves away from the surface of the
earth, the pressure of the air gradually decreases. w The word atmosphere comes from the Greek
There is no line in the atmosphere which shows terms ‘atmos’ means vapour and ‘sphaira’ means
where the space begins, it does not happen sphere. It means the air between the earth’s
surface and outer space.
suddenly, it’s gradual.
w The terms ‘greenhouse effect’ and ‘global warm-
When we go up a mountain, it gets colder.
ing’ are often used. Both these terms are linked to
carbon dioxide. Carbon dioxide level in the
EXOSPHERE
500 atmosphere, affects the earth’s weather and
climate.

THERMOSPHERE Troposphere
This is the lowest yet the most important layer of
HEIGHT IN KILOMETRES 50 Stratopause km at the Poles and 18 km at the Equator. Being
the atmosphere. It extends to a height of about 8
MESOSPHERE
80
closest to the earth, this is the densest layer of the
atmosphere. All weather phenomena such as
clouds, fog, rainfall, hail, storm, lightning, etc.,
take place in this layer. Here, the temperature of
STRATOSPHERE
air decreases with increasing height at the rate of
Aeroplane
OZONE
1°C for every 65 meters. This is known as ‘nor-
10 Tropopause mal lapse rate’. The uppermost edge of this layer
TROPOSPHERE
is called ‘tropause’ and beyond this the tempera-
Atmospher ture stops decreasing. The troposphere absorbs

Social Science-7 136