The vast expanse of air which envelops the earth all around is called the atmosphere. It extends to thousands of kms. It protects the earth’s surface from the sun’s harmful, ultraviolet rays. It also regulates temperature, preventing the earth from becoming too hot or too cold. The major constituents of air in the atmosphere are Nitrogen (78%), Oxygen (21%), Argon (0.93%) and Carbon dioxide (0.03%). Besides water vapour, dust particles, smoke, salts and other impurities are present in air in varying quantities. On the basis of temperature, the atmosphere is divided into five layers:
1. Troposphere : It is the lowest layer and lies close to earth’s surface. The average height is 18 km over equator and 8 km over the poles. It is the zone where all atmospheric processes leading to climatic and weather conditions take place. Temperature decreases with increasing height at the rate of 1°C for 165 metres of ascent which is known as ‘normal lapse rate’.
2. Stratosphere : The zone separating troposphere from stratosphere is called tropopause. The air temperature at tropopause is about -80°C over equator and – 45°C over the poles. The stratosphere lies beyond the troposphere. This layer extends upward from the tropopause to about 50 km. Ozone layer is present in stratosphere.
3. Mesosphere : It extends up to a height of 80 kilometres over the stratosphere. Temperature decreases with the height and reaches up to -100°C.
4. Ionosphere : It is located between 80 kilometres and 400 kilometres. Radiowaves transmitted from the earth are reflected back to earth by this layer. Temperature starts increasing with the height.
5. Exosphere : It is the topmost layer of the atmosphere and extends beyond the ionosphere above a height of 400 kilometres. This layer is extremely rarefied and gradually merges with the outer space.
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