The mercury is in free fall.

Calcutta recorded a minimum temperature of 10.2° Celsius on Tuesday — the lowest in 13 years, or nearly 5,000 days — as cold northwesterly winds asserted their dominance across Bengal.

The sun remained hidden for most of the morning, and the city found little respite from the piercing cold even at the height of the afternoon. The maximum temperature settled at 18°, around 7° below normal based on the 30-year average.

Wednesday is likely to be as cold, if not colder.

All of Bengal shivered on Tuesday, with the mercury sliding to 6° in Birbhum, where the Met office declared a cold wave.

“What has amplified the effect of the chill is the low day temperature,” said H.R. Biswas, head of the Regional Meteorological Centre in Alipore. “Usually, when the minimum drops below 10°, the maximum is also on the lower side, but typically by 2-3°. On Tuesday, the maximum was around 7° lower than average.”

January 9, 2013, was the last time Calcutta recorded a lower minimum temperature. On that day, the minimum had dropped to 9°, though the maximum was 20°. The following day saw a minimum of 9.6° and a maximum of 23.4°.

The last time the city recorded a minimum in the 10° range was January 6, 2023, when the minimum was 10.9° and the maximum 22.7°.

On Tuesday, the maximum temperature remained below 20° for the third consecutive day. A shallow but persistent fog dragged down day temperatures, while the bite in the northwesterly winds helped keep conditions cold.

A tour of the city made the severity of the chill evident. Attendance thinned at schools and cricket camps. Tennis players warmed up in full-length track pants. Banks, usually crowded, were deserted. Tea, kachoris and kebabs sold briskly, while small fires — lit in violation of norms — burned even in broad daylight.

After sundown, the wind felt like needle pricks on the skin. Neighbourhood shops shut early, and roads emptied much sooner than usual.

According to a Met bulletin: “Cold day and cold wave conditions are very likely to occur at one or two places in Birbhum and East Burdwan districts. Cold day conditions are very likely to occur at one or two places in Hooghly, Murshidabad, Nadia, West Burdwan, North and South 24-Parganas districts.”

A cold wave in the plains is usually declared when the minimum drops below 10° and the departure from normal is 5° or more. The classification is determined by how cold the night has been. In Met parlance, a “cold day” refers to conditions where the minimum falls below 10° and the maximum remains around 5° lower than average.

This winter has lived up to its name. The minimum has stayed at or below normal for an extended stretch, bringing an uninterrupted and unusual spell of cold.

The chill is not going anywhere, the Met office said.

The forecast pegged Wednesday’s minimum at around 10°. “Later this week, even if the minimum rises, the increase will be marginal. The maximum will not shoot up either,” a Met official said.

Dense fog enveloped north Bengal on Tuesday and is expected to persist. In south Bengal, dense fog is likely in Nadia, Murshidabad and Birbhum on Wednesday, with shallow fog expected elsewhere.