Calcutta got its first real taste of monsoon 2010 with Monday emerging as the wettest day of the year under the influence of a cyclonic circulation.
Three months after the monsoon had entered the state, on June 13, the city received 99.2mm of rainfall till 5.30pm on Monday, beating the July 31 mark of 96.2mm.
Tuesday is likely to be cloudy, with spells of rain and thundershower.
“The city experienced its wettest day this season. Nearly 100mm of rainfall in 24 hours is something that hasn’t been witnessed in Calcutta since the monsoon of 2008. By the look of it, the monsoon has finally come alive in Calcutta in its last lap this year,” said an official of the India Meteorological Department (IMD).
With spells of heavy rain lashing the city since late on Sunday and continuing in the form of light rain or a drizzle through Monday, the rainfall count went past last monsoon’s record of 93.1mm but fell short of the September 13, 2008 high of 100.8mm.
Meteorologists attributed the rainfall to a cyclonic circulation over the Bay of Bengal, which formed overnight and induced the formation of thunderclouds over south Bengal (see graphic).
“A cyclonic circulation formed over the Bay early on Monday resulting in the incursion of abundant moisture into the atmosphere over south Bengal and the formation of thunderclouds,” said Gokul Chandra Debnath, director of the Regional Meteorological Centre at Alipore.
Meteorologists said the circulation’s influence has been strengthened by the axis of the monsoon trough — a line marking a zone of instability between the westerly and easterly flows that induces heavy rainfall in areas around it — which is passing through south Bengal now.
The downpour dragged the maximum temperature down by three degrees to 29.1 degrees Celsius, three notches below normal. The minimum too was a degree below normal at 24.6 degrees Celsius.
“The weather has not been comfortable this monsoon primarily because there haven’t been many such rainy days,” said a Met official.
This was the fifth time since 2004 that the wettest September day in the city has been on the 13th of the month.
“September 13 was the wettest September day in 2004, 2005, 2007 and 2008. There is no special factor that can be singled out for this phenomenon. It is a strange coincidence,” said O.P. Sharma, the chief meteorologist of a private Met agency.
The circulation is expected to influence the weather over south Bengal for another 24 hours at least. “The spells of heavy rain in parts of the city and its outskirts are likely to continue through Tuesday,” said Debnath.
Will the rejuvenated monsoon showers do enough to make up for the rainfall deficit of July and August?
“That can only be said after a few days of heavy rain. As of now, that looks unlikely. The city has less than a month left before the monsoon officially departs (around October 7),” said Sharma.
On an average, the city receives 295.9mm of rain in September over 13.4 rainy days. This month. the city has so far received 179.82mm of rain.