The atmosphere can be described as “a blanket of air” surrounding the earth. This enveloping mixture of gases contains huge numbers of solid and liquid particles collectively called aerosols.

Some of the gases may be regarded as permanent atmospheric components that remain in fixed proportions to the total gas volume. Other constituents vary in quantity from place to place and from time to time.

If the suspended particles, water vapour and other variable gases were excluded from the atmosphere, we would find that the dry air is very stable all over the earth up to an altitude of about 80 kilometers.

As shown in table 21.1, two gases, nitrogen and oxygen, make up about 99 per cent of the clean, dry air. The remaining gases are mostly inert and constitute about 1 per cent of the atmosphere.

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The deep layer through which the gaseous composition of the atmosphere is generally homogeneous is called the homosphere. At higher altitudes, the chemical constituents of air change considerably. This layer is known as the heterosphere.

At sea level the following principal gases comprise the dry air; nitrogen, oxygen, argon, carbon dioxide, neon, helium, ozone, hydrogen, krypton, xenon and methane.

Out of these gases, argon, neon, helium, krypton and xenon are so inert chemically that they are never found in any chemical compounds. They stand completely alone.

Besides these gases, large quantities of water vapour and dust particles are also present in the atmosphere.

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These solid and liquid particles are of great climatic significance. Different constituents of the atmosphere, it may be noted, have got their individual characteristics as briefly discussed below.