Three factors indicate the withdrawal of monsoon and weather scientists have spotted two of them in Calcutta.

Weather scientists generally treat three to four rain-free days in this part of the year as a prerequisite to announcing the withdrawal of monsoon.

It did not rain on Friday and Saturday and Calcuttans were greeted by bright sunshine on both days.

A significant factor in the change of seasons that has started is the wind blowing from north India instead of southwest, the direction of the Bay of Bengal. That means dry winds from north India have replaced the moisture-laden winds from the Bay of Bengal.

The minimum relative humidity, a measure of moisture in the air in the daytime, has decreased from above 70 per cent, barely two days ago, to 49 per cent on Saturday.

The monsoon’s withdrawal from Calcutta is likely to be officially declared in two to three days.

“There are three indicators that the monsoon is on its way out — no rain, a change in direction of the flow of wind and a decrease in the moisture content in the atmosphere,” G.K. Das, director, India Meteorological Department, Calcutta, said. “While the last two signs are already visible, we are waiting for another couple of rain-less days before announcing the withdrawal of the southwest monsoon from Calcutta.”

The monsoon has lingered beyond October 10, the date by which it normally withdraws from the city.

The downpour that lashed the city on Wednesday was probably the last of this monsoon for Calcutta, Met officials have said after observing the change in the direction of the flow of wind. This marks the change of seasons.

The Met office had on Thursday recorded 6.2mm of rain in Alipore.

The moisture content in the air has “decreased reasonably” and people are not sweating like they had been a few days ago.

“The southwest monsoon has withdrawn from parts of Jharkhand, Bihar and some parts of Bengal,” a bulletin from the IMD’s regional office in Calcutta said.

“Conditions are becoming favourable for further withdrawal of the southwest monsoon from most parts of eastern and some parts of northeastern India in the next two to three days,” according to the bulletin.

Director Das said the monsoon’s withdrawal had already started from the western districts of Bengal — Bankura, Purulia and parts of West Burdwan and Birbhum.

“The wind flow from the Bay of Bengal is weakening. So, unless there is a cyclonic circulation or some such strong weather system near the city, rain will start drying up from now,” a weather scientist said.