Armed with exhaustive data on rainfall from several more stations in the past 30 years, the Indian Meteorological Department is working on an analysis that will see a change in the date of monsoon onset and withdrawal over the country. Since 2006, the monsoon’s retreat has been commencing between September 21 and September 30 against a normal date of September 1. A report on India’s climate profile by IMD director-general Ajit Tyagi and Dr S D Attri reveals that between 1941 and 2000, there has been a slight shift in monsoon activity with late onset and late withdrawal and a general increase in duration by about a week. As IMD set specific criteria for onset and withdrawal four-five years ago, making it mandatory for all conditions to be met before they declare onset or withdrawal, the withdrawal date has shifted by two-three weeks. Between 1971 and 2000, Jammu and Kashmir saw a standard deviation of 14 days in the onset from the normal date. West and northwest India have seen onset delayed by an average of 1.5 weeks. Officials said a change had also been recorded in the total rainfall across various regions with east getting lesser and west getting more rainfall. However, there had not been a significant change in the overall rainfall for the country.
Conditions for onset: Rainfall for two consecutive days over specified regions, wind field, outgoing longwave radiation
It is withdrawal if: No rain for five consecutive days, formation of anticyclone over Rajasthan, reduction in water vapour