The Celsius struck 40.1 on Thursday, six degrees of separation from the maximum temperature recorded barely two days ago. And the news from the weather office is that April could get crueller with a heatwave over the weekend.

The forecast says that the maximum temperature would remain in the range of 39 to 40 degrees Celsius on Friday and possibly rise to 41 degrees or more by Sunday.

The 40.1 degrees recorded on Thursday is, of course, the highest of the season and four notches above normal.

The Met department's yardstick for a heatwave is a maximum reading of 40 degrees Celsius or higher, translating into 4.5 degrees or more above normal. In Calcutta's case, the cut-off is 40.5 degrees Celsius since the "normal" for this time of the year is 36 degrees.

The last time the mercury had crossed 40 degrees in the city was 687 days ago: on May 21, 2014.

The temperature didn't touch 40 degrees in 2015.

The weather had been hot even on Tuesday, with AccuWeather.com pegging the RealFeel temperature for that day in the region of 50 degrees Celsius.

But the nature of the Celsius rise has since changed. The warm and humid conditions at the start of the week have made way for drier heat, making the Celsius soar to its highest of the summer.

RealFeel is a reading calculated using multiple factors, including temperature, humidity, cloud cover, sun intensity and wind, to arrive at an index of how much your skin feels the heat outdoors.

The western districts are currently experiencing even hotter conditions than Calcutta, with Bankura topping the Celsius charts at 45 degrees, followed by Asansol at 42.2 degrees and Sriniketan at 41.9 degrees Celsius.

"By Saturday, we expect Calcutta to be in the grip of this heatwave, which will continue at least till Sunday," said Gokul Chandra Debnath, deputy director of the India Meteorological Department (IMD) in Calcutta.

The official IMD forecast states that the atmosphere will become even more hot and dry since winds from neighbouring Bihar and central India were blowing into south Bengal, including Calcutta.

According to Debnath, a trough of low pressure lingering over the Odisha coast until Wednesday had moved east towards Bangladesh, leading to a drop in the moisture content of the air.

A trough is an imaginary line joining the low-pressure points across the breadth of the country. Since troughs have atmospheric pressure lower than there surroundings, they draw air towards themselves.

The trough over the Odisha coast had been drawing moisture-laden air from the Bay of Bengal into Calcutta and its surroundings.

With the trough gone, relative humidity has declined rapidly. This has had an effect on the conditions as well as the impact of heat. "The temperature has risen because moisture is no longer resisting the flow of hot winds from the west," a Met official said.

Parts of the city received a drizzle late on Thursday, but the weather office said this wouldn't help. A thundershower in the next couple of days isn't ruled out, though.