Monsoons , What causes Monsoon ,South-West Monsoon,Progress on Land,India and Monsoons


Origin and Scope

  • The English word monsoon has been derived from Portuguese monçãom which probably originated from a Dutch word which is said to have derived from the Arabic word ‘Mausim’, which can be translated as ‘Season’.
  • In modern parlance, Monsoon means a Periodic rainy season. Here the word periodic means – which is active during a decided period.
  • Historically monsoons are significant because it’s fixed south westerly direction allowed sailing vessels to cross the Arabian sea from the African and the Arabian lands towards India.
  • Initially the word monsoon applied only to the seasonal winds in the Arabian sea, now it has acuired generic connotation. It now has a broader meaning to include almost all of the winds which have a definite annual cycle in the tropical and sub-tropical regions of the earth.

What causes Monsoon

  • Monsoons are caused by the cyclical and large temperature differences between the land and the  nearby oceans.
  • The pressure and the wind systems which cause monsoons are:-
    • The permanent low over the equator called the “Doldrums”. (1012 hPa)‏
    • The permanent Oceanic High around 30° North and 30° South. (1020hPa – 1025 hPa)‏
    • Seasonal highs and lows over Asian land mass of Siberia and Thar desert .
    • Trade winds which blow from the tropical highs to the equator as follows:-
      • North Easterly in the NH
      • South Easterly in the SH.

South-West Monsoon

  • The southwest monsoon occur from start of June till end of September.
  • Northern Hemisphere starts getting warmer from Spring Equinox (March) and reaches its peak at by Summer Solstice (July)‏
  • Atmospheric Pressure in NW of the Indian sub-continent (Thar desert)  drops upto 994 hPa. The Equatorial region generally   remains at 1012 hPa  and due to pressure gradient, winds from the Equator blow towards the Thar low.
  • In the SH, the SEly trade winds, blowing from SH High (30°S), cross the equator and continue their journey into the NH.
  • Because of the Coriolis force, the winds turn right and augment the SWly wind. 
  • The combined winds pick up moisture from the Indian Ocean and the Arabian sea before arriving on the Indian Subcontinent.

Progress on Land

  • The moisture-laden winds on reaching the southern-most point of the Indian peninsula, due to its topology, become divided into two parts:
  • The Western Branch of the SW Monsoon hits the Western Ghats of the coastal state of Kerala,  moves northwards giving  rains to the coastal areas west of the Western Ghats.
  • The Eastern Branch of SW Monsoon blows over the Bay of Bengal heading towards North-Eastern India and Bengal, picking up more moisture from the Bay of Bengal.
  • Himalayas act like a high wall and do not allow the winds to pass into Central Asia.
  • After hitting  the Eastern Himalaya  the rain bearing clouds turn towards West, giving rains to Indo-Gangetic Plain.
  • In the Northern part of the Arabian sea, extensive clouds and precipitation occur due to presence of a depression called subtropical cyclone.
  • Monsoons are also experienced in China but not as intensive or extensive as in Indian sub-continent.

Winds during SW Monsoon

  • In the Arabian sea, wind force excess of force 6 prevail for 10 days of a month.
  • The roughest seas are east of Socotra wind force excess of force 7 prevails 50% of time in July.
  • In Bay of Bengal, Force excess of 7, for about 5 -10 days of a month in July and August.
  • Between Equator and 5 deg North and East of 60deg E, winds are milder between force 3 and direction is from South or west. .

North East Monsoon

  • NE Monsoons occur during winters in the Northern Hemisphere.
  • The cooling of NH starts after the sun crosses the Equator at Autumn Solstice (September)‏
  • High pressure prevails over Siberia (1036 hPa).
  • The Equatorial region generally   remains at 1012 hPa  and due to pressure gradient, winds from the Siberian High  blow towards the equatorial regions.
  • Anti-cyclonic winds from the around the Siberian High travel over Bay of Bengal and reach India as NE Monsoon.
  • Heavy rains on the Eastern Coast and moderate rains in Kerala
  • In China sea, NE monsoons are stronger and may reach upto force 7.
  • The NE-ly winds subsequently cross the equator and continue their journey into the SH.
  • Because of the Coriolis force, the wind turn left and overcome weaker SE trades.   (But not as strongly as the SW monsoons)
  • The wind picks up moisture from the South Indian Ocean as they reach the NW coast of Australia giving them monsoonic rains.

In Indian Weather bulletins classification of monsoons is as follows.

  • Weak                           Force 3 and less                     (< 11 Kt)‏
  • Moderate                     Force 4 & 5                             ( 12 – 22 Kt)‏
  • Strong                         Force 6 & 7                             (23 – 34 Kt)‏
  • Vigorous                      Force 8 and over                     (34 Kt & above)‏

India and Monsoons

  • The South Asian monsoon affects a larger number of people due to the high density of population in this part of the world.
  • The monsoon accounts for 80 percent of the rainfall in the country[citation needed]. Indian agriculture (which accounts for 25 percent of the GDP and employs 70 percent of the population) is heavily dependent on the rains, especially crops like cotton, rice, oilseeds and coarse grains.
  • A delay of a few days in the arrival of the monsoon can, and does, badly affect the economy, as evidenced in the numerous droughts in India in the 90s.
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