With the mercury in rise rather than slide mode this winter, Tuesday turned out to be the warmest December 21 the city has witnessed in six years. The minimum temperature touched 20.4 degrees Celsius, six degrees above normal.
'This warm weather is expected to remain for at least the next two or three days,' said an official at the Alipore meteorological office.
The weatherman added that the minimum temperature was likely to hover around 20 degrees till the warm spell lasts, raising the spectre of a Christmas minus the chill.
An upper-air 'anti-cyclonic' circulation over central Bay of Bengal had taken shape gradually since Monday, leading to a heavy inflow of warm moisture-laden air to south Bengal, Orissa and Andhra Pradesh.
'As a result, the sky has remained hazy and the changed wind pattern has blocked the North Wind,' observed an official, adding that the sky was expected to remain partly cloudy on Wednesday.
Officials said the upper-air circulation has a tendency to shift slowly in a north-easterly direction, weaken and then dissipate.
'But this system has just formed, so we will have to watch it and see how long it takes to weaken,' they added.
Officials at the regional meteorological centre in Alipore could not remember the last time it was so warm in the third week of December. 'This afternoon, it really felt like spring. In fact, I felt the south wind brush against my face,' said an official.
This December, the minimum temperature is yet to drop below 16 degrees Celsius. Over the past 10 days, the minimum temperature has been over 17 degrees and rising.
This is the result of a number of systems over land and sea, blocking the cold wind from the north-west and the north. A week ago, an upper-air trough line of low pressure, spread across Jharkhand and Gangetic Bengal, had effected a change in the wind pattern and sucked in moisture-laden wind from the sea, blocking the North Wind.
'Once the low-pressure trough petered out, we thought the passage was clear for the North Wind. But on Tuesday, we found this upper-air circulation in the middle of Bay of Bengal,' said a weather official.
Former deputy director-general of the regional meteorological centre R.N. Goldar termed the prevailing situation 'abnormal'.
'The minimum temperature is six degrees above normal, which is quite abnormal around this time. From my records, I find that on December 20, 2002, the minimum temperature had risen to 19.8 degrees. But Tuesday's minimum was even higher,' Goldar added.
Once the sky clears, the North Wind will come rushing in from Jammu and Kashmir, Uttar Pradesh and the adjoining areas, assured officials at Alipore.