What Exactly is Wind Chill?

by | Jan 31, 2019


Yes Yesterday at about 6:30am I took my dog on a walk. The temperature was -6 degrees and the wind chill was -23 degrees. Wow -23 degrees! I bundled up, including wearing a balaclava on my face. Surprisingly, while it felt cold, it didn’t feel anywhere close to -23 degrees cold. So, what exactly is wind chill?

Wind chill, as published by the National Waether Service, is supposed to relate the frostbite risk for exposed skin. Wind causes objects to lose heat faster than being in no wind because wind blows away the warm(er) air that surrounds the object. Related IFOD on why sweat cools us.

Wind chill, as a frost bite risk factor, was originally expressed as kilocalories per hour per square meter. Because that is only useful to scientists, it was converted to degrees. Thus, yesterday’s wind chill of -23 suggested that my frostbite risk on exposed skin would be similar to -23 degrees with no wind.

However, how people lose heat varies based on all sorts of factors, such as body shape and size, hairiness, fatness vs. leanness, etc. the differences from person-to-person can be dramatic. Wind chill is calculated on a “worst case scenario” for the 5th percentile of heat retainers. Thus, for 95% of the population, wind chill overstates frostbite risk. Plus, wind chill doesn’t tell you how long you can stay outside in the wind without risking frostbite. Knowing the wind chill was -23 degrees didn’t tell me whether I could stay outside for two minutes or 20 minutes with exposed skin. I had no idea.

Some private weather services have developed their own proprietary indexes that approximate how cold it feels. For example, AccuWeather has its “Real Feel” Index that supposedly does a much better job than wind chill of expressing how cold it feels outside. From AccuWeather:

“Unlike the Wind Chill index, the RealFeel Temperature measures how the weather really feels by factoring in not just ambient temperature and wind speed, but also solar intensity, humidity, precipitation, elevation and cloudiness. As an example, consider a winter day with a temperature of 30 degrees, no wind, and a clear, sunny sky. The RealFeel Temperature accounts for the sunshine and correctly indicates that it will feel like a more temperate 44 degrees. Since the Wind Chill index ignores the sunshine, it only reports the apparent temperature as a cold and discouraging 30 degrees. Add a 10 mph wind to the scenario, and the Wind Chill estimate of perceived temperature drops from 30 to 21. The RealFeel Temperature properly balances the offsetting effects of wind and sun and correctly estimates the perceived temperature as a more moderate 26 degrees.



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