An overcast sky over the Maidan at 4pm on Thursday. With high heat and humidity, the conditions were ripe for rain but the clouds flattered to deceive. Picture by Pradip Sanyal
A miserable mix of mercury and moisture made Thursday afternoon the most uncomfortable this season as the discomfort index rose to a dreary 66.4 degrees Celsius, 12 degrees above the comfort level.
Moisture incursion from the Bay of Bengal turned the sky grey by 3pm but an anticipated summer squall eluded the city for the third day on the trot.
The weather office raised hopes, though. A Met official said a weather system has been drawing moisture from the sea since Wednesday and it was expected to remain strong at least for the next 24 hours.
“We can expect a thunderstorm in Gangetic Bengal on Friday evening but the day would remain hot and humid.”
The maximum temperature climbed from 37.1 degrees Celsius on Wednesday to 38.7 degrees the day after, a gruelling four degrees above normal. It was just a notch less than the benchmark of five degrees above normal or 40-plus maximum temperature that prompts the Met office to declare a heat wave.
“The weather was intolerable and I thought a Nor’wester was on the way when the sky darkened towards late afternoon. That was not to be and the storm deceived us again,” said Indrani Pal, 32, of Salt Lake.
Calcutta was pushing towards a heat wave around 2.30pm when the maximum temperature read 38.4 degrees Celsius. Relative humidity was rising in tandem too and driving up the discomfort index — which reflects the effect of heat and humidity on the human body — to 66.4 degrees Celsius.
The discomfort level was way above the 61-62 degrees Celsius registered over the past two-three days.
“Humidity and the discomfort index are correlated. When humidity increases in this heat, the discomfort level rises simultaneously,” a senior Met official said.
The sultry condition was attributed to high humidity that prevents sweat from evaporating. “The heat makes you sweat. The air saturated with moisture doesn’t allow the sweat to dry up. Which is why we feel sticky and stifling in this weather,” a weather scientist said.
If the Bay was to be blamed for Thursday’s sweaty condition, it could be thanked for holding out hope for Friday. Gokul Chandra Debnath, the director of the India Meteorological Department, Calcutta, said: “There is a trough of low pressure from north Bengal to Odisha, which is pulling moisture inland. As there is adequate heating during the day, the conditions are suitable for the formation of thunderstorms on Friday in Gangetic Bengal.”
There was a “good chance” of rain in Calcutta, he said.
On Thursday, a thunderstorm developed over the western districts of Bengal and Odisha and started travelling east but fizzled out before the clouds reached Calcutta.
“The sky became overcast in Calcutta because of peripheral clouds while the cloud core that was more than 10km tall stayed away,” said a weather scientist. The irony was that less moisture in the Calcutta air during the first half of the day kept the thundercloud away.
Moisture incursion via the trough could make conditions ripe for rain in the city on Friday. A low-pressure trough pulls air towards itself and the one that could turn into the city’s saviour is drawing moisture from the Bay because of its position.