State the important characteristics of each of the layers.

Troposphere

1. It is the lowermost atmospheric layer. Under normal conditions, the height of the troposphere is about 8 km at the Poles and about 18 km at the Equator.
2. All weather phenomena takes place in this layer.
3. In addition to pure air, it contains water vapour and dust.
4. There is decrease in temperature with increasing altitude at the rate of 6.4° C per km in troposphere.
5. Tropopause marks the upper boundary of troposphere.

Stratosphere

1. The lower most limit of stratosphere is at tropopause. It extends from 18 to 80 km.
2. It is free from water vapour and dust.
3. Temperature does not change with altitude. In certain situations slight increase in temperature (with elevation) may be noticed.
4. No weather phenomena is seen accept Cirrus cloud formation in lower part.
5. It has Ozone layer. There is a maximum concentration of ozone, a vital gas between 20-30 km.

Ionosphere

1. Ionosphere extends from 80 km to about 480 km.
2. The ionisation of molecules and atoms occurs mainly as a result of ultraviolet rays, X-rays and gamma rays.
3. The temperature decreases with increase in elevation until a low point of -100℃ is reached. Beyond this level, the temperature increases again as a result of absorption of short wave solar radiation by the atoms of oxygen.
4. Nitrogen in the extremely rarified air of ionosphere.
5. Ionosphere reflects low frequency radio waves, but absorbs medium and high frequency waves, making it especially important in long distance radio communication.

Exosphere

1. It is the outermost layer of atmosphere.
2. It contains rarified Hydrogen and Helium gases.