Page 115 - SST Class 07

changes related to weather conditions occur. This
is the lowest and the most important layer of the Activity
atmosphere. It extends to a height of about 8 km Draw a neat diagram of structure and composition
at the poles and 18 km at the Equator. Its average of atmosphere.
height is 0 to 12 km. This can vary with season. Thermosphere
Almost all weather phenomena such as clouds,
rainfall, fog, storms and hailstorms etc. occur in Lying above the mesosphere, this layer extends
this layer. Troposphere protects us from the between 80 to 700 km. Here, temperature rises
excessive heat of the sun during the day and keep rapidly with increasing height. This layer contains
the earth’s surface warm at night. It is the densest electrically charged particles called, ions which
layer of the atmosphere. The upper most edge of help in transmitting communication signals. Radio
this layer is called tropopause . It marks the end of waves are sent back to the earth from this layer. It
the troposphere and beyond this, the temperature is also called the ionosphere because of the
stops decreasing. It works as a boundary between presence of ions.
the troposphere and the stratosphere.
Exosphere
Info Bits This is the outermost layer of the atmosphere
Info Bits
which extends to a height of about 700 to 10,000
km and merges with the interplanetary space. The
The summit of Mt. Everest is about 9 km above the
earth’s surface. It lies in the troposphere. air present here is very thin containing gases like
helium and hydrogen, that escape into space. This
Stratosphere is the layer where many man-made satellites orbit
Stratosphere lies above the troposphere and the earth.
extends to a height of around 50 km above the
troposphere. Due to the warm temperature and F ind Outind Out
dry air, this layer is almost free from clouds and What is meant by air pollution?
other weather phenomena. Therefore, it is the
ideal layer for flying aeroplanes. WEATHER AND CLIMATE
The word strato means straight . The layers of the Weather and climate are two different terms.
air in this sphere are almost straight or horizontal However, they are mistakenly used
and undisturbed. The temperature in the lower synonymously. Weather is the atmospheric
part of the stratosphere is constant while there is a condition in a small area over a short period of
gradual increase in the upper part. The time while climate is the average weather
stratosphere is very rich in ozone and also called conditions of a large area over a long period of
ozonosphere . This gas absorbs the ultraviolet time spanning 25-30 years. The weather of any
radiation of the sun and thus protects the earth area may change at short intervals. The weather
from its harmful effects. The upper part of this may be hot or humid. It may be sunny, cloudy,
layer is called stratopause which separates it from rainy, cyclonic etc. It may be changed from day-
the mesosphere. to-day. Atmospheric
Mesosphere temperature, rainfall,
a t m o s p h e r i c
The mesosphere extends upto a height of 80 km. pressure, humidity,
Here, the temperature starts decreasing with wind direction and
height and reaches about 100°C at the height of
precipitation etc. are
80 km. Meteorites burn up in this layer as they the major elements
enter from space due to friction and presence of
of weather and
some oxygen. The mesopause , the upper part of
climate.
this layer separates it from the ionosphere.
The Atmosphere
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