The heat wave that has left Calcutta gasping is the worst the city has experienced, according to the Met office record books that date back to the summer of 1950.

The city has been simmering since April 18, with the mercury setting a cruel record by crossing the 40-degree mark eight times in 10 days (see graphic).

On Monday, the temperature touched 41.1 degree Celsius, five degrees above normal, coupled with a discomfort index of 64 degrees Celsius, nine degrees above normal.

With not a cloud in the sky for the 10th day on the trot, Metro got G.C. Debnath, the director of the Regional Meteorological Centre at Alipore, to decode the city’s longest hot spell.

How many days in April has the mercury crossed 40 degree Celsius?

Eight days so far. The first was on April 18, when the mercury touched 40.8 degree Celsius. April 19 was the hottest April day in 29 years, with the temperature rising to 41.2 degree Celsius.

Is this a 40-plus record?

This is a new record. Calcutta has never experienced such a long spell of 40-plus temperatures in the past 59 years (the records start from 1950).

Eight out of 10 days has no precedent, not just in April, but in all of the city’s recorded summers.

There were three-day spells of temperatures breaching the 40-degree mark in 1969, 1972 and 1980. The hottest ever April day was April 25, 1954, when the mercury touched 43.3 degree Celsius.

What were the factors at play during the previous hot spells? Were they different from the factors now?

The conditions and factors were remarkably similar. Like this April, on all these occasions, hot, dry northwesterlys and westerlys were blowing in towards south Bengal because of the heat wave conditions prevailing over central and north India. This wind flow, over the years, has mainly been responsible for the Celsius surge. This time there has also been a conspicuous absence of sufficient moisture in the lower troposphere over south Bengal, which is a must for Nor’westers, the prime source of relief in April.

Will this dry and hot April be a watershed in Calcutta’s meteorological study?

Definitely. This year is a watershed. This April is remarkable because of a lot of factors, like the cyclone Bijli bypassing us and the relentless northwesterlys from central and north India, which have greatly destabilised the atmospheric conditions. For instance, the monsoon this time could get delayed because of what we are seeing this April.

This April hasn’t followed the normal course and that is also because of the global pattern. Recovery from such destabilisation takes time, two weeks or more.

Does the April hot spell have anything to do with Pujas being early this year?

No. Not at all. There is no scientific basis to that.

What do you blame for your suffering this summer? Tell ttmetro@abpmail.com