A trough of low pressure over south Bengal led to an overcast sky on Diwali and a few spells of rain in the morning.

Met department officials said the rain might dry up on Thursday but the sky could remain partly cloudy over the next one or two days.

With the sun blocked out, the Alipore Met department recorded a temperature of 25.6 degrees Celsius at 2.30pm on Wednesday, almost eight degrees lower than what had been recorded around the same time 24 hours before.

“The temperature is expected to climb up to 30 degrees on Thursday as the cloud cover will thin,” an official in the department said.

The nip in the air that was being felt in the morning in the last week of October would take a while to come back but a dip in the minimum temperature, which read 24.5 degrees on Wednesday, three degrees above normal, is likely over the weekend.

Drizzles started a little after 8.30am and continued in phases till noon. The Calcutta Municipal Corporation’s drainage pumping station in Mominpur recorded 9.4mm of rain, the highest. The ones in Patuli, Joka, Ultadanga and Behala Flying Club recorded 8.6mm, 8.4mm, 0.3mm and 5.6mm, respectively.

“Moisture was flowing into the city’s skies under the impact of a north-south trough of low pressure that stretched from east Bihar to Gangetic Bengal. The trough has started weakening, which is why there was no rainfall post-noon,” said Sanjib Bandyopadhyay, deputy director general, India Meteorological Department, Calcutta. “The sky will remain partly cloudy on Thursday but rain is not expected.”

Weather watchers expect a change in the wind flow after Friday, when a low-pressure area is likely to form over the Bay of Bengal, triggering a fall in the mercury.

“The minimum temperature is expected to drop to 20 or 21 degrees over the weekend. The dip could have come about earlier but did not happen because a well-marked, low-pressure area that was supposed to stay over the Bay of Bengal had entered Tamil Nadu. So, it did not pull the north-westerly wind that ferry cold to Calcutta and its surroundings as expected,” a Met official said.

“We expect a cyclonic circulation that is hovering near Thailand to develop into a low-pressure area and move close to the Andamans by Friday. That should draw the cold north-westerly wind from Kashmir towards Bengal and its neighbouring states,” the official said.

How much the minimum temperature dips will depend on the path the low-pressure area, which may intensify further, takes.