Warming Up Wind Chill

By: 
Charles Seife
2000-01-06 18:00
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The traditional method for calculating wind chill factor--the apparent temperature during frigid windy weather--may soon be blown away.

How cold the air feels to our skin depends on both the temperature and the speed of the wind that helps wick heat from the body. The formula used to put numbers on this sensation is based on World War II army experiments, says Maurice Bluestein, an engineer at Indiana University-Purdue University in Indianapolis. "They did a primitive study where they took a container of water and measured how long it took to freeze under different conditions," he says. This yielded an approximation of heat transfer under different temperatures and wind conditions, which, in turn, led to a rough estimate about how much heat the body loses in the same environment.

But the experiments were crude, says Bluestein: The researchers didn't take into account the container's insulating properties, and they assumed a steady skin temperature of 30 degrees Celsius, rather than skin that cools with the ambient temperature. This means the wind chill factor has been over-estimated.

To right this wrong, several researchers have been beavering away at a better formula. According to Bluestein's numbers, for instance, a -21 degree Celsius day with a steady 40-kilometer-per-hour wind will make it feel like it's -36 out--not -42 degrees, as current calculations would suggest. Such a revelation may not make snow shoveling in Nunavut any more fun, but weather watchers should enjoy the precision.

Robert Quayle, who works for the National Climatic Data Center in Asheville, North Carolina, has gathered a group of experts to vet Bluestein's and other new models. "We'll be comparing the indices and making a recommendation to the Weather Service," says Quayle, who expects to unveil his team's findings in May at the annual meeting of the American Meteorological Society.

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