The atmosphere consists of a blanket of gases
that surrounds the earth. It absorbs harmful
ultraviolet radiations from sun. Atmosphere has
mass and exerts pressure. It has a total mass of
5x108 Kg. Life is not possible
without atmosphere. It contains oxygen which is
essential for life. Carbon dioxide in atmosphere
is essential for Photosynthesis. The atmosphere
is composed of various gases and water vapors.
The dry air in the lower atmosphere contains
nitrogen, oxygen, argon, carbon dioxide, neon,
helium, hydrogen, methane, ozone, carbon
major constituents of dry air are
% by Volume
Structure of atmosphere:
atmosphere can be divided into five different layers based on
Troposphere: It is the lower most portion of the
atmosphere which extends up to about 8 km at poles and 16 km at the
equator with some variation due to weather. In troposphere, the decrease
in temperature with increase in altitude is fairly uniform. The end of
troposphere boundary is marked by tropopause. Troposphere consists of
80% of the atmosphere mass.
Stratosphere: It extends roughly from 12 km to up to 50
to 55 km above earth surface. It is separated from Troposphere by
Tropopause. It contains ozone layer. It defies a layer in which
temperature increases with increase in altitude. It is due to the
absorption of ultraviolet radiations from the sun by ozone layer. The
outer limit of stratosphere is stratopause having temperature of 0°C.
Mesosphere: It extends up to 85 km from earth and
30-35 km beyond stratosphere. In Mesosphere, the temperature decreases
slowly with altitude and then sharply to a minimum of about -75°C. In
mesosphere most meteorites burn upon entrance due to experience of
Thermosphere: This layer lies in between Mesosphere and
Exosphere. It is the biggest layer in the atmosphere. It begins at 85 km
from earth surface. The temperature increases with altitude to a very
high value and it approaches 2000°C and even more depending upon solar
Exosphere: Exosphere is the outmost layer of earth’s
atmosphere. It contains lightest gases like hydrogen. It also contains
some gases like atomic oxygen, helium and carbon dioxide near its base (exobase).
There is no fixed boundary between the outer space and exosphere so
sometimes it is considered as part of outer space.