New Delhi: Kerala, Delhi and many other
parts of the country will soon have new onset dates for
A team of 40 weather scientists are in the process of
revising the normal onset dates for monsoon across the
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," 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," Tyagi said.
These changes are still being debated among weather
scientists and could come into effect from next monsoon
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.