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| Air |
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| It is an inexhaustible natural resource. It is very essential for the survival of all the living organisms on earth. In atmosphere, about 95% of the total air is present upto a height of 20 km above the earth's surface. The remaining 5% of air is present up to a height of about 280 km. Air is a mixture of different gases; nitrogen and oxygen are the major components. Thus, total volume of air present in atmosphere consists of 78% nitrogen 21% oxygen and remaining 1% is made up of other gases such as argon, neon, helium, krypton, xenon and radon. |
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| A thick gaseous envelope surrounding the earth is called atmosphere. The density of air in the atmosphere varies with altitude. |
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| Atmosphere is divided into five different layers or zones: |
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 Troposphere |
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 Stratosphere |
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 Mesosphere |
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 Ionosphere |
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 Exosphere |
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| Composition of atmospheric air |
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| Oxygen is present in the first zone, 10 to 12 km from the surface of the earth. This zone is called troposphere. Therefore, at higher altitudes, some of us find it difficult to breathe. Above the troposphere is the stratosphere. This zone is rich in ozone. Hence, it is also called ozonosphere. |
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| In ozonosphere, the sunlight converts oxygen to ozone by photochemical dissolution. The ozonosphere completely absorbs short wave solar radiations, such as ultraviolet (uv) radiations, x-rays, gamma rays etc. from the Sun. Oxygen and ozone of stratosphere provide protection from harmful solar radiation. It protects us from the bad effects of ultraviolet rays of sunlight. The effects of ultraviolet radiations are: |
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 Cataract - eye lens becomes opaque causing blindness |
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 Inflammatory diseases |
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 Skin cancer |
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| Scientific studies have indicated that a normal human being requires about 250-265 kg air/day for doing various activities. Air is also necessary to grow crops, support animal life and also is a medium of communication. Atmosphere filters sunlight reaching the earth, affects climate and serves as a reservoir of elements essential for life (eg. oxygen, nitrogen, CO2 etc.) |
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| Note: |
| Cloud formation, lightning, thunder etc all take place in the troposphere. Air temperature in troposphere decreases with height at the rate of about 6150C per km. |
| Stratosphere is free from clouds. Aeroplanes usually fly in this zone. |
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