Rainfall

Orographic or Relief Rainfall

When the warm moist winds are obstructed by a mountain range, they are forced to ascend along its slopes. These moist winds get cooled, while ascending. When their temperature falls below dew point, clouds are formed. These clouds cause widespread rain on the windward slopes of the mountain range. This type of rain is called orographic rainfall. However when these winds cross over the mountain range and descend along the leeward slopes, they get warm and dry. Therefore, they cause little rain. Areas lying on the leeward side of the slopes are called rainshadow areas.

Most of the rainfall which occurs in India is orographic. The south-west monsoon winds are forced to rise-up due to the obstruction caused by the Western Ghats. Consequently the western slopes of the Western Ghats receive very heavy rainfall. But when the same winds descend on the eastern slopes of the Western Ghats, they become hot and dry and cause little rainfall. The rain shadow areas of the Western Ghats are almost dry. For example, Mangalore located on the western slope of the Western Ghats receives mean annual rainfall of above 200 cm whereas Bengaluru situated in the rain shadow area receives only 50 cm mean annual rainfall.

Cyclonic or Frontal Rainfall

Rainfall associated with a cyclone is known as cyclonic or frontal rainfall. In the temperate lands, when a warm air mass converges with a cold air mass, it rises up along the slope of the front. Saturation and condensation take place and cause rainfall. This type of rainfall normally occurs along the paths of temperate cyclones. Winter rainfall in north-west India is also caused by cyclones. In tropical cyclones, there is convergence of air from all sides towards the centre which cause heavy rainfall.

Distribution of Rainfall

Heavy Rainfall

Areas receiving over 2000 mm of annual rainfall are termed as area of heavy rainfall. These areas include the equatorial belt, the windward slopes of the mountains along the western coasts in the cool temperate zone and the coastal areas of the monsoon lands.

Moderate Rainfall

This includes areas having 1000 to 2000 mm of annual rainfall. Areas adjacent to heavy rainfall regime are the areas of moderate rainfall. The coastal areas in the warm temperate zone also receive moderate amount of rainfall.

Inadequate Rainfall

These areas receive 500 to 1000 mm of annual rainfall. The central parts of the tropical lands and eastern and interior parts of temperate lands are included in this category.

Low Rainfall

Areas receiving less than 250 to 500 mm of annual rainfall are known as areas of low rainfall. Western margins of the continents in the tropical lands and the arid deserts come under this category. Areas lying in the rain shadows, the interior of continents and high latitudes also receive low rainfall of less than 500 mm per annum.

Extremely low Rainfall

There are vast tracts in the hot and cold deserts which receive extremely low annual rainfall of less than 250 mm. These include Sahara desert, Arabian desert, Atacama desert, Kalahari desert, ar desert, Australian desert and cold deserts of Central Asia and Tundra.

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