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Onset of monsoon

Date: 16 May 2022 Tags: Geography & Environment


The India Meteorological Department (IMD) has announced that southwest monsoon is likely to set in over Kerala on May 27.



Since 2009, this will be the earliest onset of the monsoon over Kerala, if the forecast turns out to be true.



  • The onset of monsoon will signal arrival of the four-month, June-September southwest monsoon season over India. The season contributes to about 70 percent of India’s rainfall needs.

  • The monsoon onset marks significant changes in the large-scale atmospheric and ocean circulations in the Indo-Pacific region.

  • The onset is announced only after certain newly defined and measurable parameters are met. These parameters were adopted in 2016.


Parameters for declaring onset

  • If 60% of 14 designated meteorological stations in Kerala and Lakshadweep must witness at least 2.5 mm of rain for two consecutive days at any time after May 10. 

  • The depth of westerlies must be upto 600 hectopascal in the area bound by the equator to 10ºN latitude, and from longitude 55ºE to 80ºE. 

  • Outgoing Long-wave Radiation (OLR) value should be below 200 watt per sq m (wm2) in the region confined by 5-10ºN latitude and 70-75ºE latitude.


Unusual nature

Early or late onset is not unusual. Previously, the onset has been either late or early, depending on conditions.



  • The onset process happens during the progress of the monsoon over the Indian subcontinent. Delay or early arrival is not a concern.

  • It has no bearing on the quality or amount of rainfall, or its regional distribution over the country.

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