Percentage by Volume
The atmosphere is a mixture of many discrete gases, in which varying quantities of tiny solid particles are suspended. Pure dry air constitutes mainly of nitrogen 78% and oxygen 21% which together make up 99% of the air by volume.
Their proportion remains almost constant within lower layer of the atmosphere close to earth. Remaining 1% is accounted for by gases like argon (0.93), carbon dioxide (0.03), hydrogen, helium and ozone. Besides water vapour, dust particles, smoke, salts and other impurities are also present in the air, in varying quantities.
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. It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface. It is largely responsible for the green house effect.
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 per cent of the air by volume, while in the dry and cold areas of desert and Polar Regions, it may be less than one per cent 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.
Atmosphere has a sufficient capacity to keep small solid particles, which may originate from different sources and include sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors. Dust particles are generally concentrated in the lower layers of the
atmosphere; yet, convectional air currents may transport them to great heights. The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds in comparison to equatorial and Polar Regions. Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce clouds.
Atmosphere can broadly be divided into four layers- the troposphere, the stratosphere, mesosphere and the thermosphere.
Troposphere: - it is the lowest layer of the atmosphere. It extends roughly to a height of 8 km. near the poles and about 18 km. at the equator thickness of the troposphere at the equator is the greatest because heat is transported to great heights by strong convictional currents. Temperature decreases with height in this layer, roughly at the rate of 1°C for 165 metres of ascent. This is known as Normal lapse rate. This layer contains dust particles and over 90% of the earth’s water vapour. All vital atmospheric processes leading to various climatic and weather take place in this layer.
Stratosphere:- beyond the troposphere lies, the stratosphere. The zone separating the two layers is known as the tropopause. Temperature ceases to fall with the increases in height at this level. The air temperature at the troposphere is about 80° C over the equator and about 45°C over the poles. The stratosphere extends up to a height of 50km. in the lower part of this layer i.e. Up to a height of 20 kilometers, temperature remains constant Afterwards it gradually increases up to a height of 50 km because of the presence of ozone layer which absorbs the sun’s ultra violent lays. Clouds are almost absent and there is very little dust or water vapour. The air movements are almost horizontal.
Mesosphere:- it is over stratosphere and extends up to a height of 80 kms. Temperature decreases with height again and reaches up to 108°C at the height of 80 mms.
Thermosphere:- it is located over the mesosphere. In its lower part, there is an electrically charged layer called the ionosphere. Radio waves transmitted from the earth are reflected back to the earth by this layer. Temperature again starts increasing with height because of radiation from the sun. The upper part of the thermosphere is called the exosphere. Here the atmosphere gases are very thin. This part is extremely rarefied and gradually merges with the outer space.