Hot, hotter, hottest: no relief in sight

Rith Basu April 10: The mercury shot up to 36.7 degrees Celsius today, making it the hottest in 303 days. The sultry spell is likely to continue for two more days, the Met department has said. The last time the city had experienced hotter weather was on June 11 last year, when the maximum temperature had touched the 40-degree mark and the city was reeling under a heat wave.

The moisture in the air, coupled with the heat, took the RealFeel temperature calculated by weather portal Accuweather.com up to 46 degrees Celsius this afternoon.

"We expect similar conditions to continue because the maximum temperature would remain in the range of 37 degrees Celsius for the next two days and humidity is also likely to stay as high," said Sanjib Bandyopadhyay, deputy director general, India Meteorology Department, Calcutta.

He said there was a trough of low pressure - an imaginary line that connects low-pressure points - extending from north Gangetic Bengal to south Chhattisgarh at a height of around 0.9km from the surface. Having atmospheric pressure lower than its surroundings, such weather systems draw air towards themselves.

The trough over Bengal was positioned in such a way that it was drawing moisture from the Bay of Bengal into Calcutta and its surroundings. "The current trough is such that it is drawing moisture from the Bay but there isn't enough to help form rain-bearing clouds," a senior Met official said.

The relative humidity in the city varied between 30 and 89 per cent today. When both heat and humidity are high, sweat does not evaporate from the skin, causing discomfort. "Sweating is the body's mechanism of losing heat. The body cools when sweat evaporates from the skin. But when humidity is high this evaporation does not happen," a senior Met official said. As a result, the body does not cool down and keeps producing more sweat, giving that sticky feeling.

What added to the discomfort was a two-degree climb in temperature in just 24 hours. The maximum temperature had been 34.6 degrees Celsius yesterday.

According to records, the city gets around four squalls accompanied by rain in April and around six in May and these bring reprieve from the heat. But the first 10 days of April this year have seen no such activity in Calcutta although there have been thunderstorms in the western districts of Bengal.

Most of the thunderclouds that bring these storms to Calcutta are generated over the Chhotanagpur plateau but for that to happen it must be both hot and humid, a criterion that is not being fulfilled at the moment. "There needs to be a low-pressure system near Jharkhand for moisture supply to the area, something that is not there at the moment," according to Bandyopadhyay.

Last April, too, had been scorching with no rainfall all month and six days of heat wave when the temperature crossed 40.