The minimum temperature remained below normal in Calcutta for the 17th consecutive day on Wednesday, an unusually long spell brought about by uninterrupted flow of cold winds alternately from the north and the north-west.

The Alipore Met office recorded a minimum of 11.9 degrees Celsius on Friday — a notch below normal — falling a little from 12.6 the day before.

Officials said the temperature could increase slightly on Monday before starting to fall again from Tuesday.

The current spell of sub-normal minimum temperature started on December 19 and had its lowest point on December 29, the day the Celsius plunged to 10.6 degrees, the season’s lowest so far.

The weather has got warmer since but the minimum has remained below normal.

Meteorologists say it is rare for Calcutta to have so many days of below-normal minimum temperature at a stretch during this period.

In the last season (2017-18), the minimum had dropped below normal only once between December 19 and January 4. In the three seasons before that, the maximum number of days the minimum had stayed below normal was eight.

“Calcutta gets its coldest days between December 15 and January 15. But the spells are usually shorter. This year the cold spell has been continuing for a longer duration because the two winds that bring cold to Calcutta — the northerly and the north-westerly — are complimenting each other,” said Sanjib Bandyopadhyay, deputy director general, India Meteorological Department, Calcutta.

The intensity and speed of the north-westerly wind depend on snowfall in Jammu and Kashmir, and that of the northerly on the conditions in Sikkim and Darjeeling.

The coolest phase comes when the two winds blow together.

The minimum dropped below normal on December 19 under the impact of the north-westerly wind. Just when the wind started weakening, the northerly gained steam following snowfall in Sikkim and Darjeeling.

The north-westerly packed punch again at the turn of the year.