A trademark summer-evening squall is expected by the weekend as multiple sources have started injecting moisture into Calcutta and its neighbouring areas.
Clouds appeared in the sky over Gangetic Bengal and Jharkhand and dragged down the Celsius on Wednesday because of the accentuated moisture incursion from the Bay of Bengal. But these clouds would blow away in the next two days, aiding the required heating in eastern India for Nor’westers to hatch in the Chhotanagpur Plateau.
“We are expecting a Nor’wester over the weekend,” said Ashish Kumar Sen, director of the India Meteorological Department, Patna.
Satellite pictures showed three cyclonic circulations encircling the region: one over Bengal with a trough till Odisha, another over Chhattisgarh and the third over Assam, which was dissipating. These systems were sucking vapour from the Bay where a high-pressure belt was pushing moisture inland.
The wispy clouds blocked the sun but unleashed another summer scourge — humidity — that made the weather sultry and miserable. The maximum temperature on Wednesday was 36.3 degrees Celsius, climbing down to normal for the first time in five days, while relative humidity jumped to 85 per cent.
“We need high temperature and a good supply of moisture for a thunderstorm to develop. Over the past weekend, heating was adequate but moisture was in short supply. On Tuesday, the moisture incursion intensified but the temperature reduced,” said a senior weather official in the city.
The sun, though, was poised to make a comeback. “The clouds wouldn’t stay and the heating will return,” said a weather scientist.
“The forecast says most of the weather systems injecting vapour inland on Wednesday will stay active for the time being. We will have a combination of heat and humidity over Jharkhand, an ideal situation for the formation of Nor’westers,” he said.
Nor’westers are born when moist winds from Bengal rush to fill the void in Jharkhand where the air heats up and rises. The heating kindles a cycle of cool, moisture-laden air dashing to Jharkhand, rising in the heat and forming cumulonimbus clouds in the process.
Some of these clouds — tall, menacing and heavy with water droplets — hurtle towards Calcutta and a thunderstorm occurs normally towards evening.
The city usually starts getting such squalls from late March but this year has been an exception. Barring one apology of a tempest on March 25 — that too triggered by a local system — the city hasn’t got a true-blue Nor’wester yet this summer.