The Earth is a unique planet in having conditions favourable for life. The oxygen layer surrounding it is essential to all life forms. The air envelope also moderates the temperature on the Earth. So the day-night and summer-winter temperature variations are not very high unlike on other planets. The presence of water favours the growth and evolution of different living specie. Thus the earth is unique in having a biosphere or life-bearing layer on it.
The atmosphere, held to the Earth by gravitational force, is the air blanket, which surrounds the Earth. Of the atmosphere, 99 % is within a height of 32 kilometre from the earth surface. Most atmospheric changes take place within this layer. An average sample of pure dry air consists of
Nitrogen (78 %)
Oxygen (21%) and
Argon (0.9 %)
Carbon Dioxide, Hydrogen, Helium and Ozone - Minute quantities
Though water vapours are not more than 3-4 % of the total air volume, they are important in atmospheric processes.
The lowest layer of the atmosphere is the troposphere. All weather phenomena take place here. It extends upto 18 km. over the equator and about 8 km. along the poles. Here, the air temperature decreases with height at an average rate of 1OC for 165 metres, called the lapse rate. The upper limit of this layer is known as the tropopause.
Above it lies the stratosphere, with a thickness of 50-55 km. Here, the temperature is constant and then increases with height. This layer, being free from clouds, is ideal for flying jet aircraft. The ozone layer is present here, which absorbs harmful ultra-violet radiations from the sun. Above the stratosphere are the mesosphere and the ionosphere. The ionosphere contains charged particles called ions. This layer reflects radio waves back to the earth‘s surface making wireless communication possible. The ionosphere is followed by the uppermost layer, the exosphere, with no fixed upper limit.
The atmosphere is an important part of the physical environment. The differential heating of the atmosphere by the sun rays produces circulation of air, leading to winds, clouds and precipitation.
The “Lithosphere” is the layer of rock material on the earth’s surface, on continents and ocean floors. With an average thickness of about 60 km., it supports diverse life forms. The crustal layer is of lighter density and contains rocks rich in aluminium and silica, hence leading to its name the sial layer. Below it is the mantle, upto a depth of 2900 km. The mantle has an inner silicate layer, rich in magnesium and aluminium, and a transitional zone of mixed metals and silicates.
The core of the earth contains metals in liquid or plastic shape due to high temperature and pressure. The core, with a radius of 3400 km., contains great amounts of nickel and iron, leading to its name Nife layer.