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| Air |
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| Air is an inexhaustible natural resource, essential for the survival of all the living organisms on earth. Air is a mixture of different gases; nitrogen and oxygen are the major components. 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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| Scientific studies have indicated that a normal human being requires about 250-265 kg air/day. Air is also necessary to grow crops, support animal life and is a medium for the sound waves to travel. |
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| About 95% of the total air in the atmosphere, 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. |
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| A thick gaseous envelope surrounding the earth is called atmosphere. The density of air in the atmosphere varies with altitude. It 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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| 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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| We are concerned here only with the first two layers. |
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| Troposphere |
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| The first zone, 10 to 12 km from the surface of the earth is called troposphere. Oxygen is present here. At higher altitudes, some of us find it difficult to breathe, as the oxygen content is reduced. |
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| Stratosphere |
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| Above the troposphere is the stratosphere. This zone is rich in ozone. Hence, it is also called ozonosphere. In ozonosphere, oxygen in the presence of sunlight converts to ozone by photochemical dissolution. The ozonosphere completely absorbs short wave solar radiations, such as ultraviolet (uv) radiations, x-rays, gamma rays etc. Oxygen and ozone of stratosphere provide protection from harmful solar radiation and the bad effects of ultraviolet rays. Stratosphere is free from clouds. Aeroplanes usually fly in this zone. |
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| 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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| Cloud formation, lightning, thunder etc all take place in the troposphere. |
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