You wake up on a winter morning, grab your phone, and check the day’s weather. Despite your weather app reporting it’s 40˚F outside, because winds are blowing at seven miles per hour, your body will feel as if it’s 35˚F. The difference between the reported temperature and how the wind affects it is known as the “wind chill.” The highly-calculated formula was developed “solely to assess the risk of frostbite on unclothed parts of the body," according to Krzysztof Blazejczyk, a researcher who studies the thermodynamics of the human body. Read on to take a deep dive into wind chill, how it’s calculated, and how humans experience it differently.
Wind Chill Lowers Body Heat
If you're outside on a cold, windy day, blowing wind lifts away the heat generated by your body. This heat loss leaves you more chilled than if you'd been out on a similarly cold day without wind. Wind chill calculations incorporate air temperature and wind speed to provide an approximation of how cold your skin will feel.
Wind chill is only determined when the temperature outside is 50˚F or colder, and when wind speeds are above three miles per hour. Wind chill decreases the time it takes for a person to develop frostbite (freezing of skin and tissue), or hypothermia (when body temperature, typically around 98.6˚F, goes below 95˚F).
How Is Wind Chill Calculated?
The wind chill temperature is often referred to as the "feels like” temperature. In the 1970s, the National Weather Service calculated wind chill using the Siple-Passel index, based on work by Antarctic researchers Paul Siple and Charles Passel, who in 1945 had noted that water bottles froze faster in windy conditions.
But the Siple-Passel index didn't deliver perfect results since humans are obviously very different from water bottles, which were placed 33 feet above the ground, a lot higher than any person reaches. Following the Siple-Passel index ultimately resulted in forecasted wind chill temperatures that were too cold, particularly at higher wind speeds.
It wasn’t until almost three decades later that a new wind chill formula would take flight. In 2001, the Joint Action Group on Temperature Indices (JAG/TI), made up of experts from the United States and Canada, debuted a new system for calculating wind chill. Their work took into account the heat loss experienced by 12 men and women who walked on treadmills in a wind tunnel. Wind speeds were calculated at five feet, the average height of a human face. It was also assumed a person's hands and face would be exposed to the elements.
The formula that resulted was Wind chill (ºF) = 35.74 + 0.6215T - 35.75(V^0.16) + 0.4275T(V^0.16), where T stands for temperature and V for velocity (wind speed).
When the previous scale would have given a wind chill temperature of -60˚F, the new one calculated it would be -38˚F.
But for those of us who aren’t scientists, instead of using the above formula you can determine wind chill temperature with this National Weather Service calculator. In addition, you can check this NWS chart for information about wind chill temperatures. The chart also indicates how long it would take for frostbite to occur.
Issues With the Wind Chill Index
The updated formula for determining wind chill temperatures still has its flaws. For one thing, it doesn't include the impact of the sun. A bright sunny day can feel 10˚F to 18˚F warmer than nighttime or a cloudy day. In addition, the participants in the original study were walking and generating body heat versus simply standing still.
The new formula also used a consistent wind speed. In general, wind doesn't blow at a steady rate; there are gusts and lulls on any windy day. And location will affect how a person feels wind. Tall buildings or trees can blunt some of the wind's force, resulting in less of a wind chill effect.
Some systems, such as the Universal Thermal Climate Index, try to factor in additional variables. The UTCI takes into account “ambient temperature, ambient humidity, solar radiation levels, wind speed, and combines it all with the level of clothing you'd expect someone to be wearing,” according to physiological researcher George Havenith, who aided in developing the model.
But including more variables makes any calculation more complicated, sometimes unnecessarily. The National Weather Service and most other countries continue to stand behind the 2001 formula, and the majority of U.S. broadcasters follow suit.
Everyone Experiences Wind Chill Differently
Even if all meteorological conditions could be taken into account, wind chill is still an approximation of what your body will feel in certain conditions. And the truth is that everyone experiences wind chill differently. This is due, in part, to clothing: If you're dressed warmly, it will take longer to be chilled by the wind.
Wind chill can only affect warm-blooded creatures and different body types have different experiences with wind chill. People with more fat cells are better able to retain heat around their core. But while this means their internal body temperature stays higher, they'll feel colder because less heat reaches their skin. Plus their risk of frostbite is higher, again due to heat not reaching their skin.
The updated wind chill index uses a heavier person as a model due to their heightened risk for frostbite. Randall Osczevski, an environmental physicist in Canada who helped update wind chill calculations in 2001, told the New York Times in 2004, "There's no average person. You have to decide who you're going to represent. We chose to represent the person who would have the most trouble keeping their skin warm in cold wind."
Wind can speed up the time in which exposed skin can freeze. Yet no matter how much the wind is blowing, if temperatures are above freezing, your skin and tissue can't freeze and you won't experience frostbite. However, even when it's above freezing outside, wind chill can still lower your body temperature and put you at risk of hypothermia.
Featured image credit: Leo Schindzielorz/ Shutterstock