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John Wheeler, WDAY, Published December 29 2013

Weather Talk: Coldest weather usually arrives in January, February

Although this December has been remarkably colder than average, during most winters our coldest weather comes in January and February. The first two months of the year are the time of big high-pressure areas from the Arctic.

During winter, in the perpetual darkness of the higher latitudes, night after night of clear skies over snow-covered ground causes a tremendous loss of heat into space through a process known as radiational cooling. Air gets colder and colder under these conditions.

Simultaneously, this cold air grows very dense while the atmospheric pressure increases proportionally. As these areas of dense, cold air drop southward, they lose characteristics of their source region and begin to warm. But as long as the Canadian prairies are snow-covered (and they usually are), they only warm a little.

By March, increasing daylight in the Arctic causes these cold highs to become less cold. And, of course, if the upper-level winds do not blow them our way, we get fewer of them and our winter cold is less severe.


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