KOLKATA: A 12km tall cloud, stretching from north to south, is waiting to burst on a city that is still soggy from last week's four-day drencher.

Darkness descended on Kolkata suddenly on Monday afternoon — almost as quickly as someone had drawn a curtain. Streetlights and headlights came on at 2pm. As people wondered what was happening, lightning streaked across the city and killed at least four people, including three schoolchildren. For 45 minutes, the menacing shroud hung over Kolkata but just when the residents were bracing for a deluge, the rain petered off. But the cloud isn't gone and Tuesday is going to be wet, says the weatherman.

The last time such a huge cloud formed over the city was 10 years ago, said regional meteorological director G C Debnath. Monsoon clouds often stand 8-9km tall but Monday's cloud was unusual, he said, adding, "Rarely do we witness such tall columns."

A sudden incursion of moisture from the Bay of Bengal added to the residual moisture left by last week's low pressure, and "up and down wind movements" led to the formation of the cloud. "It swelled further under the influence of a cyclonic circulation and contained large drops of rain and thunderclouds. The friction led to lightning," said Devendra Pradhan, deputy director, meteorology.

It proved deadly for students of a school in Deganga, North 24-Parganas. Five friends from Class VI had finished their school test early and sheltered under a construction site when they were hit by lightning. Chandana Mondal, Rupa Sardar and Shipra Parui died on the spot while Shampa Khatun and Mandia Kahar are in critical condition in a hospital. In Durgapur, Ranjan Sona, a farmer, was struck by lightning while working at the fields and died.

"We stopped the test midway because of a power cut and huddled all students in the school hall. We had no idea that these five students and some guardians had take shelter in a nearby building. We are shattered by the deaths," said headmaster Sirazul Islam.

The weatherman has forecast heavy showers for Tuesday as the cyclonic circulation is expected to intensify. "It could develop into a low-pressure area by Tuesday evening," said Debnath. The sudden influx of moisture was not associated with the impending low-pressure, he pointed out.

"This was a localized phenomenon. So, it was followed by a short spell of shower. But with the cyclonic circulation forming, a wet spell is likely to follow. The showers could turn heavier from Tuesday," said Pradhan.

Kolkata received 19.7mm of rain till 8pm, but it was enough to cause waterlogging in central and north Kolkata. Scores of trees were uprooted along EM Bypass and on the Maidan and Southern Avenue. Muktarambabu Street, CR Avenue, Rabindra Sarani, Ultadanga, Bentinck Street, College Street and the Park Circus connector were in ankle-deep water.