Thursday afternoon was extremely hot and sweaty as the mercury had jumped more than three notches in a day to reach 34.6 degrees Celsius.

The Met office expects the weather to remain like this till Sunday when showers are likely.

Accuweather.com says the RealFeel temperature - which indicates the effect of temperature, humidity, angle of the sun and winds on the human skin - was 48 degrees Celsius around 4pm because of high moisture content in the air.

"The weather will be similar to Thursday's for the next two days. But from Sunday rain is expected to make a comeback in Calcutta and cool things down," G.C. Debnath, the deputy director general of meteorology, IMD, Calcutta, said. "A cyclonic circulation is brewing over the Bay of Bengal. It is near the Myanmar coast at the moment but is expected to veer towards the Bengal and Bangladesh coast slowly."

Weather systems like depressions or cyclonic circulations have atmospheric pressure lower than their surroundings. This causes them to draw air from surrounding areas towards itself.

The cyclonic circulation would inject moisture-laden air from the Bay of Bengal into the city air, leading to clouding and rainfall, he said. "At the moment, there is a deep depression over east Madhya Pradesh and Chhattisgarh, which is drawing moisture and causing heavy rainfall there. Calcutta falls on the outer reaches of this weather system."

Had the depression been closer to Calcutta, the city would have received showers as well. The city is getting limited supply of moisture from the Bay because of the distance.

This has pushed up the levels of humidity and discomfort without forming rain-bearing clouds, a Met official said.

The relative humidity varied between 57 and 75 per cent throughout the morning and afternoon on Thursday. Whenever the sun shone bright, the temperature increased, leading to sweat and discomfort, he said. There were odd showers in some parts in the afternoon.