Calcutta may have shivered over the weekend, with Sunday being the coldest day of the season, but the city has seen days far colder over the past five decades.

On December 22, 1966, the mercury had plummeted to 7.2 degrees Celsius. In the next decade, the temperature twice touched the eight-degree mark — in December 1974 and January 1978.

In the 1990s, too, the mercury had dropped below 10 degrees — in the first month of 1990 and 1992. Even in the new millennium, the minimum temperature had hovered around nine degrees in consecutive years.

“In all the cases, the drop in minimum temperature appears to be associated with an extreme event, such as a western disturbance,” said G.C. Debnath, the director of the weather section at Regional Meteorological Centre in Alipore.

Met officials said that the rain last Thursday was caused by a western disturbance. As the sky cleared on Friday and the North Wind blew unhindered into the city, the minimum temperature began to drop, touching 10.8 degrees on Sunday.

Another western disturbance has crossed the north-western border of the country. It lay over Jharkhand and adjoining areas on Monday, leading to formation of rain clouds over parts of Gangetic Bengal.

“We expect the sky over Calcutta and its neighbourhood to remain partly cloudy. There may be light rain in the districts,” said Debnath.

The minimum temperature, which rose to 12.4 degrees on Monday, is expected to rise further to around 15 degrees on Tuesday.