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Kolkata experienced a record-breaking rainfall of 180 mm in just three hours.
This led to severe waterlogging and the tragic death of 10 people, mostly due to electrocution.
The city's drainage system struggled to cope, exacerbated by high tides and urban expansion.
Authorities and experts are debating the causes, with some attributing it to a cloud shift rather than a cloudburst.
Kolkata is limping back to normal amid intermittent showers after being deluged by record-breaking rainfall from the wee hours of September 23, 2025. The continuous torrential downpour caused acute waterlogging throughout the city. Some pockets remained under knee-to-waist-deep water for as long as 10 hours after the major spell had stopped.
Experts pointed out that disfigured drainage and choked canals, coupled with heightened hightide, magnified the impact of record rainfall.
The situation vindicates the findings of the Sixth Assessment Report (AR6) published by the United Nations Intergovernmental Panel on Climate Change. The analysis had predicted in 2021 that Kolkata would be receiving sharply increasing short rainfall episodes in the near future.
The crisis has been exacerbated by the rapid decline of the city's waterbodies, which act as natural sponges, due to unchecked and often illegal urban expansion.
In the 24 hours from September 22 morning to September 23 morning, the city received 251.4 millimetres rainfall — the highest for any September day in the last 39 years and sixth highest in the city's history. Of this, 180 mm of rain fell during 2.30 am to 5.30 am of September 23 alone.
Officials of the India Meteorological Department admitted that the total rainfall on September 23 was more than double the predicted volume. Kolkata Municipal Corporation (KMC) sources, however, said the total rainfall volume during the 24-hour period was 332 mm.
The situation has further deteriorated as a wide part of the city was out of electricity for a long duration, many places for an entire day, as the Calcutta Electric Supply Corporation (CESC) had cut off electricity to prevent electrocution on roads.
However, till evening of September 23, 10 persons had died in the city and its fringes, mostly being electrocuted. Many pointed out that the live wires linked to provide temporary power supply to thousands of pandals might have been responsible, though CESC declined the allegation.
Both Chief Minister Mamata Banerjee and City Mayor Firhad Hakim have termed the rain “unprecedented”, and advised people not to venture out of their homes on Tuesday.
While Banerjee pointed to many issues leading to the situation, from union government not dredging Ganga properly and power distributor CESC not taking enough precautions to huge unmanaged water coming from upstream, Hakim claimed that the rising hightide in River Ganga and canals already filled to the brim were spoilers as they did not allow the accumulated water within the city to be drained out.
Officals of the Damodar Valley Corporation dismissed the CM’s claim and CESC denied charges of duty dereliction. “We have 5,000 people working around the city and will investigate each of the deaths,” said Abhijit Ghosh, executive director of CESC.
“I have not seen such intense rainfall within such a short time during my lifetime. We could not open the lockgates for a long time, as the canals, already filled up to the brim, were back flowing in the city instead of the accumulated water being drained out,” said mayor Firhad Hakim to this correspondent.
Tarak Singh, who heads the drainage department of KMC, concurred with the chief minister and the mayor. “We expected the accumulated water to drain out during low tide, but were shocked to see the volume of water still in the river during low tide. Hardly any water could be drained out in the morning, although we used around 900 pumps. As a result, hardly six inches of accumulated water receded even four and a half hours of the rain stopping in areas like Ballygunge and Palmerbazar,” explained the official.
IMD data showed how the downpour became many times more intense between 2am and 5 am, but the event wasn’t caused by a cloudburst, the met department said. Several independent experts agreed.
From 48.6 mm during 11.30 pm (September 22) to 2.30 am (September 23), the quantity of rainfall increased to 185.6 mm in the following three hours. “City’s existing drainage can absorb around 10-12 mm of rainfall per hour,” pointed out Arunabha Majumdar, a drainage and public health expert and former director-head of the All India Institute of Hygiene and Public Health. On September 23 morning, more than 60 mm of hourly rainfall pummelled the city for several hours straight.
The sources in KMC said the southern Kolkata received more rainfall compared to its northern part. Garia in the city's southern fringe recorded the highest precipitation (340 mm), followed by Kalighat with 297 mm, Jodhpur Park with 290 mm, Topsia with 284 mm and Ballygunge with 279 mm during the 24 hours. In comparison, northern parts recorded rainfall less than 200 mm on average.
A section of experts, however, said there was a cloudburst. “The heavy rainfall that happened in the early morning hours of September 23 was due to a cloudburst from the low-pressure system lying over Kolkata,” claimed Nairwita Bandopadhyay, a geographer and academic.
“It was neither a cloudburst nor any role played by cumulonimbus clouds, as many claimed. Our finding showed that the cloud was 4-6 km long, instead of 18 km as mentioned by some analysts. A cloudburst occurs when there is 100 mm or more rainfall per hour. We had less than that. Moreover, the cloud structure was also different from what we find in cloudburst cases,” explained Habibur Rahaman Biswas, head of IMD Alipore.
Biswas accepted that the rainfall prediction for September 23 was around 115 mm, less than half of what the city actually received. “Sometimes the low pressure combines with continuous inflow of large volumes of water vapour to create such a scenario,” explained the scientist.
“We predicted about 115 mm of rainfall for Kolkata and 200 mm for adjoining south Bengal districts. But part of the south Bengal cloud got disintegrated and moved towards Kolkata, triggering the record downpour.”
IMD chief Biswas predicted that there would be spells of rain in the days leading to and during Durga Puja, the city's iconic festival. “On September 25 and 26, there may be some passing showers, while stronger rainfall and cloud spread is expected on September 27 and 28. The situation is likely to clear up after that.”
According to the expert, the cyclonic circulation has already formed close to Myanmar coast and is expected to move towards Odisha. The low pressure is set to form on September 25 and turn into a depression the following day. “As a result, there will be rain in the coastal districts of West Bengal, as well as in Kolkata, though not as heavy as we witnessed this time,” said Biswas.