For example, a temperature of 0 degree F and a wind of 20 mph creates wind chill of –22 degrees F and skin can freeze in 30 minutes. The wind chill charts do show generally how long it will take skin to freeze at lower and lower values. And yet, frostbite times don’t provide very useful clues about how much clothing to wear. If viewers know how quickly their exposed skin will freeze, they might be more cautious. Some meteorologists say it would be more useful to report “minutes until frostbite” rather than wind chill values. If wind chill is not the actual temperature on your skin, why bother reporting it? This is a fair question. AccuWeather’s “RealFeel” index adds in effects such as cloud cover and sun angle, but because the formula is patent-protected outside scientists cannot evaluate the math. The National Weather Service wind chill chart uses only those two quantities, and runs them through a model based on the tissue in a prototypical human face as well as rates of heat loss for the body. Army Research Institute of Environmental Medicine in Natick, Mass. Different wind chill indexes use different formulas, but in all of them the most important factors are air temperature and wind speed, says Catherine O’Brien, a research physiologist at the U.S. Wind chill is all about perception, and the wind chill index is an attempt to gauge that perception. Scientists are not sure why this occurs, but they think it is a signal to close down blood vessels in the skin and extremities so more blood can flow to the body’s core, to keep your organs warm and keep you alive-even if you lose a finger or toe to frostbite in the process. Your nerve endings and brain perceive the rapid drop in skin temperature as extreme, however. Once the wind surpasses 25 mph or so, it whisks away heat more quickly than your body can emit it, leaving your skin exposed to the full low temperature. Wind carries some of that heat away, however, and the faster the wind, the faster the heat loss. If you stand still in air that is 20 degrees F and there is no wind, your skin will be warmer than 20 degrees F. Thanks to blood in your skin and underlying tissue, your body constantly radiates heat, generating a thin boundary layer of warm air on the surface your skin that helps insulate you from the cold. So what’s the point of wind chill, then? Should we worry about it? Is it deceiving? Wind chill is a mathematically derived number that approximates how cold your skin feels-not how cold your skin actually is. The coldest your uncovered face could get would be 15 degrees F whether the wind is calm or howling at 40 mph. Your skin temperature cannot drop below the actual air temperature. But if the air temperature is, say, 15 degrees F, and a 20–mile per hour wind makes the wind chill –2 degrees F, would the temperature of your exposed skin drop to that temperature? No. Alarmed weather forecasters are now routinely displaying big maps that show the extremely low wind chill values: –34 degrees Fahrenheit in Minneapolis, –36 degrees F in Chicago, –39 degrees F in Fargo, N.D., last night alone. and Canada in a deep freeze several times already this winter. The infamous polar vortex has put the U.S. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today. If you're enjoying this article, consider supporting our award-winning journalism by subscribing.
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