Why I Hate Wind Chill Reports

I once counseled a friend (actually, a relative, but that's neither here nor there) to beware of taking wind chill reports too seriously. He apparently thought I meant there was no such thing and in the course of wanting to clarify my sentiments I put pen to paper and worked out the following:

I didn't say there was no such thing as wind chill; it is a very real effect, and should be considered when engaging in or contemplating any sort of cold weather outdoor activity. What I do have a problem with is when some weather people get excited about reporting wind chill as an absolute number and referring to records, etc.

Things such as temperature and wind velocity are directly quantifiable. One can stick a thermometer out the window and read directly the temperature. One can do the same with the wind by using an anemometer. There is, however, no instrument to measure wind chill—it is a factor that is derived from a table and is calculated from temperature and wind.

The second problem with wind chill is that the value reported is only good for a particular temperature/wind velocity combination. And although temperature tends to change relatively slowly over time, wind is almost always changing, often in dramatic increments. There is almost never a steady state wind velocity. Therefore reporting that the, “wind chill in Podunk is -30°,” is only correct at the particular wind speed (and temperature) that produce that wind chill value. In real conditions that wind chill factor may be jumping plus or minus 10 to 20° from the reported value, depending on how variable the wind is.

Some reporters have gone so far as to report or predict record wind chills. Of course there can be no such thing for a number of reasons. The most important is that no records of a calculated value are kept since it's an immeasurable quantity. Moreover, the circumstances related above regarding variability in wind velocity render a resolution of a specific wind chill factor subjective, at best. Finally, the formula used to calculate wind chill isn't static; it is revised from time to time to better illustrate the effect that wind and temperature have.

However, don't take my word for it. Here are some comments as reported in USA Today on 19 August 2001:

“This whole romance with the wind chill factor is just a bunch of hype so the TV weatherman can scare you,” says Edwin Kessler, former director of the National Severe Storms Laboratory in Norman, Okla. He says crying wolf with exaggerated wind chill factors can cause people not take the threat of cold seriously.

[To illustrate a point, Kessler suggests that one] consider two days with a wind chill of 20°. One day is breezy and 35°; the other is windless and 20°. “If you dress the same on these days, you'll pay the price. At 20°, you can get frostbit but at 35° you can't, no matter how hard the wind blows,” Kessler says.

“The wind chill index is like asking me how tall I am and my answer is 150 pounds. It doesn't accurately measure anything,” says Dennis Driscoll, a meteorologist at Texas A&M.

By the way, wind chill is only valid for the skin of live animals. There is nothing to be gained by putting a blanket over the radiator of your car on a cold, windy night. Unless, of course, you wanted to cool your blanket.

27 January 2009

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