Monster clouds enveloped the sky above the city and its adjoining areas on Monday afternoon. Heavy rain and a storm followed.

Each column of cumulonimbus cloud that started gathering over the city around 3pm was eight to 10 km tall and spread over several km, said officials of the Alipore Met department.

Around 4pm, it became so dark that the Calcutta Municipal Corporation had to order the street lights switched on. Drivers, too, put on headlights and foglights. At the airport, two Calcutta-bound flights were diverted because of low visibility. Suburban train services were also disrupted.

The clouds dispersed around 5pm after the downpour and storm, that sent people scurrying for cover. The rainfall was measured at 19.8 mm. The maximum wind speed recorded was 83 kmph. Two trees fell on DL Khan Road during the storm, said police.

The weatherman attributed the convergence of the gigantic clouds to a trough of low pressure in the upper air (3.1 km above the sea level) that formed near Bihar and passed over Gangetic Bengal to the north-east Bay of Bengal and a cyclonic circulation (about 1.5 km above the sea level) from Gangetic Bengal to Bangladesh. These caused incursion of moisture from the bay.

An Alliance Air flight from Silchar was diverted to Bhubaneswar because of the storm, said airport officials. The flight was scheduled to land at 4.45pm. It returned to Calcutta at 6.40pm. A non-scheduled flight from Jamshedpur, too, could not land and returned to the Steel City.

Train services on Sealdah and Howrah divisions were disrupted from 3.50pm. The storm damaged overhead wires between Titagarh and Palta on the Sealdah division. A power failure affected services on the Howrah-Burdwan Main and Chord lines between 3.50pm and 4.30pm, holding up long-distance trains.