Calcutta has a remarkably intense high-pressure zone called Siberia High more than 6,000 km away in north Russia to thank (or curse) for its coldest day in more than two decades.
The cold north wind that brings chill to the city originates in that massive collection of very cold, dry air on the Eurasian terrain for most of the year. This high-pressure belt has been unusually active this year, say meteorologists.
The last time the city had recorded anything below nine degrees was 24 years ago, on January 14, 1989, when the minimum recorded temperature was 8.4 degrees Celsius.
While the actual minimum reading in Alipore was nine degrees, the feel-like temperature ' a measure of the effect of the chill on the human body ' was 8.1 degrees Celsius, the Met office said.
Dum Dum was colder at 6.6 degrees Celsius on Wednesday morning but the higher reading in Alipore is the official one for the city in the records of the India Meteorological Department (IMD).
"This year, Siberia High has been very active. So the intensity of the cold wind is much more. The north wind hits Calcutta after blowing across Russia, Afghanistan, Pakistan, Kashmir, Delhi and Uttar Pradesh. All these places have also been colder than usual," a senior scientist with the IMD said.
"The minimum temperature in Delhi hit a five-year low recently (1.9 degrees Celsius on January 6) and places like Lucknow and Kanpur in Uttar Pradesh have experienced sub-zero temperatures," he added.
The tendency of a high-pressure zone is to drive the wind away, as opposed to a low-pressure area or cyclonic circulation that sucks in moisture.
Local weather conditions played their part in ensuring the chill in Siberia would send Calcutta scurrying for cover.
"The chill would have been nowhere this biting had there been a cyclonic circulation or a western disturbance. But with nothing to check the run of the chill-laden wind from the north, the night temperature slumped overnight," said G.K. Das, the acting head of the regional meteorological centre in Alipore.