18-km tall cumulonimbus over city caused 3-hour deluge: Weatherman
The Statesman | 24 September 2025In the weatherman’s version, the excessive precipitation was caused by a towering cumulonimbus cloud accumulation 18 km in height standing above south Kolkata around 2.30 a.m, resulting in extremely heavy showers over eastern, central and parts of northern Kolkata. The precipitation was far greater in the southern regions of the city, with between 100 mm and up to 300 mm of rainfall in some places in the space of three hours.
Very heavy rainfall that has occurred so far this monsoon has not exceeded 100 mm in the same period of time, weather observers said. An official in the weather department described the deluge as one of the city’s heaviest single-spell rainfall in the last 30 years.
According to data from the IMD, Kolkata received 2,663 per cent more rainfall in the 24 hours ending 8.30 a.m. than the long-term average rainfall for the city. Adjoining Howrah was a far-away second, with 1,006 per cent more rainfall than the long-term average. North 24-Parganas received 857 per cent more rain than the average. The numbers point to the scale of rain the city has received in a short duration.
The main culprit was a low pressure area over northwest Bay of Bengal. A low-pressure area formed over the Bay on Monday, which triggered the heavy rain the city experienced early this morning. The IMD said under the influence of an upper air cyclonic circulation, a low-pressure area formed over northeast Bay of Bengal in the morning.
Rainfall exceeding 100 mm in an hour over a 20 to 30 sq km area is defined as a cloudburst, explained the weatherman. Another similar system is likely to form on 25 September and bring showers to the region, the Met office warned. It said the rainfall intensity was highest in south and east Kolkata. Garia Kamdahari recorded 332 mm in a few hours, followed by Jodhpur Park (285 mm), Kalighat (280 mm), Topsia (275 mm) and Ballygunge (264 mm). North Kolkata’s Thanthania recorded 195 mm.
Heavy rain occurs due to atmospheric instability, or sudden changes in atmospheric pressure, from western disturbances and monsoon currents interacting with moisture-laden air, a process intensified by climate change. Warming oceans add more moisture, while jet streams alter the path of western disturbances, leading to more frequent and intense rainfall, explained a Met official.
This ranks as the second-highest single-day rainfall for September in the city, behind 369.6 mm on 28 September, 1978. “All of Kolkata seems to have been affected. Bombay too suffers for one or two days every year in few areas, particularly on high tide days. But this was far worse,” pointed out a Mumbai resident who grew up in Kolkata.
