Winds howl like banshees loosed from arctic imprisonment, and snow fills the sky, a swarm of ermine moths stampeding under the windsangerscream whip. Those snow crystals reaching the ground hiss along the icy surface formed under the frigid boreal breath, the broad trough between frozen wave crests: snow dunes, snow drifts. A blizzard wraps its fury around the town and closes it down.
While the media rush to call any severe or heavy snowstorm a blizzard, true blizzards have a strict definition according to meteorologists, and snow need not even fall in a blizzard. Blizzard conditions in the United States occur, according to the US National Weather Service, when a winter storm has wind speeds that exceed 35 mph (56 km/h) and visibility reduced by falling or blowing snow to a quarter mile (400 m) or less. Such conditions must last for at least three hours. A severe blizzard boasts winds in excess of 45 mph (72 km/h). There is no specific temperature criterion in the US definition, but blizzards commonly have air temperatures below 20 oF (-6.7 oC) and severe blizzards below 10 oF (-12.2 oC).
In Canada, blizzards take on a slightly different character which varies with the province where the storm occurs. The official Environment Canada definition states a blizzard, in general, is a winter storm lasting for at least three hours with winds exceeding 40 km/h (25 mph) and visibility reduced by falling or blowing snow to under a kilometre (0.625 miles). All regional definitions contain the same wind speed and visibility criteria but differ in the required duration and add a temperature criterion. A coastal British Columbia blizzard need not even have its temperature below freezing but must last for six hours!
Blizzards take on different characteristics when they rage around North America, combining wind and temperature and snow into a variety of recipes, though they all start as low pressure systems. In the Arctic and northern Canadian Prairies, blizzard snowfall is generally light with required visibility reductions caused by blowing snow under the gale-force winds. In the Pacific Northwest and in its coastal mountains, blizzards result when strong Pacific storms drive relatively warm, moist air against extremely cold air masses pushing out of the interior plains and valleys through the mountain passes, and thus they may not be very cold.
The Rocky Mountain chain spins up blizzards with the names of Alberta Clipper and Colorado Low sending wind, snow and severe cold southeastward from Arctic latitudes to as far south as Texas, where they are known as Blue Northers. Snowfall may become very heavy in regions when the arctic air plunges into tropical Gulf of Mexico air. Such storms may eventually spin across the Mississippi and Ohio River valleys, the Great Lakes and Atlantic Coast burying them in feet of snow.
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