The Celsius soared to a season high of 40.5 degrees on Wednesday, prompting the weather office to sound the summer’s first heat-wave warning for Calcutta.

The hottest day of the year so far was also the second hottest May day of the decade after a 42.1-degree scorcher on May 9, 2009. Only thrice in the past 10 years have May temperatures hit 40-plus in the city.

The Celsius surge contrasted with a sharp fall in the minimum relative humidity from 64 to 28 per cent, putting Calcuttans through the other extreme of dry heat after a sweat-filled Tuesday.

The day’s maximum temperature was six degrees above normal for this time of the year, which in meteorology is a point more than the threshold for a heat-wave alert. The norm is to issue such a warning whenever the maximum summer temperature shoots five degrees above normal.

“The loo-type weather on Wednesday is likely to continue for at least the next 24 hours. Rain isn’t likely before Friday afternoon,” said Gokul Chandra Debnath, the director of the Regional Meteorological Centre in Alipore.

“A heat-wave warning has also been issued for Malda and the two Dinajpurs, but there the weather might improve sooner,” he added.

Tuesday had started with an overcast sky that promised some rain relief but the “localised clouds” blew away overnight, allowing the sun to blaze down on the city on Wednesday. “My skin was burning after a few minutes in the sun. I was wearing sunscreen, but the only way to escape being tanned was to wrap the dupatta I was wearing around my face,” said Tanaya Ray, a mother of two who steps out of home twice in the afternoon to pick up her kids from a drop-off point on the Bypass where their school buses stop.

Debaprasad Roy of Baguiati, employed with a private bank, couldn’t remember the last time he experienced the “burning feeling” on a summer day in Calcutta. “A gust of hot wind hit my face the moment I stepped out of home to head for my workplace. I haven’t experienced anything like this in the city lately,” he said.

The discomfort index on Wednesday was 68 degrees Celsius, 13 notches above normal. Experts said it would have been much higher had the moisture content in the air not declined in the past 24 hours.

“The long absence of rain has made temperatures shoot up in Calcutta and other parts of Bengal. But we expect the weather to improve in a couple of days. We might have rain around Friday,” said Ajit Tyagi, the director of the India Meteorological Department.