The rainiest August in at least a decade brought to Calcutta almost double the rainfall it normally gets and September is in no mood for dry statistics either.
A cyclonic circulation in the making over the Bay of Bengal could bring 48 hours of rain or more from Monday, the weather office warned even as afternoon clouds blanketed the sunny Sunday sky.
“There is a cyclonic circulation in the northwest Bay of Bengal and rainfall is likely over the next two days. If the circulation intensifies, the wet spell can stretch longer,” said a senior official of the India Meteorological Department.
The city received 684.6mm of rain last month against an average aggregate of 352.4mm, based on data collected over 30 years between 1971 and 2000.
The showers that flooded large parts of Calcutta and its suburbs thrice in as many weeks largely contributed to the surplus of 94.3 per cent. The surplus for the season so far is 37.4 per cent despite a low contribution from July, historically the wettest monsoon month in Calcutta.
“There has never been more rainfall in Calcutta in recent memory than this August. Three wind systems with low atmospheric pressure blew in the moisture from the sea leading to cloud formation and the showers,” Gokul Chandra Debnath, director of IMD Calcutta, told Metro.
The decadal rain count for August (see chart) shows a big gap between 2013 at the top of the table and second-placed 2007, when the city received 471.1mm of rainfall. In 2002, it rained 459.3mm in August.
While the number of wind formations in August isn’t unusual — three low-pressure zones is said to be par for the course — the key to the excess rainfall is the path these weather systems follow.
“What made the difference this August was the trail of all the weather systems. Normally, they move towards Odisha after hitting the Bengal-Odisha coast. But this year, two low-pressure areas and a more intense depression in between hit the Bengal and adjoining Bangladesh coasts and stayed on instead of moving away to Odisha,” Debnath said.
For those singing rain-rain-go-to-Spain after an excess of August rain, September is just as likely to be a wet month. Weather scientists expect the heavy rainfall to continue in September as that has been the trend of monsoon rain in recent years.
Last year, September recorded the highest aggregate among all the monsoon months. The aggregates for June, July, August and September tell the story — 178mm, 248mm, 251mm and 441.2mm.
July is usually the rainiest month, followed by August and June.
Experts say it is difficult to predict if the latest weather system forming over the Bay of Bengal will bring as much rain as those in August because its movement depends on current atmospheric conditions.
“There are many atmospheric conditions that influence the movement of a system. Between August 19 and 21, a depression had brought almost 250mm of rain in three days. Due to wind and pressure conditions, it had not been able to advance much beyond Calcutta, which increased the rainfall here,” Debnath explained.
The one benefit of the excessive rainfall in August was the Celsius drop. Taking the average of maximum and minimum temperature over the past five years into account, the city was cooler by one degree this August than in the past six years. By the same token, if the sum of the mean maximum and minimum temperature between 2008 and 2012 was around 59 degrees Celsius in August, the figure was 57.2 degrees Celsius this year.
The renewed clouding because of the cyclonic circulation that developed on Sunday means solar radiation will be less and the day temperature will remain under check. “It should be around 32 degrees Celsius,” an IMD official said.