A polar vortex has descended upon the United States, plunging temperatures across much of the Midwest into the negatives—as low as minus 33 degrees Fahrenheit in Fargo, North Dakota (colder than Antarctica). The cold may be difficult to endure, but the relentless wind is making conditions even more miserable, driving perceived temperatures down to ungodly levels. In Chicago, for example, wind gusts of up to 24 miles per hours will generate a temperature with wind chill of minus 50 or 60 degrees Fahrenheit.
But what exactly is wind chill, and how does it work?
According to Gene Brusky, the Science and Operations Officer at the National Weather Service office in Green Bay, Wisconsin, wind chill models were derived from research by Antarctic explorers during World War II. Those venturing near the South Pole wanted to figure out how much heat the human body loses when exposed to wind. The wind chill index was adopted by NWS in the 1970s and then updated in 2001 to reflect modern understandings of human body temperature.
The actual air temperature, which is unchanged by the wind, is not the only way to determine if your body is at risk in the cold. A wind chill index accounts for a combination of air temperature and how fast the wind is blowing. Higher winds strip heat away from the body more quickly. In the United States, wind chill is calculated with a formula using degrees Fahrenheit and wind speeds in miles per hour, but wind chill values can also be calculated using degrees Celsius and meters per second. (Using the metric system, the wind chill value is expressed in watts per meter squared, which is a measure of irradiance, or the flux of radiant energy over a given surface area—the amount of heat change applied to the surface area of a body.)
The resulting values are assigned rough temperature estimates to represent what the conditions “feel like,” but there is no way to actually measure the subjective wind chill temperature. The measurement is nonetheless important, as wind can increase the risk of bodily damage from the cold. If the body enters a hypothermic state—temperatures below 95 degrees Fahrenheit—it can be deadly.
“The wind chill value is important because it gives the user an idea of how quickly the body is susceptible to frostbite or hypothermia,” Brusky says. “You basically look at the temperature, compare it to the winds, and if that number is, say, negative 25 degrees, that’s when frostbite can occur in about 30 minutes.” A wind chill index that dips into the negative 60s, as the Midwest is expected to do this week, can strip heat away from the body at an incredible rate. Without protective clothing, hypothermia or frostbite could set in within minutes.
Consider two days with the same air temperature. Both days have dipped down to 10 below zero. One day has 5 mph winds while the other has 35 mph winds, giving them a wind chill index of minus 20 and minus 40, respectively. This measurement means that frostbite or hypothermia could set in within 30 minutes on the day with milder wind, but a person could start freezing to death in as few as 5 minutes on the windier day. The temperature of the air is the same on both days, but gusts of wind strip heat away from the body, making a person physically colder than they would be on a less windy day.
The fact that the index is tied to human body temperature is important. Let’s say it’s 33 degrees Fahrenheit outside, and winds are 30 mph. The human body will experience a 19-degree wind chill temperature, and prolonged exposure could be a health threat. However, the wind will not change the temperature of something that doesn’t produce its own heat. No matter how hard cold winds pummel a lake, if the air temperature stays above freezing, so will the water. And plants, which don’t produce heat, may be susceptible only to water loss in high winds.
“Plants don’t generate their own heat, so it doesn’t matter how windy it is,” Brusky says. “Heat isn’t going to get taken away from something that doesn’t produce heat.”
Wind doesn’t change the ambient temperature, but it does steal heat from our bodies and change how we experience the temperature outside. The best way to protect against wind chill is to dress appropriately and limit exposure. Even if wind chill temperatures are a fairly subjective phenomenon rather than an objective scientific temperature measurement, the wind can still put you at risk in frigid temperatures.
“The main things to protect yourself are to stay dry, stay covered in layers, and keep in mind that half the body heat is lost through the head,” Brusky says, echoing an old mountaineering adage:
If your feet are cold, put on a hat.