Kerala, Delhi and many other parts of the country will soon have new onset dates for monsoon.

A team of 40 weather scientists are in the process of revising the normal onset dates for monsoon across the country.

However, according to Ajit Tyagi, Director General of India Meteorological Department (IMD), there will not be any significant difference and the new normal dates may vary only by one or two days.

As per the current datasets, June 1 is the normal date for the onset of monsoon over Kerala, while in Delhi it is June 29. When the new datasets are introduced this could vary by one or two days.

Scientists felt the need for revising the normal onset dates for south west monsoon as they were based on limited data of less than 40 years.

“The earlier dates were based on monsoon data between 1932 and 1970. Now we have a more comprehensive analysis of the seasonal rains till 2009,” Mr. Tyagi said.

Experts believe that the changes would be of little consequence as monsoon was a highly dynamic phenomenon.

“There is nothing sacrosanct about the dates. They are only pointers and are subject to variation of plus or minus seven to eight days,” Mr. Tyagi said.

These changes are still being debated among weather scientists and could come into effect from next monsoon season.

The changes would give a more accurate picture on the onset and progress of monsoon as they have been drawn after an analysis of data accumulated nearly 80 years.

Usually, south-west monsoon sets in over Andaman Seas by mid-May and under normal conditions reaches Kerala by June 1.

Under normal conditions, its onward surge begins with Kochi getting monsoon showers by June 2 followed by Mangalore (June 3), Vishakhapattanam (June 8), Mumbai, Nagpur, Raipur, Ranchi (all June 10), Varanasi (June 15), Lucknow (June 18) and Delhi (June 29).

Southwest monsoon covers the entire country by July 15, according to current dates used by the IMD.

The IMD declares onset of monsoon over Kerala if 60 per cent of the listed 14 weather stations report rainfall of 2.5 mm or more for two consecutive days and if other criteria like windfield and outgoing longwave radiation are favourable.

Under such conditions, the onset over Kerala is declared on the second day of the region getting rains.

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