Composition Of The Atmosphere
The atmosphere is composed of gases, water vapour, and dust particles. The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth.
- Carbon dioxide is meteorologically a very important gas as it is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation.
- By volume, air contains 78.09% nitrogen and 20.95% oxygen. Hence, both together make up 99% of the gases of the atmosphere.
- It absorbs a part of terrestrial radiation and reflect back some part of it towards the earth’s surface. It is largely responsible for the greenhouse effect.
- The volume of other gases is constant but the volume of carbon dioxide has been rising in the past few decades mainly because of the burning of fossil fuels.
- Ozone is another important component of the atmosphere found between 10 and 50 km above the earth’s surface and acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth.
- Water vapour is also a variable gas in the atmosphere, which decreases with altitude. In the warm and wet tropics, it may account for four percent of the air by volume, while in the dry and cold areas of desert and Polar Regions; it may be less than one percent of the air.
- Water vapour also decreases from the equator towards the poles.
- It also absorbs parts of the insolation from the sun and preserves the earth’s radiated heat. It thus acts like a blanket allowing the earth neither to become too cold nor too hot. Water vapour also contributes to the stability and instability in the air.
It is the source of all kinds of precipitation.
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The atmosphere has a sufficient capacity to keep small solid particles, which may originate from different sources and include sea salts, fi ne soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors.
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Dust particles are generally concentrated in the lower layers of the atmosphere; yet, convectional air currents may transport them to great heights.
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The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds in comparison to equatorial and Polar Regions.