As an arctic air mass lingers over large swaths of the Northeast and Midwest, the National Weather Service has issued wind chill advisories across the region, and the Mount Washington Observatory in New Hampshire recorded a wind chill of minus 46 degrees. What is wind chill, anyway?
Despite what you might hear on a weather report, wind chill isn’t actually a measure of how cold the air feels. A wind chill temperature is, rather, a calculation of how cold it would have to be to cause the same rate of heat loss from your skin if there were no wind blowing. Wind chill approximates how cold it feels because the rate of heat loss corresponds with skin temperature, and skin temperature is what our nerves sense.
Wind chill was first measured in 1941, when two explorers, Paul Siple and Charles Passel, spent a long Antarctic winter measuring how long it took water to freeze in plastic containers left outside, hanging in the wind. The stronger the wind, the faster the water would freeze. Using this data, they created the first equation to calculate wind chill. Their original formula, published in 1945, was revised slightly in the intervening years, but was always found, especially by those who lived in very cold places, to be a poor measure of how cold the air actually felt to the skin. In parts of Canada where the temperature regularly fell below minus 40 degrees, for example, people reported that minus-40-degree wind chill felt a lot warmer.
So, in 2001, a gaggle of agencies from the United States and Canada—collectively, the Joint Action Group on Temperature Indices—agreed on a new formula for calculating wind chill and conducted experiments to measure exactly how much heat is lost from human skin in a windy environment. Twelve volunteers donned winter coats and walked on treadmills in a freezing wind tunnel, all while wearing thermal sensors on their faces, inside their mouths, and in their rectums.
The catch with this new model, which is still in use, is that heat loss from skin—known as skin tissue resistance—varies greatly from person to person. Those with thicker skin have greater skin tissue resistance, for example. But high resistance, oddly, puts one at greater risk of frostbite because the skin conducts less heat from inside the body to warm its surface. To be on the safe side, the new model is calibrated to unusually high skin resistance—95th percentile—and therefore overestimates how cold it feels for those with more average skin.
Explainer thanks Randall Osczevski of Defense R&D Canada-Toronto and Maurice Bluestein of Indiana University-Purdue University Indianapolis.