The atmosphere that surrounds the earth is not of the same thickness at all heights. There are concentric layers of air or regions and each layer has different density.

The lowest region of atmosphere in which the human beings along with other organisms live is called troposphere. It extends up to the height of $\sim 10 \mathrm{~km}$ from sea level.

Troposphere is a turbulent, dusty zone containing air, much water vapour and clouds. This is the region of strong air movement and cloud formation.

The region between $10 \mathrm{~km}$ to $50 \mathrm{~km}$ above the sea level lies is called stratosphere.

Stratosphere, on the other hand, contains dinitrogen, dioxygen, ozone and little water vapour The presence of ozone in the stratosphere prevents about $99.5 \%$ per cent of the sun's harmful ultraviolet ($UV$) radiations from reaching the earth's surface and thereby protecting humans and other animals from its effects.