Celsius-scarred Calcutta has been deprived of Nor’wester relief this April because the Chhotanagpur Plateau where such storms originate hasn’t been heating up enough and receiving adequate moisture at the same time, say weather scientists.
The city usually gets three squalls, accompanied by rain, between the second half of March and the first week of April. It has received just one this year, a 58kmph storm that felled trees and stripped billboards a fortnight ago on March 25.
Thunderstorms or rain have since been forecast on a few occasions, only for the atmospheric conditions to change at the last moment.
The result? Calcuttans are stewing in the mix of Celsius and humidity that make April the “cruellest month”.
“For a Nor’wester to develop, you need wet air to be drawn into the landmass from the south-easterly direction. The confluence of this moist wind and the hot and dry air over the Chhotanagpur Plateau triggers a disturbance that leads to the formation of tall thunderclouds,” L.S. Rathore, director-general of the India Meteorological Department in Delhi, told Metro.
Jharkhand, a part of the Chhotanagpur Plateau, has alternated between sunny and partly cloudy skies over the past couple of weeks because of Western Disturbances — storms that blow into north India from the Mediterranean — following a predominantly southern trajectory.
The cooling-off phase has prevented the build-up of heat for periods long enough to trigger thunderclouds over the plateau.
“Thunderclouds that develop in Jharkhand naturally travel east towards Bengal, though not all of them reach Calcutta. This hasn’t happened this season, which is why the city is still deprived of its share of Nor’westers,” a Met official said.
Another factor has been the absence of anti-cyclonic circulations or high-pressure belts that form in the Bay of Bengal at this time of the year, pushing moisture inland to create conditions for rain.
While rain has stayed away, minimum relative humidity has shot up in Calcutta — it was 46 per cent on Tuesday, almost trebling in 48 hours — because of a cyclonic circulation over Gangetic Bengal.
The trough, extending till northern Karnataka, could cause a stray shower in town in the next two days but is unlikely to trigger a thunderstorm.
The only dark cloud with a silver lining is a squall alert in Jharkhand that might translate into shared relief for Calcutta. “The wind pattern is favourable and should result in moisture incursion. The thundersquall will hit with winds at 50-60kmph in isolated pockets, accompanied by rain,” A.K. Sen, the director of the Regional Meteorological Centre in Patna, said.
He explained that the wind pattern was fluctuating between westerly and easterly, filling the air with moisture. “A cyclonic circulation over Chhattisgarh and Odisha has also resulted in the formation of clouds over Jharkhand.”
The Ranchi Met office too announced the possibility of Nor’wester activity. “We are expecting a kalbaisakhi in pockets of the state in the next 48 hours,” an official said.
The maximum temperature was over 41 degrees at many places on Tuesday, including Jamshedpur and Daltonganj.
- Western Disturbances are dipping south, bringing unseasonal rain to north India. Winds from there have been cooling Jharkhand before the plateau heats up enough for thunderclouds to form
- No sign of high-pressure bel over the Bay, cutting down moisture flow to Jharkhand
- No mingling of hot and dry winds over Chhotanagpur Plateau