The city has already recorded 70 days of rain this monsoon, exceeding the usual 60-day quota for the entire season, according to Met office archives.

The Regional Meteorological Centre in Alipore, whose readings represent Calcutta, shows that June, July, August and September normally experience about 60 “rain days” (days with measurable precipitation). However, between June 1 and August 31 this year, the Met office recorded 71 rain days in Alipore alone. September is just four days in, three of which have been rainy.

The figures confirm what Calcuttans have been experiencing. The city has received approximately 1,200mm of rainfall between June and August — 20% above the average for this period.

This year, the monsoon arrived in south Bengal on June 17, about a week behind the usual June 10 onset. In north Bengal, it reached early, on May 29, ahead of the date usual arrival date around June 5.

Bay systems

Met officials attribute the heavy monsoon to two key factors: an unusually high number of weather systems forming over the Bay of Bengal and the favourable positioning of the monsoon trough throughout the season.

Monsoon rainfall in south Bengal depends on these weather systems developing over the Bay and adjacent areas. Typically, the Bay experiences about 10 such systems from June to September, according to Met officials. But this year has already seen 11 low-pressure areas emerge since June, with more expected in September.

The latest system, which formed on Monday, began as a cyclonic circulation before intensifying into a low-pressure area by Wednesday morning. This triggered widespread rainfall across Calcutta since Monday night. On Wednesday morning, the system was on the northwest Bay, off the Odisha coast. Many parts of Calcutta received sharp spells of rain through the day.

Monsoon trough

The other main factor is the monsoon trough — an imaginary line connecting low-pressure zones stretching from western to eastern India. The trough has predominantly remained positioned over south Bengal this season. Currently, it runs through Sambalpur in Odisha to meet the weather system over the Bay.

The trough’s location is critical for regional rainfall distribution. When it is in the Himalayan foothills, north Bengal and Sikkim receive more rain. When it moves southward, as it has this year, south Bengal experiences more showers.

H.R. Biswas, head of the weather section at the Regional Meteorological Centre in Alipore, explained: “Last year, the monsoon trough was stationed at the Himalayan foothills for much of the season. This year, it has remained
over south Bengal for a prolonged period, keeping monsoon currents active and strong here. Between August 1 and 13, the trough briefly moved north, causing a deluge in north Bengal, but otherwise, it has largely stayed in the south.”

This year’s rainfall also highlights a regional disparity (see chart). South Bengal received 1,034mm of rain between June and August — well above the usual 900mm. In contrast, north Bengal recorded 1,279mm, which is below its average of nearly 1,500mm for the same period.

Additional reporting by Samarpita Banerjee