1234 Kolkata: There could be a nip in the city's air from Thursday, and the minimum temperature could drop by four degrees, taking it below 20°C for the first time this season by Sunday. The temperature drop will be triggered by the cool northwesterly wind, which is set to flow into Gangetic Bengal at multiple levels. It could pull the mercury down by three to four degrees and make it dip to 18°C by the weekend, even as the southeasterly wind, which carries moisture from the sea and prevents a temperature drop, is cut off by a stronger northwesterly, said the Met office.
To begin with, the mercury may drop by a degree on Thursday, said meteorologists. Kolkata recorded a minimum temperature of 22.8°C on Monday, which remained unchanged from Sunday, even as the maximum temperature dropped by a degree to 30.5°C. The minimum temperature could climb by a degree on Wednesday and then dip.
"The northwesterly has been flowing in but not at all levels. But it is now set to gain strength. The southeasterly has so far been active, so there has been no significant drop in temperature. It could now start changing slowly, and we are likely to see the minimum temperature drop below 20°C by Sunday. It could slide to 18°C," said Regional Meteorological Centre scientist HR Biswas.
It is the northwesterly that pulls down the temperature in the region, and the more unobstructed it flows, the lower the temperature would be. But the northwesterly usually does not have a significant impact before the end of November. It has already started flowing in, though. "It has been active but not at all levels of the atmosphere, which is necessary for a temperature drop. The city remained partially cloudy on Sunday and Monday due to the incursion of southeasterly winds. Since there was moisture in the air, the mercury has not dropped over the last several days. But with the northwesterly getting stronger, it could now start dipping slowly and touch 20°C in a few days," added Biswas.
Another factor that helps pull the temperature down is western disturbance. These are systems that form in the northwestern part of the subcontinent and float eastward, triggering rain. "They form in the northwestern hills and travel to the north Indian plains. The cloud cover generated by them pulls the mercury down and helps to usher in winter," said a weather scientist.
The first mercury drop, however, is triggered by winds flowing in from the chilly, snow-covered peaks of Kashmir. "Once it starts snowing in the Himalayas, the chill travels fast with the northwesterly across the north Indian plains. But that is still some weeks away," said the weather scientist.