12. Rainfall

Rain is liquid precipitation of water droplets falling from the sky. Raindrops fall to Earth when clouds become saturated with water droplets, and they become heavy enough to fall under gravity.

 

Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It is the primary source of water for almost all purposes. There is a close relationship between temperature and rainfall distribution. Generally, rainfall is high in the equatorial region and decreases gradually towards the poles. A rain gauge is used to measure rainfall.

Convectional rainfall occurs in the regions where the Earth surface is intensely heated through the sun's rays during the daytime or in the summer.  When the air near earth's surface is heated, it rises and expands as the heated air becomes lighter, resulting in the formation of convectional air currents. On ascending, its moist vapour cools and turns into a cumulo-nimbus cloud with a great vertical extent and results in convectional rainfall.

This rainfall occurs regularly in the equatorial region in the evenings. Convectional rainfall is also experienced in tropical, sub-tropical and temperate regions in the summer months and on warmer days.

This type of rainfall is independent of relief or convection. It is purely associated with cyclonic activity. Basically, it is due to the convergence of two different air masses with different temperatures and other physical properties, i.e. due to adiabatic cooling (due to upward movement of air mass). It occurs in tropical as well as temperate regions. When warm and cold air masses converge, the warm air rises, condenses, and precipitation takes place on the boundary between warm and cold air masses called Frontal rainfall.

The relief rain is formed wherever moist air is forced to ascend a mountain barrier. The ascending moisture-laden air cools and results in the formation of clouds and rain. In this type of rainfall, the windward side gets maximum rainfall. This is called a rainfed region. The leeward side gets little or no rainfall. This is called a rain shadow region.