The monsoon is set to retreat from Calcutta for this year by Wednesday, the Met office said.

Weather scientists have spotted telltale reversal of wind flow that marks the withdrawal of the monsoon, which stayed back longer than usual this year.

The normal date of the season’s withdrawal is October 12. A low-pressure area had formed over the Bay of Bengal last week and delayed the retreat.

But over the past couple of days, winds from north India are entering the city, instead of winds from the southwest, the direction of the Bay of Bengal. That means dry winds from north India have replaced the moisture-laden winds from the bay.

“The southwest monsoon is expected to withdraw from Calcutta and the rest of the state by Wednesday. The rain has stopped. The northwesterly winds are coming into the city. The moisture content in the atmosphere is low,” said G.K. Das, the director of the India Meteorological Department, Calcutta.

Weather scientists generally consider three to four rain-free days during this time of the year as a prerequisite to announcing the withdrawal of the monsoon. A dip in the moisture content in the air is another marker of the end of the monsoon.

The last three days in the city have been rainless and a clear sky and bright sunshine have been greeting Calcuttans.

The minimum relative humidity — a measure of the moisture content in the air during the driest part of the day — dropped to 36 per cent on Monday and stayed there on Tuesday. Till days ago, the minimum relative humidity used to hover above 60 degrees.

As the moisture content in the air has dropped, people are not sweating like they had been a few days ago.

The winds blowing in from northern and western parts of India are mild now. They will become stronger in the days to come, heralding the onset of winter.

But a hint of chill is expected in the city in the early hours of the day, Das said.

The minimum temperature slid to 21 degrees on Tuesday, a notch below normal. It is likely to remain on the lower side, said a Met official.

The retreat of the southwest monsoon from the city is likely to coincide with the onset of the northeast monsoon in Tamil Nadu, the Met office has said.

“The northeast monsoon activities might inject some moisture-laden winds into south Bengal, including Calcutta, but not before five-six days. The temperature will then be on the higher side because the moisture content will impede the flow of the winds from northern India. But it is unlikely to have a lengthy impact on the city,” said a Met official.