The atmosphere has evolved as a significant component of biospheric eco-system through the process of degassing or volcanism.
- Nitrogen (78 per cent) and Oxygen (21 per cent) together constitute 99 per cent of the total gaseous composition of the atmosphere.
- Oxygen is essential for the survival of living organism since they require it to convert food into energy.
- Carbon dioxide acts as a 'Green House Gas' as it absorbs most of the radi-ant long wave energy from the earth and re-radiates it back to the earth, thus adding to the warming of the lower atmosphere.
- Ozone, less than 0.0001 per cent by volume, is another important gas. It protects the life on earth from the harmful effects of ultraviolet rays by absorbing it. It is concentrated between the altitude of 20 and 30 km.
- About 90 per cent of the total water vapour present in the atmosphere is found up to the height of 5km. Its content ranges between 0 and 5 per cent in the atmosphere.
- The blue colour of sky, the dawn and twilight are the result of scattering by the rest particles.
Structure of The Atmosphere
- About 50 per cent of the atmosphere lies below the altitude of 5.6 km and 97 per cent of the atmosphere is confined to the height of 29 km.
- Based on temperature and pressure, following layers of atmosphere are recognised:
- This is the lowermost layer.
- Extends to an average altitude of 10 km which varies between 18 km above the equator and 8 km above the poles.
- The height is more above the equator due to convective movement of the air.
- In the troposphere the temperature decreases with increasing height at the rate of 6.50 centigrade per km which is called Normal Lapse Rate.
- Most weather phenomenon takes place in the troposphere.
- The boundary line separating troposphere from the next layer is called "Tropopause".
- This layer lies above the troposphere.
- Extends upwards to a height of about 50 km.
- The temperature ceases to decrease in the lower part of the stratosphere after which it starts increasing.
- Strong and persistant winds blow in stratosphere from west to east.
- Normally holds very little water vapour or dust, so weather disturbances are absent here.
- Jet air flights take place mainly in the stratosphere.
- The existence of ozone layer between 15 to 35 km which absorbs harmful solar ultraviolet rays is an important feature of the stratosphere.
- Extends from stratopause at 50 km (the dividing zone between Strato-sphere and Mesosphere) to Mesopause at 80 km.
- In this layer, temperature decreases from 0°C at the stratopause to -80°C at the mesopause.
- It is a zone of chemical reactions.
- Bulk of the meteors are destroyed in this region.
In this sphere temperature increases rapidly. Thermosphere is divided into two layers:
- Ionosphere: Extending between 80 km and 400 km, this is an electrically charged layer. Absorption of heat by certain ions present in this layer in-creases the temperature. This layer reflects radio waves transmitted from the earth back to the earth.
- Exosphere: This uppermost layer extending beyond ionosphere impercep-tibly merges with the outer space. Temperature becomes extremely high but the highly ratified air holds little heat, so the high temperature is not felt. Hydrogen & helium predominate in this region.