Atmosphere:

The layer of the gases which surrounds the earth is known as the atmosphere.
OR
The atmosphere is a thin layer of the gases held to the earth by the gravitational attraction.
OR
Atmosphere is the huge blanket of gas that circles the entire earth.

Composition Of The Atmosphere:
The atmosphere consists of:
i) Nitrogen: 78.03%
ii) Oxygen: 20.99%
iii) Argon: 0.94%
iv) CO2: 0.03%
v) Hydrogen: 0.01%
vi) Neon: 0.0018%
vii) Helium: 0.0005%
viii) Krypton: 0.0001%
ix) Ozone: 0.000001%

Layers Of The Atmosphere:
The atmosphere of the earth is divided into following layers.
i) Troposphere
ii) Stratosphere
iii) Ionosphere
iv) Exosphere
Ionosphere is sub-divided into:
i) Mesosphere
ii) Thermosphere

1) Troposphere:
i) Troposphere is the lowest layer of the atmosphere.
ii) It extends roughly to a height of 8 kms near the poles and 16-18 kms at the equator.
iii) It is troposphere where the people, plants, animals and insects live.
iv) It is the layer where all weather occurs that’s why it is also referred as “The weather Zone”.
v) In the Troposphere temperature gradually falls with increasing altitude.
vi) There is a thin buffer zone between the troposphere and stratosphere is called tropopause.

2) Stratosphere And Ozone Layer:
i) The second layer of the atmosphere is known as the stratosphere.
ii) The stratosphere extends from the tropopause to about 30-31 mile above ground level.
iii) The important ozone layer is found in this region where heat is generated by absorption of UV.
iv) Here the temperature either remains constant or increases with altitude.

Stratopause:
It is the upper boundary of the stratosphere which occurs beyond 52 kms. Here the temperature remains constant with increase in height.

3) Ionosphere:
i) The third major layer of the atmosphere is the ionosphere.
ii) It lies above the stratosphere.
iii) It lies between about 30 and 90 miles above the surface of the earth.
iv) It is divided into mesosphere and thermosphere.
v) The ionosphere is bombarded by cosmic radiation and solar x-rays, which causes the gases in the ionosphere to ionize.
vi) Brilliant displays of colored lights in the sky called Aurora Borealis in the northern hemisphere and the Aurora Auralis in the southern hemisphere occur when streams of electrically charged particles from the sun (solar wind) ionize the atmosphere gases.

4) Exosphere:
i) The uppermost layer of the atmosphere extending beyond Ionosphere is called exosphere.
ii) Beyond 300 miles is the very rare field exosphere which consists only of scattered atmosphere of O, H and He.