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Alert for heatwave rerun as humidity drops

- Hope of rain relief vanishes with moving trough, brace for hot and dry winds over the next two days

Calcutta faces its second heatwave alert in less than a month with the Celsius primed to shoot up and relative humidity poised to plunge over the next two days.

If there was a slim chance of rain until Sunday evening, it disappeared on Monday. “There was a trough of low pressure stretching from north Bengal till Odisha. The trough has since shifted eastwards and is not causing incursion of moisture inland anymore,” said Gokul Chandra Debnath, director of the India Meteorological Department, Calcutta.

The change in direction has paved the way for cloudless skies and hot, dry winds. Monday’s maximum temperature was 39.1 degrees Celsius, four degrees above normal. While the Celsius is on the rise, minimum relative humidity, which indicates the moisture content in the air, has dropped 13 per cent over the past four days. The reading on Monday was 24 per cent, against 37 per cent on Friday.

The Met office declares a heatwave when the maximum temperature hits 40 degrees Celsius to go five notches above normal. The normal maximum temperature in Calcutta at this time of the year is 35 degrees.

A heatwave had been declared for a day on March 30 when the Celsius reached 39.9 degrees Celsius, a decimal short of the benchmark but six degrees above normal.

The Met office has forecast loo-like conditions in the western districts of Bengal on Tuesday and Wednesday. Calcutta is likely to experience a bit of the same.

A weather scientist clarified that the hot, dry winds wouldn’t be coming from the Chhotanagpur Plateau, which is known for loo at this time of the year. This is because many places in that region are cooler than usual at this time.

Even if heating increases over the plateau, the lack of moisture means there is little chance of a Nor’wester forming there for now. Heat and humidity are both required for the formation of thunderclouds.

While the temperature was high on Monday, the weather wasn’t as oppressive as it would be with humidity on the higher side.

The discomfort index, which indicates the combined effect of heat and humidity on the human body, was 62.6 degrees Celsius, eight notches above normal. That was at 2.30pm, when the temperature reading at the Alipore observatory was 38.8 degrees and relative humidity was measured at 25 per cent.

The discomfort index was higher at the same time on Sunday because relative humidity was higher, making it hard for sweat to evaporate. There is apparently no weather system in the vicinity of Calcutta that can change the pattern immediately.