
Calcutta received widespread rain in two phases on Tuesday. The wet phase will continue for two more days, the Met office has said.
The city will receive moderate rainfall on Wednesday and Thursday (7.6-35.5mm), a Met official said.
On Tuesday, the first spell of rain came around 11am. It lasted for about an hour, after which the skies began to clear and the sun came out. But around 2pm, grey clouds reappeared and there was a fresh bout of shower.
Most parts of the city got 10-20mm rain. The only exception was Birpara on the northern fringe, which received 50mm rain because of a tall cloud.
The Alipore Met office recorded 8.5mm rain in 24 hours till 5.30pm on Tuesday.
Satellite pictures showed thick clouds over the west-central Bay of Bengal and the adjoining areas of north Andhra Pradesh and south Odisha, at a fair distance from Calcutta.
But the clouds have been feeding moisture into the city and its surroundings from the sea.
"The cyclonic circulation is very strong.... It is positioned between 4.5km and 7.6km above the surface," the official said. "It will turn into a low-pressure area by Thursday."
The trough of low pressure passing over gangetic Bengal is connecting this circulation with one over sub-Himalayan Bengal and Bangladesh.
The trough is an imaginary line joining various low-pressure points across the country.
Both systems have drawn moisture inland from the sea, leading to clouding and rain.
The low-pressure area would not lead to heavy rainfall in Calcutta as it is unlikely to move towards the city, the official said. He said the clear skies in the morning led to high temperatures that resulted in thunderstorms.
On Tuesday, the city temperature had reached 30 degrees Celsius with 78 per cent humidity around 11am when the first spell of rain came. Before the second spell at 2pm, the mercury had soared to 33 degrees Celsius while humidity had to 67 per cent.