The mercury didn't climb above 36.1 degrees Celsius on Thursday but sweat-drenched Calcutta had to contend with conditions that felt close to 50 degrees Celsius, 10 notches hotter than desert cities like Dubai and Jaisalmer.

At noon, with the Celsius peaking and relative humidity at 60 per cent, the RealFeel temperature, as calculated by AccuWeather.com, was almost 45 per cent higher than the Met office's reading. The heat hardly eased over the next three hours, still reading 49 degrees Celsius at 3pm.

RealFeel is a formula devised to determine how hot your skin feels outdoors, taking into consideration weather parameters like the day's temperature, relative humidity, wind and angle of the sun.

Dubai and Jaisalmer were hotter in terms of maximum temperature but felt more comfortable than Calcutta. The maximum temperature in Dubai was 40 degrees Celsius while Jaisalmer recorded a maximum of 41 degrees. The RealFeel temperature in these two cities was negligibly higher at 42 and 40 degrees respectively.

Pawan Tibrewalla, 52, knew it would be a stiflingly hot day in Calcutta even at 5.30 in the morning "I usually jog near the Victoria Memorial for 40 minutes every morning but I gave up within 20 minutes on Thursday. It was that bad," the businessman said.

The reason for the sweaty and stifling feeling over the past couple of days is the combination of blazing sun and high relative humidity. There is plenty of moisture in the air but no weather system in the vicinity that could lead to cloud formation and cause rainfall, the weather office said.

"The moment I stepped out of the house in the afternoon, a gust of hot wind hit my face. I was sapped of all energy in the 10 minutes that it took me to get to the bus stop and escort my son back," said Abira Basak, a homemaker from Kasba.

According to weather scientists, humidity soars at the ground level in the absence of a cyclonic circulation or trough of low pressure to push the moist air into the upper atmosphere, leading to cloud formation.

Clouds not only filter the sun's rays and reduce the surface temperature but also increase the possibility of rainfall to cool the temperature and suck out the moisture in the air.

"In the absence of favourable weather systems, moisture hasn't been able to reach the upper atmosphere. It is staying in the lower reaches, in the 60 and 75 per cent range, even during the hottest part of the day, which is extremely high," a Met official explained.

So, even as the maximum temperature in the city peaked at 36 to 37 degrees Celsius on Wednesday and Thursday - the normal reading for this time of the year is 35 degrees - the level of discomfort suggested a much higher temperature.

"Perspiration is the body's mechanism of cooling down. A person would sweat the same amount at a particular temperature, irrespective of the relative humidity. But when relative humidity is high, sweat doesn't evaporate easily and this makes it uncomfortable for us," a weather scientist said.

Since ample moisture is stuck in the lower atmosphere, perspiration has been lingering on the skin, making anyone not in an air-conditioned room feel extremely uncomfortable.

According to the Met office, relative humidity is high because of two factors apart from the absence of weather systems close to the city.

The wind changed direction on Wednesday and began blowing from the south, further stacking the ambient air with water vapour from the Bay of Bengal. Another factor that has kept the moisture content in the air intact is Calcutta's missed date with the last couple of Nor'westers that hit south Bengal but didn't travel to the city.

So, how can this weather pattern change?

The Met department has reported a cyclonic circulation in the making over Bangladesh but its position is unfavourable for rain in Calcutta until Thursday. On Friday, it is likely to move towards the city and get stronger, which would be ideal for thunderstorms.

Nor'westers, which are storms generated in the Chhotanagpur Plateau, aren't expected anytime soon as Jharkhand is very dry at the moment.