Composition of Atmosphere and Variations in Temperature
It is convenient to divide the atmosphere into two broad thermal zones. The lower region is known as the ‘troposphere’, in which the temperature decreases with height. The upper region, in which the temperature is constant or increases slightly, is called the ‘stratosphere’. The surface of separation between the troposphere and stratosphere is known as ‘tropopause’. The height of the tropopause has an interesting variation with latitude. It is highest at the equator where it is located at about 18 km, but decreases as we proceed towards the poles. In the polar region it is found at about 6 km above the earth. Over India, the height of the tropopause is generally 16 km, while in the extra-tropical latitudes the tropopause is observed near 11 km. The tropopause often changes its level sharply across a jet stream. The height of the tropopause may even become discontinuous near the axis of the jet stream.
The depth of the stratosphere is of the order of 50 km. Between 50 and 85 km we have another region where the temperature decreases fairly rapidly with height. This is named the Mesosphere, where photochemical actions between ionized particles become predominant. Above the mesosphere, we have the Thermosphere where temperatures begin to rise with height.
It is interesting to note in passing that some of the earliest concepts of the atmosphere came rather close to the truth, as we know it now. There is evidence to suggest that Aristotle divided the sphere of air into three regions. The first and lowest region was warmed by the sun’s rays and the earth’s heat. The second region was cold, where the ‘watery meteors condensed into clouds’. Lastly, the uppermost region was hot because of its contact with the ‘sphere of fire’. The modern picture of the atmosphere is more complex than Aristotle’s view, but, historically speaking, it seems remarkable how close he was to reality with very little visual aid.
There are two principal features of the normal temperature pattern during the monsoon months. Firstly, we find a zone of high temperatures over the semi-arid regions of northwest India. This region of high temperatures is built up gradually during the pre-monsoon months of May and June. The interesting feature is that the geographical location of the thermal high appears to coincide fairly well with the position of an area of low barometric pressure. Indeed, the coincidence is so remarkable that the quasi-static low-pressure system over northwest India, and its extension along the Indo-Gangetic plains in the form of a monsoon trough, is attributed to the thermal high. The seasonal low-pressure zone is a heat induced low.
A second feature is the appearance of another zone of warm temperature over the Tibetan plateau at 500 hPa (hecta pascal). The Tibetan plateau has an average altitude of 4.5 km and an area of about 1.7 X 10 to the power of 6 square kms. Observations suggest that the middle and upper troposphere above the Tibetan Plateau is 6 – 8 º C warmer than the equatorial atmosphere over southern India. This makes the plateau act as a heat source, and provides the ascending branch of the Hadley Cell associated with the Asian Summer Monsoon. But, after an initial ascent the air tends to spread out southwards. In doing so it is aided by an anticyclone in the upper troposphere over Tibet.
Acknowledgement: Excerpt from ‘The Monsoon’ by Dr.P.K.Das, former Director General of the National Meteorological Service of the Government of India. Published by National Book Trust, India. Price: Rs.75/=