Write explanatory notes on the following: (a) formation of clouds; (b) relief rainfall.

Answer Details

(a) Formation of clouds

Clouds are masses of tiny water droplets or ice crystals floating in the sky. They form when warm, moist air rises and cools. As the air rises it expands and its temperature falls; when it cools to the dew point the water vapour it carries can no longer stay as a gas, so it condenses around tiny dust particles (condensation nuclei) in the atmosphere. Countless tiny droplets so formed collect together to become a visible cloud. The air may be forced to rise by heating of the ground (convection), by being pushed over mountains (relief), or where warm and cold air meet (frontal uplift).

(b) Relief (orographic) rainfall

Relief rainfall forms when moist air from the sea is forced to rise over a highland or mountain barrier. As the air climbs the windward slope it cools, its vapour condenses into cloud and heavy rain falls on that side of the mountain. Having lost most of its moisture, the air descends the far (leeward) slope, where it warms and dries; this drier side receives little rain and is called the rain shadow. Relief rainfall therefore gives heavy rain on the windward slope and a dry rain shadow on the leeward slope.