A heatwave swept Calcutta on Sunday with loo-like winds taking the mercury to 40.5 degrees Celsius and keeping the Eden Gardens galleries uncharacteristically half-empty during the Kolkata Knight Riders' first match of IPL Season 9.

The conditions are likely to remain unchanged over the next few days, with little chance of a thundershower to bring some respite from the scorching heat, the weather office said.

A heatwave is defined as a prolonged period of excessively hot weather. In meteorological terms, a place is said to be in the grip of a heatwave when the maximum temperature exceeds 40 degrees Celsius and is 4.5 degrees or more above the normal reading for that time of the year.

The Eden Gardens is usually packed for the first home game featuring KKR, going by previous editions of the IPL.

The galleries had filled up quickly last year too, when defending champions KKR faced the eventual winners, Mumbai Indians, on April 8. The heatwave appeared to have blown away much of the home fans' enthusiasm in this edition's first game against the Delhi Daredevils.

"I and my wife tried to persuade our son Sarthak to watch the match at home in the comfort of the AC. We mentioned that there would be a constant supply of finger food and he would not miss going to the ground. He wouldn't be lured and finally we gave up," said Pradip Bhattacharya, sales manager at PepsiCo India, sweating it out with Sarthak in the upper tier of the BC Roy Club House.

Abhijan Chakraborty spent the entire day in an air-conditioned room but did not mind braving the heat to support his favourite team. "I had to come for KKR's first match at Eden!" said the IT professional, who was at Eden with friend Poulami Maity.

But many other die-hard fans could not convince their families and friends to escort or accompany them to Eden. The result? Rows of empty seats and a crowd count of between 25,000 and 30,000 fans.

Heat, the spoilsport, has been building up in the city since the middle of last week. The maximum temperature shot up by 1.9 degrees Celsius in 24 hours, thanks to the increased flow of hot winds from central India to south Bengal.

Since the normal maximum temperature in Calcutta at this time of the year is 36 degrees Celsius, Sunday's 40.5 just breached the heatwave barrier. The Met office had seen this coming, of course, after the western districts of Bengal came under a heatwave spell a couple of days ago.

The maximum temperature had first exceeded 40 degrees on Thursday. But the 40.1 reading that day didn't qualify for a heatwave because the temperature wasn't at least 4.5 degrees above normal.

While Calcutta has recorded 40-plus temperatures twice at the start of summer this year, 2015 didn't have a single day when the reading touched 40. The highest temperature recorded last year was 38.5 degrees Celsius on April 3. Even when there was a "hot phase" in May, the maximum temperature didn't exceed 38.2 degrees Celsius.

At 3pm on Sunday, the RealFeel temperature was 54 degrees Celsius. According to AccuWeather.com, RealFeel is a measure of how your skin feels the combined effect of heat and humidity at any given point in time.

The RealFeel reading is calculated taking into account weather parameters such as temperature, humidity, winds and the angle of the sun.

"Humidity has dropped in Calcutta. That is why there is no resistance to the hot winds blowing in from the northwest and the temperature is skyrocketing," said a senior official of the Met department.

Weather scientists said a cyclonic circulation over Assam and Meghalaya was directly drawing moisture from the Bay of Bengal, forcing the city out of the radar. The maximum temperature is tipped to rise further to 41 degrees on Tuesday, according to the website of IMD, Calcutta.

Calcuttans on the move were spotted wrapping dupattas around their face or using handkerchiefs to keep the hot winds from hitting the nostrils and face.

The Mamata Banerjee government has already announced that state-aided and state-run schools would remain closed from Monday till further notice to spare students the "intense heat".

Private schools have been requested to do the same.

South Point has suspended classes on Monday and Tuesday for Nursery I and II, Transition and Class I. Some other schools, including La Martiniere for Boys, said they would take a call on this on Monday.

And what chance of rain coming into the equation? "Since the temperature in and around Calcutta has gone up, atmospheric pressure has fallen. A high-pressure belt is needed in the Bay of Bengal so that some moisture-laden air comes the way of south Bengal. This will resist the loo-like winds from the northwest and increase the prospect of a thundershower or two," a weather scientist said.

Is this the hottest you have ever felt in Calcutta? Tell ttmetro@abpmail.com