On the basis of its chemical composition, the atmosphere is divided into two broad spheres- homosphere and heterosphere.
Homosphere:
This is the lower part of the atmosphere which extends up to a height of about 88 kilometers. It is characterized by uniformity in composition. In other words, the proportions of the component gases of this sphere are uniform at different levels.
In fact, the term ‘homosphere’ means the zone of homogeneous composition. The homosphere has been subdivided into three sub-layers: (a) troposphere, (b) stratosphere, and (c) mesosphere.
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Each sublayer is separated from the adjoining one by a very shallow transition zone. Tropopause, stratopause and mesopause represent such transition zones.
Heterosphere:
The atmosphere above the homosphere is not uniform in its composition. It has a heterogeneous composition, hence the name ‘heterosphere’. Different layers of the atmosphere in this part differ from one another in their chemical and physical properties.
The heterosphere is also referred to as the thermosphere, for in this layer temperature goes on rising up to the outermost boundary of the atmosphere.
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It may be remembered that in the upper parts of the atmosphere, where the gases are so rarefied, the high temperatures are caused exclusively by the photochemical actions of the ultraviolet solar radiation.
In this sphere the gases are said to be arranged into the following four roughly spherical shells, each of which has its own distinctive composition: nitrogen layer, oxygen layer, helium layer, and hydrogen layer.
The lowermost layer dominated by molecular nitrogen is called the nitrogen layer. Its average height above the earth’s surface is said to be from 88 to 200 kilometers. Above this layer is the oxygen layer consisting of atomic oxygen whose average height above the surface is estimated to be from 200 to 1120 kilometers.
This layer is overlaid by the helium layer which is dominated by helium. It average height above the earth’s surface is from 1120 to 3520 kilometers. At the top there is the hydrogen layer extending up to the outermost boundary of our atmosphere.
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Its principal component part is hydrogen. The various layers of gases have been found to be arranged in accordance with the weight of different gases.
Nitrogen being the heaviest makes up the lower layer and hydrogen being the lightest is the fourth and the highest layer.