What is "Wind Chill?"

Winter 1996-97 BOZEMAN -- Perhaps "no man is an island," but each of us exists with a little boundary area around us that is part of what keeps us warm in winter.

Usually.

That can change quickly if you are in the wind. Even a gentle breeze can make zero degrees feel like 20 below zero, and it does that by blowing away that thin boundary layer of warm humid air around us, says Jon Wraith, assistant professor of Montana State University's Department of Plant, Soil and Environmental Science.

"This layer serves to insulate our body from heat loss," says Wraith. When the wind blows it away, the body ends up constantly trying to reheat the surrounding area.

The reason a down jacket -- or any jacket -- keeps us warm is that it creates still air trapped between our bodies and the outside air. Our body is able to heat the still area as if the wind were not blowing. That's also why it feels warmer on the leeward side of an evergreen than out in the open, or warmer anywhere there is a slowing or blocking of the wind.

Still air is a very good insulator, which is why a double-pane window keeps a home warmer than a single pain. The layer of air between the two panes acts as insulation.

Officially, the "wind chill factor" quantifies the effects of heat loss from a body, and we use this as a measure of how cold it feels, says Wraith. Though there are ways of calculating wind chill, usually weather reporters rely on a series of charts that give air temperature, wind speed and the resulting wind chill.

For instance, at 20 degrees Fahrenheit, a 10 mile per hour wind will chill us as if it were 2 degrees above zero. The faster the wind, the greater its chilling affect. So at zero degrees and a 10 mph wind, it feels like minus 22 degrees, but make that a 20 mph wind and it feels like minus 40.

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11/99 Addendum: A wind chill formula was listed on The Math Forum web site (http://forum.swarthmore.edu/dr.math/problems/gunderson2.1.96.html) which included the following, which was credited as "the National Weather Service's equation for calculating weather."

T(wc) = 0.0817(3.71V**0.5 + 5.81 -0.25V)(T - 91.4) + 91.4

T(wc) is the wind chill, V is in the wind speed in statute miles per
hour and T is the temperature in degrees Fahrenheit.)

You can send questions or comments to Carol Flaherty at carolf@montana.edu.

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