Geology - The Exterior of the Earth

We know that the gaseous layer encircling the Earth outside it's surface is called the atmosphere. This gaseous environment extends to about 500 km beyond the Earth surface. Although , extending to far such large distances , this atmosphere weighs only about one millionth part of the weight of the Earth. This is because of its gaseous composition.
This atmosphere has been established to be consisting of layers of different environments. As such , it can be divided into the following four zones :-

(1) Troposphere :- It is the nearest gaseous zone extending from the surface of the earth upto a average distance of about 11 km at the poles and 17 km at the equatorial regions.
Temperature decreases with the height in the troposphere , since the main heat source is the solar radiations which is absorbed at the ground level. The rate of decrease of temperature , called the lapse rate , is about 5* C/km. The troposphere contains most of the water vapour , clouds , and storms of the atmosphere Winds tend to be the strongest at the tropopause , the level of the jet streams. This is also the level at which the jet aircrafts cruise. The troposphere contains about three-fourth of the total weight of the atmosphere, and is responsible for the most of the meteorological processes. Thus it is the troposphere , in which winds are generated and clouds are formed , which cause precipitation.

(2) Stratosphere :- Above the tropopause (at about 10-16 km altitude) lies another zone , called the stratosphere , extending upto about 50 km altitude , to a point called the stratopause. Between the tropopause and stratopause , the temperature increases gradually with height , from about - 80*C to about 0*C or so. At the stratopause , the temperature , infact , is about as heigh as at the ground level. It's warmth due to the absorption of ultraviolet radiation from the sun by oxygen and ozone. Most of the worlds ozone is found in the stratosphere , where it may exceeds 5 ppmv (parts per million by volume). Hence , stratospheric air is lethal to human beings. There is very little water vapour at these heights. Ozone is , infact , formed in this zone due to the splitting of an oxygen molecule into the atomic oxygen and it's subsequent union with an oxygen molecule to form Ozone. Ozone is , however , destroyed naturally by ultraviolet radiation , as it helps in preventing ultraviolet radiation from the reaching the ground. In the stratosphere , ozone is , thus , constantly being formed and destroyed , and is , therefore , maintained in a rough equilibrium , under natural unpoluted environment.
Since , ozone is formed in the stratosphere , this zone is also sometimes called as the ozonosphere. The ozonosphere is of utmost importance to life on the Earth. As it absorb most of the ultraviolet radiations from the sun , which in its absence , would possibly have burnt all the life on Earth.

(3) Mesosphere :- The Mesosphere extends from the stratopause at about 50 km altitude and at about 0* C to another temperature minimum of about - 110* C at about 80 km altitude. This top boundary of Mesosphere is called mesopause. The atmosphere in this zone is windy and turbulent , but there is usually too little water vapour for clouds to form.

(4) Thermosphere :- Above the mesopause ( 80 km altitude ) the temperature increases indefinitely upward into the Thermosphere ( I.e the hot upper atmosphere ) , which zone may extend upto about 500 km altitude.
The Mesosphere and the Thermosphere zones contains many ions and free electrons. Cosmic rays and radiations from the sun produces these ions. Both these zones together are , therefore , usually referred to as the ionosphere. The ionosphere is very light in mass , despite having a large volume.
The region of atmosphere beyond about 500 km , is called exosphere , and till today , very little is known about it. It is perhaps a low density , high temperature region , with minimum atomic collisions.
As far as the chemical compositions of the atmosphere is concerned , it has been found to be mainly a mixture of approximately 78% Nitrogen and 20% oxygen by volume. Other gases like carbondioxide , argon , neon , helium , ozone and water vapours will be constitute the balance 2 % . This consumption is fairly constant in the region of troposphere and a large part of stratosphere. Dust particles and liquid droplets are also found present in the lower atmosphere , and water vapours may be present in large variation.