The atmosphere

The vast expanse of air which envelops the earth is called atmosphere. Among the four elements of environment, the atmosphere is the most dynamic as changes takes place in it not only from one season to another but also over shorter periods of a few hours.The atmosphere extends to thousands of kilometres above the Earth's surface but it has no well defined upper limits and gradually merges with the outer space. Of the total mass of the atmosphere, about 99% is within a height of 30 kilometre from the Earth surface.It is within this layer that most of the atmospheric changes take place.

           The atmosphere contains life giving gases like oxygen for man and animals, and carbon dioxide for plants to be used in manufacture of food. It acts like a greenhouse by trapping the heat.like the glass in a greenhouse, the atmosphere allows shortwave radiation to enter it and reach the earth surface.atmosphere protects the earth from the harmful radiation from the sun. It also serves as a storehouse for water vapour which leaves the precipitation fairly distributed over land and sea. The presence of air and water on the earth makes it a unique planet in the solar system.

Structure of atmosphere

The atmosphere consists of almost 

concentric layers of air with varying density and temperature. Density is 

highest on the earth's surface and decreases rapidly upwards. In the atmosphere,  broadly 5 layers can be identified i.e troposphere,  stratosphere, mesosphere, ionosphere and the exosphere.

Troposphere

This is the lowest layer of atmosphere, thus laying closest to the Earth's surface. it extends roughly to a height of 8 kilometre near the poles and about 18 kilometre at the equator. It is thickest at the equator as heat is transported to great heights by strong conventional currents.temperature in this layer decreases with increasing height, roughly at rate of 1 degree Celsius for 165 metres of ascent.this is known as normal lapse rate. Troposphere contains dust particles and over 90% of the earth's water vapour. All important atmospheric processes leading to various climatic and weather condition take place in the troposphere, hence most significant layer.aviators of jet aeroplanes are often avoid this layer due to presence of bumpy air pockets.

Stratosphere

It lies beyond the troposphere, and the zone separating the two is called tropopause. Stratosphere extends upto a height of 50 km. Temperature ceases to fall, and remain constant up to a height of 20 kilometre.afterwards it gradually increases up to a height of 50 kilometre due to presence of ozone layer which absorbs the ultraviolet radiations of sun. Clouds are almost absent and very little dust or water vapour. The air movements are almost horizontal. air temperature at the tropopause is about - 80 degree Celsius over the equator and about -45 degree Celsius over the poles. 

Mesosphere

this is the third layer over the stratosphere extending up to a height of 80 kilometre. Temperature decreases with height again reaching up to -100 degree Celsius at the height of 80 kilometre.

Ionosphere

The fourth layer, ionosphere is located between 80 and 400 kilometre. it is an electrically charged layer, in which ions reflect radio waves back to the Earth surface and enable wireless communication. Temperature again starts increasing with height due to radiation from the 🌞.

Exosphere

This is the uppermost layer of atmosphere extending beyond the ionosphere above a height of about 400 km. This layer is extremely rarefied and gradually merges with the outer space.