Describe the structure of the atmosphere.

The atmosphere consists of concentric layers of air. On the basis of the characteristics of temperature and air pressure, it can be divided into four thermal layers — Troposphere, Stratosphere, Mesosphere and Thermosphere. The Thermosphere is divided into Ionosphere and Exosphere. The four layers are briefly described below:

1. Troposphere — It is the lowermost and densest layer. Its height varies from 18 km at the Equator to 8 km at the Poles.
2. Stratosphere — This layer lies above the Troposphere. It extends to a height of 50 km from sea level.
3. Mesosphere — This layer lies above the Stratosphere. It extends to a height of 80 km from ground level.
4. Thermosphere — It lies above Mesosphere and consists of two parts:
   1. Ionosphere — It extends up to a height of about 400 km.
   2. Exosphere — It is the uppermost layer of the atmosphere. It lies between 400 to 1500 km above the earth.