The chill Calcuttans felt early on Tuesday was because of a 2.5-degree drop in the Celsius in 24 hours.

The mercury dipped to 16.6 degrees Celsius on Tuesday, the coldest so far this season, from 19.1 degrees on Monday. The minimum temperature is normal for this time of the year but the effect was pronounced because the mercury had been on the higher side for several days.

A deep depression over the southwest Arabian Sea drifted towards the Somalia coast, clearing the path for the cold northwesterly winds in Gangetic Bengal and leading to the fall in temperature.

“A low-pressure area had formed on Sunday. It intensified into a depression and then into a deep depression. The system was pulling the cold northwesterly winds towards itself. Since the system has now moved quite far away from the Indian coast, the cold winds are getting passage into Gangetic Bengal,” Ganesh Kumar Das, director, India Meteorological Department, Calcutta, said.

The minimum temperature is likely to dip further, touching 16 degrees Celsius over the next 48 hours. But the cool phase will not herald the onset of winter, Das said. Winter in Calcutta is declared in the second week of December, when the minimum temperature remains 16 degrees Celsius or lower for a few days at a stretch.

Weather scientists expect another low-pressure system to form over the Arabian Sea later this week and push the mercury up again in Calcutta by the end of this week.

The dip in the moisture content in the air contributed to the dip in the mercury. The minimum relative humidity was around 40 per cent on Monday and Tuesday. It had been in the 50-plus region the past week.

“The northeasterly winds flowing in from Bangladesh had led to the rise in the moisture content. That flow has since ebbed,” a weather scientist said.

Most Calcuttans were caught by surprise, albeit pleasant, by the dip in the temperature. “I stepped out to drop my son to his playschool around 7.30am. It was cold and windy. I was missing a jacket,” Anupam Ghosh, a Behala resident, said.

The cool spell comes after a rather warm spell. The minimum temperature had remained over 19 degrees Celsius for eight days at a stretch since November 25. The mercury had touched 20 degrees Celsius on November 26. On Monday, it was 19.1 degrees, three notches above normal.