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Idaho Voices

Quick pressure change brings high winds

Sun., Oct. 31, 2010, midnight

When you feel the wind outside blowing, what you’re feeling is the movement of air from high to low pressure. It is the same concept whether it is a gentle breeze, stiff winds behind a cold front, damaging winds from a tornado, or gale force winds from a hurricane. The strength of the wind is directly proportional to the pressure gradient, or the change in air pressure over a distance. When air pressure changes rapidly over a short distance, for example, in the vicinity of a tornado, hurricane, or even just a strong area of low pressure, winds will be strong.

Speaking of strong areas of low pressure, beginning on Oct. 26, a strong storm system that swept across the Midwest was associated with nearly 500 reports of wind damage and at least 28 tornadoes! Nonthunderstorm wind gusts of over 70 mph were reported in Illinois, Indiana, Michigan, South Dakota and Wisconsin, while as much as 9 inches of snow were added to the winds in Minnesota and the Dakotas resulting in blizzard conditions. An all-time low pressure record was set in the state of Minnesota as the storm moved through. A minimum barometric pressure (adjusted for true mean sea level) of 28.20 inches was recorded at Bigfork in Itasca County. If you were looking at a weather map with isobars, which are lines of constant pressure, you would have seen a bull’s-eye of 954.96 millibars. Millibars are the units most meteorologists use to measure air pressure, though the worldwide standard unit unit is actually the pascal (Pa). The Minnesota reading also ranks as one of the lowest pressures for a nontropical storm in the whole United States! To give you an idea just how strong the area of low pressure was, it would be equivalent to a category 3 hurricane!

Locally, the wind has not been quite as dramatic as the change in weather pattern from mild and dry for the first three weeks of October, to … winter. Temperatures over the past week have been consistently 3 to 8 degrees below normal, and anywhere from 3 to 19 inches of snow were measured in the mountains of northeastern Washington and North Idaho. In the valleys, it wasn’t quite cold enough for snow (thank goodness) but we did get a healthy dose of rainfall, which boosted October precipitation totals to above normal status. As of Oct. 27, Coeur d’Alene had measured 2.41 inches of rain, nearly a half inch above normal. In Spokane, 1.11 inches had fallen, about a quarter inch above normal. The upcoming week looks to be unsettled as well, with another big dip in temps by the end of the week.

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