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Onset of Indian Monsoon (The Summer Season) – Principles of Indian Geography – Free PDF Download


  • Apparent movement of the sun towards tropics of cancer.
  • This intense heating creates a low-pressure trough that extends from the Punjab plains in the northwest to the Bengal delta in the east.
  • Along with low-pressure we also have
  1. The presence of the subtropical jet streams blows to the south of the Himalayas.
  2. The presence of a subtropical high-pressure belt over the northern plains.

The onset of Southwest Monsoon

  • No specific date for the onset of the monsoon, the onset is marked by the movement of ITCZ to 20-25 °N latitude and the STJ moves completely over the Himalayas.
  • The difference in pressure gradients becomes so high during the months of may and June that it starts attracting wind from as far as south of the equator.
  • The southwest monsoon winds are divided into two branches namely the bay of Bengal branch and the Arabian sea branch.

The Bay of Bengal Branch

  • The main part of the Bay of Bengal branch strikes the windward slopes of the west-coast Mountains of Myanmar (the Arakan and Tenasserim mountains)
  • big portion of this branch gets deflected towards the Indian subcontinent due the orographic/mountain barrier.
  • One of the branches moves along the Ganga plains reaching as far as Punjab.
  • The other branch moves along the Brahmaputra valley in the north and northeast causing rains.
  • A part of these current strikes the hills (Khasi Hills)of Meghalaya and results in heavy rainfall in Mawsynram (near Cherrapunji).

The Arabian Sea Branch

  • One of the branches of the Arabian sea branch strikes the Western Ghats at almost right angles i.e. orographic precipitation.
  • Upon crossing the Western Ghats, the rain-bearing air currents descend the eastern slopes where they are warmed up adiabatically.
  • The higher the mountains, the larger the rain-shadow effect.
  • 2nd branch moves along the Narmada and Tapi river valleys. They cause extensive rainfall to areas of central India.

  • The third branch of this monsoon wind enters India via the Saurashtra Peninsula and the Kachchh region.
  • The re-inforced branches of the Bay of Bengal and Arabian sea cause rainfall over the western Himalayas.

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