PUBLISHED BY: SURENDER KUMAR
OCTOBER 25, 2012

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THE ATMOSPHERE

The Earth is a unique planet as it has conditions favourable for life. The oxygen layer surrounding 5t is essential for all living beings. The air envelope also moderates the temperature on the Earth. So the day-night and summer-winter temperature variations are not very high unlike other planets. The presence of water favours the growth and evolution of different living species. Thus the earth is unique in having a biosphere or life-bearing layer.

The atmosphere, held to the Earth by gravitational force, is the air blanket surround5ng the Earth. Of the atmosphere, 99 percent is within a height of 32 kilometre from the earth surface. Most atmospheric changes take place within this layer. An average sample of pure, dry air consists of Nitrogen (78 %), Oxygen (21%) and Argon (0.9 %). Other gases - Carbon Dioxide, Hydrogen, Helium and Ozone are present in minute quantities. Though water vapours do not exceed 3 to 4 percent of the total air, they are important in the atmospheric processes. 

The lowest layer of the atmosphere is called the troposphere. All weather phenomena take place here. It extends up to 18 km. over the equator and about 8 km. along the poles. In this layer, the air temperature decreases with height at an average rate of 1^OC for 165 metres, called the lapse rate. The upper limit of this layer is known as the tropopause.

Above it lies the stratosphere, with a thickness of 50-55 km. Here, the temperature is constant and then increases with height. This layer, being free from clouds, is ideal for flying jet aircraft. The ozone layer is present in this region, which absorbs harmful ultra-violet radiations coming from the sun. Above the stratosphere are the mesosphere and the ionosphere. The ionosphere contains charged particles called ions. This layer reflects radio waves back to earth‘s surface making wireless communication possible. The Ionosphere is followed by the uppermost layer, the exosphere, which has no fixed upper limit.

The atmosphere is an important part of the physical environment. The differential heating of the atmosphere by the sun’s rays produces circulation of air leading to winds, clouds and precipitation.