The infamous polar vortex has put the U.S. and Canada in a deep freeze several times already this winter. 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. 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. Your skin temperature cannot drop below the actual air temperature. The coldest your uncovered face could get would be 15 degrees F whether the wind is calm or howling at 40 mph.
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. 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. 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. Wind carries some of that heat away, however, and the faster the wind, the faster the heat loss. 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.
Your nerve endings and brain perceive the rapid drop in skin temperature as extreme, however. 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. Wind chill is all about perception, and the wind chill index is an attempt to gauge that perception.
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. Army Research Institute of Environmental Medicine in Natick, Mass. 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. 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.
If wind chill is not the actual temperature on your skin, why bother reporting it? This is a fair question. Some meteorologists say it would be more useful to report “minutes until frostbite” rather than wind chill values. If viewers know how quickly their exposed skin will freeze, they might be more cautious. And yet, frostbite times don’t provide very useful clues about how much clothing to wear.
The wind chill charts do show generally how long it will take skin to freeze at lower and lower values. 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. If the wind rises to 55 mph, the wind chill drops to –32 degrees F, and skin can freeze in 10 minutes. Remember, the only reason skin freezes faster at lower wind chill is because the body’s heat envelope is removed more quickly. Skin doesn’t freeze until its temperature is well below 32 degrees F, because its cells contain salts and other compounds that lower its freezing point below that of water. Exact times vary depending on an individual’s blood flow, fat layers and underlying tissue.
Still don’t believe wind chill temperatures aren’t real? Try an experiment: Put two thermometers outside, one in the wind and one shielded from it. When you return they will read the same. Or just ask yourself a simple question: If you are driving your car at 20 mph and you read the dashboard thermometer, then speed up to 60 mph, does the temperature drop? No. Because the air temperature has not changed. There is no wind chill for your car—even if you have given your vehicle a human name.
Realizing that coldness is all about perception raises other interesting observations. People who have a lot of body fat may actually feel colder than those who do not, despite the notion that fat acts as insulation. More fat under the skin can actually prevent heat, generated in underlying muscles, from reaching the skin, O’Brien says. Because the perception of cold comes mostly from nerves in the skin, as the air temperature drops, people with high body fat might feel colder.
Women may often feel colder than men because they tend to have less muscle mass (less heat generated) and more body fat (blocking heat to the skin), and because their generally smaller size gives them a larger ratio of surface area to mass they tend to lose heat faster. Every person is different, of course.
Whether or not wind chill is a useful number, meteorologists on television and radio are running wild with it. After all, reporting wind chills of –36 and –39 sounds more dramatic than temperatures of 0 and a paltry –2. “It’s getting a little ridiculous now,” O’Brien says. “Sometimes it’s hard to find the actual air temperature.”