Structure of Atmosphere :

              On the basis of temperature and air pressure there are four layers of atmosphere from the Earth's surface to upward. These are (1) Troposphere, (2) Stratosphere, (3) Mesosphere, (4) Thermosphere.

(1) Troposphere: 

            The lower most layer of the atmosphere is known as the Troposphere. It is the most important layer because all the weather phenomenons like cloud, fog, rain, lightening etc occur in this layer. Here temperature decreases with increasing height at the rate of 6.5°C/ km. This rate of decrease of temperature is called Normal Lapse Rate. 

     The height of the Troposphere changes from equator towards the poles. The average height of the Troposphere is about 16 km over the equator and 6km over the poles. The upper limit of Troposphere is called Tropopause which is about 1.5km thick. The word Troposphere means "zone or region of mixing" where as the word means " where the mixing stop".

(2) Stratosphere:

                   The layer just above the Troposphere is called stratosphere. It is considered to be only 25 - 30 Kms in mid latitude. Average altitude of stratosphere is thus considered to be 50 km above the sea level.

             Although the stratosphere is more or less devoid of any atmospheric phenomenon as they do not occur beyond the stratosphere, but there are some feeble winds that blow in the Stratosphere, along with cirrus clouds. The temperature increases with altitude in Stratosphere. The lower portion of stratosphere having greatest concentration of ozone is known as ozonosphere . Ozone acts as a protective cover for biological communities in the biosphere because it protects the  harmful U_V radiations by absorbing.

       The upper limit of stratosphere is called stratopause and stratopause is a transitional layer.

(3) Mesosphe :

             It extends between 50 to 80km above the sea level. Here temperature again starts decreasing with the increasing in height. At the upper most limit of Mesosphe temperature becomes -80°C.

            Polar mesospheric clouds are formed in Mesosphe. The Mesosphe is also a layers where meteors burn up upon entering the atmosphere.  The upper part of the Mesosphere is known as the mesopause where occur lowest temperature in the atmosphere.  It is a transition layer.

(4) Thermosphere :

                  It  extends from 80 km to 1000 km above the sea level. Thermosphere is divided into two layers-

 

(i) Ionosphere:

                    It extends 80 to 640 km above the sea level. It contain electrically charged particles known as ions. The radio waves are reflected back to Earth by this layer. This layer also responsible for producing beautiful auroras in the sky e.g Aurora Borealis or Northern lights, producing over the Artic region.

(ii) Exosphere:

               It extends above 640 km to outer space. It represents the uppermost layer of atmosphere. It is the highest layer of atmosphere. This layer is extremely rarefied and the density is extremely low. It gradually merges with outer space.