Published on: November 8, 2021




The overnight rain in Chennai that lasted recently  was reportedly the heaviest since 2015

Why the rain

North East monsoon

  • Northeast Monsoon, also known as the ‘primary monsoon of Tamil Nadu,’ that brings sufficient rains to the state
  • Tamil Nadu’s coastal districts get 60% of the annual rainfall and the interior districts get about 40-50% of the annual rainfall from the North East monsoon.

Low pressure formation

  • The meteorological department predicted a formation of low pressure over the Bay of Bengal moving towards the northern Tamil Nadu coast with moderate rain in the coming days ahead of the formation of the low pressure.
  • A cyclonic circulation was over north coastal Tamil Nadu, southeast of Bay of Bengal and a low-pressure area was formed
  • Chances of squally winds of 40 kmph to 50 kmph gusting to 60 kmph might prevail over the sea


Northeast Monsoon

  • Though much less heard of, especially in the north of the country, the northeast monsoon is as permanent a feature of the Indian subcontinent’s climate system as the summer monsoon.
  • The India Meteorological Department (IMD) recognises October to December as the time for the northeast monsoon.
  • During this period, rainfall is experienced over Tamil Nadu, Kerala, and Andhra Pradesh, along with some parts of Telangana and Karnataka.


  • Cyclones are centred on areas of low atmospheric pressure, usually over warm ocean waters near the equator.
  • The warm moist air over the ocean rises from the surface in the upward direction, resulting in the formation of the low-pressure zone over the surface. Air from the surrounding region, with higher pressure, pushes into the low-pressure area.
  • The cool air becomes warm and moist and rises again, thus the cycle continues. As the warm air rises, the moisture in the air cools thus leading to the formation of cloud.
  • The whole system grows gradually and becomes fast with time. As a result of this, an eye is created in the centre, which is the low-pressure centre into which the high-pressure air flows from above, thus creating a cyclone.