Calcutta’s hottest April day in a decade felt even hotter because of an overnight transition from dry to sultry weather.
The day’s maximum temperature of 41.2 degrees felt like 45 degrees with moisture making a comeback — not quite adequate to induce rain but enough to make Calcuttans sweat and fret. The temperature equalled the record set on April 19, 2009, but Friday felt more uncomfortable because the minimum relative humidity was higher.
Minimum relative humidity, the index of moisture in the air during daytime, rose from 17 per cent on Thursday to 24 per cent and is expected to go higher.
“Moisture is coming in because of a trough of low pressure stretching from north Bengal to Odisha. This weather system is likely to intensify. So the weekend could be even more sweaty and stifling,” said Gokul Chandra Debnath, director of the India Meteorological Department, Calcutta.
The meteorology website accuweather.com pegged the “real feel” temperature — the one felt by the human body — in the city on Friday at 45 degrees Celsius.
The figure was calculated taking into account humidity, cloud cover, winds, sun intensity and angle of the sun, among other parameters.
Sweat evaporates easily when the relative humidity or the moisture content in the air is low. But as relative humidity rises, sweat tends to linger on the skin, making people more uncomfortable.
Weather scientists at IMD, Calcutta, take the maximum temperature and the day’s relative humidity to calculate the discomfort index, which indicates the combined effect of these two factors on the human body. The day’s discomfort index was 67.6 degrees Celsius, 13 notches above the comfort level.
The discomfort index on April 19, 2009, whose record temperature Friday equalled, was 64.8 degrees Celsius. But a five per cent increase in minimum relative humidity on Friday made a difference of three degrees to the discomfort index.
The last time an April day was hotter in the city was 34 years ago. The Alipore observatory had recorded 41.7 degrees Celsius on April 25, 1980.
The forecast is for the heatwave declared on Tuesday to continue over the weekend. Since the heatwave started, the city has recorded temperatures of 40, 40.8, 41.1 and 41.2 degrees Celsius.
This translated into an average temperature of 39.9 degrees Celsius over the last seven days. The average maximum temperature for April is 4.4 degrees lesser at 35.4.
Even Delhi’s summer, marked by occasional rain because of Western Disturbances that have adopted a more southern trajectory than usual this year, hasn’t been able to keep pace with the Celsius in Calcutta. In Bengal, places like Canning and Sriniketan recorded a lower maximum temperature on Friday than Calcutta’s 41.2 degrees Celsius.
For those hoping that a rise in humidity would bring rain, the Met office has news that will disappoint.
“Humidity will be rising over the next two days in Calcutta, so we can expect the weather to be more oppressive. But there is little guarantee of rain because of the predicted increase in the moisture content of air,” a weather scientist said.
According to the director of IMD Calcutta, a thunderstorm might hit some parts of Gangetic Bengal over the next 48 hours towards evening. Light, localised showers are expected more towards the coastal regions.
“Only if thunderclouds form during the day on Saturday will it become clear that Calcutta will get some rain,” Debnath said.
The rain, if it comes at all, will be light and the heatwave is likely to continue on Sunday, the Met office said.
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