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Trigger for moderate rain

Calcutta can expect intermittent rain in the next 24 hours, caused by a low-pressure area over the Bay of Bengal which Met officials think may travel inland and stay close to Bengal on Monday and Tuesday.

On Sunday the system was hovering close to the Odisha coast and drawing a lot of moisture from the sea. Met officials said it might make landfall on Monday and move towards Jharkhand.

“South Bengal would continue to get rain till Tuesday, while places in Odisha and Jharkhand would get heavier showers. Calcutta will get moderate rain as it is on the periphery of the low-pressure area,” said a senior official of India Meteorological Department, Calcutta.

Moderate means anything between 7.6 and 35.5mm of rain in a day.

On Sunday, Calcutta experienced showers accompanied by thunder and lightning. The heaviest spell came in the afternoon, between 2.30pm and 3.30pm.

Sources at the Calcutta Municipal Corporation said most of the civic drainage pumping stations in the city recorded 15mm of rain during the day. The Met office, however, recorded 7.9mm of rain in Alipore and 8mm in Dum Dum.

Sunday’s relative humidity was high — ranging between 92 and 71 per cent — because of moisture flow from the sea triggered by the low-pressure area.

“As the sun came out in the morning, the moist air heated up, gained height and condensed to form thunderclouds, which caused the rainfall in Calcutta,” said a weather scientist.

Sunday’s maximum temperature was 34.6 degrees Celsius, three notches above normal. The Met office has predicted the maximum to come down to 32 degrees on Monday.

Since the start of the monsoon in June, the city has received 496.4mm of rain, which was about 20 per cent excess of normal.