The Telegraph
Monday , May 19 , 2014
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‘Extremely rare’ heatwave week

Heatwave conditions for the seventh consecutive day prompted meteorologists on Sunday to call the ongoing phase “extremely rare” and warn that the hot spell would continue for some time.

Calcutta’s tryst with above-normal temperature continued as the Celsius shot up to 39.7 degrees on Sunday, five notches more than it should be for this time of May. The mercury was slightly up from the 39.2 degrees Celsius recorded on Saturday.

The mercury had risen to 41.4 degrees Celsius last Monday, the highest for May in a decade, marking the start of the current heatwave. Since then, the maximum temperature has been around 40 degrees Celsius.

A heatwave is declared when the maximum temperature climbs five notches above normal and reaches 40 degrees Celsius. “The current spell is being considered a heatwave because the maximum temperature has remained close enough to 40 degrees Celsius,” said a Met official.

Weather scientists expect the maximum temperature to continue in the 40-degree range for at least two more days since no weather system was pulling moisture towards the city at the moment.

“The city had recorded 40 degrees Celsius in the past but we cannot recall any instance of high temperatures for such a long stretch. A spell like this is extremely rare,” said Devendra Pradhan, deputy director general of meteorology at the India Meteorological Department in Calcutta.

“In Calcutta, temperatures of 40 degrees Celsius are recorded for a day or two (in a season),” he added.

The experts attributed the hot spell to unfavourable conditions over the Bay of Bengal. “A high-pressure belt over the Bay close to the Bengal-Odisha coast is a common occurrence during summer. This helps in the formation of squalls that bring rain relief to Calcutta. But the Bay system has either been missing this year or shifted position towards the south,” said Ganesh Kumar Das, meteorologist at IMD, Calcutta.

The city has received only three squalls so far this year against six to seven that it should have.

Inland low-pressure zones that pull moisture to form rain-bearing clouds usually compensate the absence of Bay-born high-pressure systems. Even those have ditched the city since most of them were positioned far away and had lost sting on the way — like the one that developed over the weekend which brought specks of cloud but no rain.

“Abundant moisture is required for the formation of rain-bearing thunderclouds. Heating without enough moisture incursion, something that we have been observing in abundance this year, will not result in rainfall,” a weather scientist said.

This explains why Nor’wester nursery, the Chhotanagpur Plateau, has failed to produce a good shower for Calcutta this summer.