In the past 72 hours, the mercury has been defying winter logic: it is going up, rather than down.
With the minimum temperature on the Celsius scale creeping up, the weather office went into a huddle to trace the trend caused by the North Wind being blocked. This warm wave is likely to continue for the next few days, observed the Alipore Met office.
Officials said the minimum temperature, pegged at 16.2 degrees Celsius on December 6, started rising gradually and had reached 18.3 degrees, four degrees above normal, on Sunday. The minimum temperature slipped to 17.5 degrees on Monday, still three degrees above normal.
'But we can expect the North Wind to blow again in the next couple of days,' said K.K. Chakraborty, director of the weather section, regional meteorological centre, Alipore.
Chakraborty said a change in the weather pattern had blocked the North Wind and led to incursion of moisture and warm air from the Bay of Bengal.
'The current phenomenon is due to an upper-air trough of low pressure over Uttaranchal, extending up to south Bengal and north Bay of Bengal. This has changed the wind pattern,' explained Chakraborty.
A similar phenomenon, he added, had pushed up the minimum temperature to 20 degrees in the third week of December 2002.
Another weather factor has been blocking the North Wind. 'An upper-air circulation over Bangladesh and adjoining Tripura, extending up to 2.1 km above sea level, has also blocked the North Wind and brought in warm air from the Bay of Bengal. We expect this upper-air circulation to move away in a north-westerly direction within two or three days,' observed an official.
The Celsius scale has had a direct impact on the sale of woollens at big stores. 'The sale of heavier woollen wear is yet to pick up. At present, there is a great demand for lighter jackets with woollen or light lining inside,' said Naveen Misra, customer care associate and manager (operations), Shoppers' Stop.
'But as the mercury dips within a week, the sale of winter wear will pick up,' added Misra, from the anchor store of Forum.
The weather office, too, is eyeing a Celsius slide by next week. Once the two systems weaken and fade out, and the passage of the North Wind from the region of Uttar Pradesh and Bihar is clear, the minimum temperature would drop below 15 degrees, probably by December 20.
'The ideal condition for the chill to set in is clear sky without any disturbance, like low pressure trough-lines capable of effecting a change in the wind pattern,' said an official at Alipore.