Alipore: The sun beat down hard on Wednesday morning and it seemed the mercury would follow the forecast of 40-plus degrees Celsius, but conditions changed dramatically in a matter of minutes around 11am. The skies quickly darkened and rain swept away several days of relentless heat and humidity.

Calcuttans had been sweating from as early as 8.30am, by which time the mercury had shot up to 34 degrees Celsius. By 10.30am, it went up to 36.1 degrees, the highest temperature of the day. The day's highest reading is usually recorded at 2.30pm.

A little after 11am, the clear sky through which the sun had been beating down was enveloped in a dark mass of clouds. By 2.30pm, supposed to be the hottest part of a day, the rain had cooled Calcutta down to 26.4 degrees Celsius.

"There was a prediction for a heatwave on Wednesday, but thunderclouds formed sooner than we had anticipated. The rate at which the temperature was increasing in the morning, it would have gone past 40 degrees Celsius by afternoon had the rain not started before that," said Sanjib Bandyopadhyay, the deputy director-general of the India Meteorological Department, Calcutta.

According to him, a trough of low pressure that the Met department had expected to dissipate on Wednesday stayed put. A cyclonic circulation that formed in Bihar made the conditions even more conducive to rainfall. The third factor was the south-westerly winds picking up, ensuring a steady flow of moisture to feed the clouds.

The rain started in north Calcutta but poured longer in the south.

An IndiGo flight from Hyderabad that was to land at 11.15am had to be diverted to Bhubaneswar. Another IndiGo flight, this one from Bangalore, could not land at 11.25am and was also diverted to Bhubaneswar. An AirAsia India flight to Nagpur ready to take off returned from the edge of the runway at 10.20am because of the strong wind. A second take-off attempt at 11.07am was also aborted. The plane finally took off at 12.39 pm.

Thundershowers are on the radar for the next couple of days. The maximum temperature will be range-bound at around 35 degrees, the weather office said.