The winter has been the coldest in the past decade if one goes by the number of successive days the minimum temperature has stayed at or dropped below normal.

Officials at the Regional Meteorological Centre in Alipore said the minimum temperature crossed the normal mark only five times in five weeks, between December 23 and January 28.

The remaining days were all “cold” in meteorological terms, a first in 10 years.

“The cold feeling was more than ever as the minimum temperature stayed normal or below normal on the trot from the last week of December, except on January 10, 11, 14, 15 and 16. There is no parallel to this kind of a prolonged cold spell this decade,” Gokul Chandra Debnath, the director of the Alipore Met office, told Metro.

The second coolest winter stretch the city experienced this decade was in January 2001, when the minimum stayed normal or slid below the mark for 18 days.

“January 2001, too, was quite cold and the minimum temperature on certain days dropped lower than this season’s lowest (10.8 on December 28) but this is the coldest winter Calcuttans have experienced in recent times, in terms of longevity,” Debnath said.

The other exceptional feature of winter 2009-10 is its “cold day phase”, a phenomenon marked by the maximum temperature dipping to at least five degrees below normal. On January 17, the maximum was 10 degrees below normal at 17.2. The cold-day phase continued from January 17 and 20.

Was the prolonged chill really unexpected, given that it followed some unusual hot spells in 2009?

“I wouldn’t call it unusual,” Debnath said. “This type of variability exists in weather phenomena and there was no reason to assume that the hot spells in summer and monsoon would usher in a warm winter.”

The weatherman said abundant snowfall in the northern parts of the country and a series of cold waves in north India as well as in Bengal’s neighbourhood added to the chill.

Winter, which arrived a fortnight early on November 22, still has “three to four weeks” of life left, according to the weatherman.

The minimum on Thursday touched 13.3 degrees Celsius, a degree below normal. Friday will be “mainly clear”, with the minimum temperature hovering around the 13-degree mark.