How IMD defines advancement of Monsoon in India

The Indian Meteorological Department, IMD, is the official Government weather forecaster for India. Monsoon is the most important season in India, which accounts for a huge chunk of the Indian economy. India’s agriculture, pisciculture, floriculture and to some extent logistics are vastly affected by seasonal variations in the monsoon. That is why, IMD makes 2 predictions on monsoon, one at the start of the season in June, and one in the middle of August. Also, IMD notes down the progress, withdrawal and activeness in almost every cities, districts, and even at village level.

How can IMD declare the onset of monsoon? There are 3 well-defined criteria to declare the onset of monsoon.

  • The first, which is the most visible one, is the rainfall. At least 60% of the stations in the area must receive rainfall of 2.5mm or more for 2 consecutive days.
  • The second one is the wind pattern, in which wind should blow more or less continuously at 27-37 kmph from the south-west direction.
  • The third one is the least visible of all, and that is the outgoing long wave radiation should be below 200 watt per square metre.

Similar to onset of monsoon, the withdrawal of monsoon also follows 3 not-so-similar criteria.

  • First one is no rainfall for 5 consecutive days.
  • The second one is development of anticyclone in the lower troposphere.
  • The last one being considerable reduction in moisture content of air.

Skymet is the other official weather station which is private. For the past 3 years, Skymet and IMD failed to agree upon the onset and withdrawal of monsoon from India. Skymet following IMD’s criteria, usually agrees with IMD, but this has failed in the last 3 years by some margin. In 2020 and 2021, Skymet announced the arrival of monsoon before IMD did, for which IMD stated the predictions as ‘factually wrong’. In 2022, IMD announced the arrival before Skymet did, and the Skymet accused IMD for ‘not following its own criteria’. However, even after all this, the arrival monsoon is after a sultry summer and a dry spell of 7-8 months brings a respite to farmers and city-dwellers alike.

Written by – Himadri Paul

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