December began on a cloudy note in Calcutta, but the winter chill is expected to set in before the week ends.

The day temperature stayed lower than usual on Monday because of the cloud cover. The minimum was, however, marginally higher than normal. According to the Met office, the skies are set to clear gradually and the minimum temperature is expected to drop towards the end of the week.

The clouds were generated by a deep depression — the remnant of Cyclone Ditwah — over the southwest Bay of Bengal. The system, now weakening, is moving northward parallel to the Tamil Nadu coastline.

The Met office, the official record-keeper for Calcutta, recorded a minimum temperature of 18° Celsius on Monday, slightly warmer than normal. The persistent cloud cover kept the sun at bay, pulling the maximum temperature down to 27 degrees, almost two notches below average.

A “two to four degree drop” in the minimum temperature by Friday has been forecast.

“Dry weather is likely to prevail over all districts of South Bengal for the next few days. Shallow fog is expected in some districts over the next day or two. After that, mainly clear skies,” a Met official said.

Cyclone Ditwah ravaged Sri Lanka, causing significant damage as it grazed the island nation before entering the Bay. Nearly 200 people have died in Sri Lanka.

“The deep depression was over the southwest Bay of Bengal on Monday morning. Around 8.30am, it was 50km east-southeast of Chennai. It was likely to move slowly northwards, parallel to the north Tamil Nadu-Puducherry and south Andhra Pradesh coasts, and weaken into a depression,” a Met official said.

Moisture-laden southerly and southwesterly winds from the system helped form clouds across the state. Light drizzles were reported in some districts, including East Burdwan, West Burdwan, Birbhum, Purulia, Bankura and Jhargram. “As the system weakens further, dry and cold northwesterly winds are expected to dominate again,” the official said.

For much of November, these cold and dry northwesterlies had been dominant in Bengal. The minimum temperature stayed below normal. But two systems in the north Indian Ocean — both of which intensified into cyclones — disrupted the pattern.

Cyclonic storm Senyar (named by the UAE), which struck Indonesia on November 26, weakened into a depression the next day.

In Calcutta, the minimum temperature is forecast to slide to around 15 degrees by Thursday or Friday.

Unlike the monsoon, the Met office does not issue a formal declaration for the onset of winter in Calcutta. However, three to four dry days paired with a minimum temperature below 15 degrees are typically seen as the markers of winter’s arrival.

The northwesterly winds from Kashmir are the usual agents of chill that reach the city through Uttar Pradesh, Delhi and Bihar.