Competitive Exams: Rainfall

When the rain-laden clouds rise up, they expand and become cooler: They also become cooler because they are moving away from hotter regions. The coolings of the air resulting in the precipitation of rains etc. may be due to three causes:

  1. Relief: Temperature decreases as we ascend a hill or a mountain. If, then a wind blowing from the sea and saturated with water-vapour comes to a mountain, it i, forced to rise. On rising the clouds reach thinner layers of air and consequently it expands. Expansion produces cooling. The moisture in the clouds is then condensed and falls as rain. Such rains are called the Relief Rains. When rain laden winds blow over a high mountain, they are forced to rise up, They are cooled, and lose some of their moisture in the form of rain. The higher the mountain, the greater the rainfall.

  2. Convection: When air is heated, it tends to rise up. As it rises, it is cooled and rain falls even though there are no mountains nearby. The air at the Equator is intensely heated almost each day and hence it rises each day, causing rain each afternoon. This rising of the wind when heated, is called convection, and the rain that is so caused is. Called the Convectional Rain.

  3. Cyclones: Cyclones or depressions are large areas of low pressure. A cyclone has low pressure at the centre, and it brings rainy or stormy weather. The rain brought about by the a cyclone is called the ‘Cyclonic Rain’ Trade Winds: These are the winds which blow from the high pressure belts, lying north of the Tropic of Cancer and south of the Tropic of Capricorn, towards the Equatorial Low Pressure Belt. Anti-Trade or Westerly Winds: These are again the low pressure belts over the cold region round the latitude of the Arctic and Antarctic Circles. Therefore, a part of the air moves northwards from the Calms of Cancer and southwards from the Calms or Capricorn. Because they blow in the direction opposite to the Trade Winds, they are also called Westerlies, because they come from the west. Polar Winds: There are high pressure regions on both the poles caused by extreme cold.

Winds, therefore, blow from the North Pole to the Arctic Circle and from South Pole to the Antarctic Circle. These winds are known as Polar Winds. Monsoon Winds: The word monsoon is derived from an Arabic word ‘mausim’ meaning “season” Monsoon may be regarded as land and sea breezes on a seasonal scale. We c. An picture the air over a great mass of land which becomes very hot during the summer. The monsoon system is associated with the lands of South-East Asia, Eastern Africa and Northern Australia.