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Chilling Out

National Weather Service defrosts its official wind chill readings

THURSDAY, Nov. 8, 2001 (HealthDayNews) -- It's official now: You were never as cold as the weatherman said you were.

The National Weather Service, it seems, has warmed up those frigid wind chill factors.

As of Nov. 1, weather forecasters started using a new temperature index they say is more accurate because it's more attuned to the latest advances in science, technology, computer modeling -- and the human condition. The result is that wind chill temperatures that are given out now are at least 15° Fahrenheit to 20° Fahrenheit warmer than the old ones.

Wind chill readings represent what the temperature feels like to people outside in the elements, rather than what is shown on a thermometer.

A year in development, the new wind chill formula was tested on 12 volunteers, six men and six women of varying ages and fitness levels, says Susan Weaver, a public affairs officer at the weather service's headquarters in Washington, D.C.

"The researchers put sensors on their face and body, put them into a climactic chamber on a treadmill walking three miles an hour in a wind tunnel for 90 minutes," Weaver says. "And based on that, we learned enough and tweaked the formula."

The original wind chill factor, first instituted by the weather service in 1973, originated out of science created by Antarctic explorers in 1945, she says. Using a plastic bucket of water suspended from a long pole, Paul Siple and Charles Passel, who were working on Army experiments to develop winter clothing, determined the heat loss from water as it froze.

"In their own way, their concept was unique at the time -- the idea that effects of wind on freezing temperatures could produce a greater impact than either one," Weaver says.

Wind chill readings reflect the fact that, as wind speed increases, heat is carried away from the body more rapidly. The result is a lower skin temperature and, eventually, a lower internal body temperature. Through a complicated formula, the wind chill factor correlates outside temperature to wind speed to give a single temperature made up of both variables.

For example, if the temperature is 5° Fahrenheit and the wind is blowing at 25 miles an hour, the wind chill under the old formula was -36° Fahrenheit. Under the new formula, those exact same conditions would produce a wind chill of -17° Fahrenheit.

"What we did was come up with an entirely new set of factors," Weaver says. "What we wanted to study was what other elements, besides wind speed and air temperature, make up wind chill."

The researchers decided that the new wind chill factor should measure wind speed at a height of five feet -- "the actual average height of a person's face off the ground," she says -- rather than the old height of 33 feet. "And then, since the face is the part of the human body most exposed to the cold, they decided to use a human face model, which they fed into a computer," she adds.

The researchers also used updated the theory on how the skin loses heat and added in calculations on skin thickness. And they based calculations for the new wind chill factor on the assumption that it was a cold, clear night.

"We took the worst-case scenario," says Maurice Bluestein, an associate professor at the Purdue School of Engineering and Technology in Indianapolis and a member of the research group. "We looked at skin thickness and used temperatures you'd feel on a cloudless night."

It turns out, he adds, that the old wind chill factor was even more inaccurate than scientists suspected.

"What we found is that wind speeds above 5 or 10 or 15 miles per hour don't influence the wind chill factor as much as we thought," he says. "So there was a much bigger discrepancy than we originally thought, as much as 15 or 20° Fahrenheit, depending on the wind speed."

Bluestein, who wrote an article last winter in the Bulletin of the American Meteorological Society criticizing the wind chill factor for inaccurate science, says the new formula is quite accurate and he's satisfied with it.

And Weaver points out an added benefit.

"The previous wind chill factor did not have a specific frostbite threshold, at which frostbite would occur on human subjects," she says. "Our [new] wind chill chart is divided into three elements and shows specifically at what point the danger zone would be for frostbite at different wind chill factors."

To develop the new formula, weather service scientists worked with experts from the U.S. Air Force, the Army Corps of Engineers, Cold Regions Research and Engineering Laboratory, the Department of Energy, the Federal Aviation Administration and the Department of Agriculture as well as members of academia, Environment Canada and the International Society of Biometeorology. The effort marks the first time the weather service has cooperated on research with another country, Weaver says.

What To Do

To check out the new wind chill chart, visit the National Weather Service online.

And to learn what to do about frostbite, go to Emedicine or the NASA Web site.

SOURCES: Interviews with Susan Weaver, public affairs officer, National Weather Service, Washington, D.C.; Maurice Bluestein, Ph.D., associate professor, Purdue School of Engineering and Technology, Indianapolis
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