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World Geography / Climatology

Distribution of Rainfall in World


    Distribution of rainfall in world depends on the climatic conditions of different regions. Precipitation or rainfall happens mainly due to the warm era. Rainfall occurs when winds carrying moisture rise up and expansion ( adiabatic ) takes place and hence cools which in turn results in condensation. If winds are descending, there will be compression which results in heating and starts absorbing the moisture. Descending winds never cause rainfall.

    Types of Rainfall

    There are three types of rainfall around the world namely, Orographic rainfall, Convectional rainfall and Cyclonic rainfall.
    • Orographic Rainfall - Warm and humid winds when obstructed by a mountain or tableland, they rise along with the slope. When they rise, there is huge amount of rainfall on the windward side. After crossing the mountain, they descend where there will be less rainfall in the leeward side. As an example, the rainfall in the windward side of Western Ghats is 200 cm whereas on the rain shadow region ( leeward side ), it is 50-60 cm.
    • Convectional Rainfall – They are very common in Equatorial region. In Equatorial region, temperature is high and winds move up. When they reach some height, they get cooled down and cause heavy rainfall. Convection is there throughout the year ( as temperature is high ) and hence there is rainfall throughout the year. 300 out of 365 days are rainy days. There will be rain everyday from 3.30pm to 4pm.
    • Cyclonic Rainfall – These are intense form of weather disturbances. They are associated with very high rainfall.

    Rainfall around the World

    • High Rainfall Regions – These are areas of per-humid climate
      • Equatorial Region – These regions are mainly Amazon basin, Congo basin, Malaysia and Indonesia. They are convectional rains which fall throughout the year.
      • Eastern Margins of Continents ( 100 – 300 ) – Trade winds reach Eastern margins of the continents from oceans and cause heavy rainfall. These areas include South Asia and Southeast Asia ( Monsoon ), Caribbean region, East coast of Brazil, Eastern part of Central America.
      • Western Margins of Continents ( 400 – 600 or Mid-latitude regions ) – The rainfall here is due to Westerlies which are fresh and virgin.
    • Low Rainfall Regions – These are the areas of arid climate
      • Western Margins of Continents in Tropics and Subtropics ( 100 – 300 ) –
        • These are called Hot or Tropical deserts. These are also called Trade Wind deserts.
        • In Australia, the regions include Western Australia deserts like Great Victoria Desert, Great Sandy Desert and Gibson Desert. These deserts are formed due to West Australian cold current.
        • In North America, the regions include Mexican or Mojave Desert. It is formed due to California cold current.
        • In South America, the regions include Atacama Desert of Northern Chile. It is formed due to Peruvian or Humboldt cold current.
        • In Africa, the regions include Sahara Desert in Chad, Libya, Mauritania, Egypt, Ethiopia and Somalia. It is formed due to Canary cold current. The other regions include Namibian Desert or Kalahari Desert in Namibia and Botswana. It is formed due to Namibian or Benguela cold current.
        • In Asia, the regions include Arabian Desert in Saudi Arabia, Kuwait and Bahrain and Thar Desert in India and Pakistan.
        • The reasons for very low rainfall in these areas are, (i) By the time Trade Winds reach these areas, they are exhausted and become dry, (ii) 300 is essentially high pressure area, where subsidence takes place. So, there is no probability of rainfall and (iii) Presence of offshore cold ocean currents.
        • In these areas, cold and dry winds blow to the continents. They absorb the moisture over the land. Rainfall in these deserts is less than 15 cm.
      • Interior Highlands of Continents ( 300 – 600 ) –
        • These are cool or temperate deserts. As temperatures are low, they are called cool or temperate deserts.
        • The regions include Gobi Desert in Western China, Mongolia and Kazakhstan, Patagonian Desert ( High Plateau region ) in Argentina, Tibetan Plateau, Iranian Plateau, Balochistan Plateau, etc.
        • These deserts are formed because (i) As they are located deep inside the continents, winds become dry by the time they reach there and (ii) Winds carrying moisture cannot enter these areas as they are inter-mountainous regions surrounded by hills and mountains.
      • Polar and Sub-Polar Regions – These are cold deserts. The areas include Greenland, Iceland and North Canada. Here low temperature causes no humidity.