It has been discovered that the wind chill index used by the National Weather Service since 1973, was wrong. It significantly overstated how cold it feels. After more than a decade of criticism from scientists, the weather service changed to a more accurate wind chill index several years ago.
Theoretically, the wind chill index measures the rate at which the body loses heat when exposed to cold and wind. The index was created as a public health tool to reduce hypothermia, frostbite, and other cold-related ailments.
As a practical matter, the wind chill index tells people how warmly to dress, a crucial decision for people who spend long periods outdoors, such as construction workers or skiers.
The former wind chill index was based on research conducted in the Antarctic in the 1940s. That study measured how long it took cans of water to freeze at different temperatures and wind speeds. The old index took measurements 33 feet above ground, where winds blow much faster than at ground level. However, human skin freezes at a different rate than water. Even different parts of the body-the face and the hands, for example-freeze at different rates.
The National Weather Service says that the new index provides a more accurate, understandable, and useful formula for calculating the potential danger from the combination of wind and cold temperatures.
The wind chill temperature represents how cold it feels outside to people and animals. Wind chill is based on the rate of heat loss from exposed skin caused by combined effects of wind and cold. As the wind increases, heat is carried away from the body at a faster rate. Therefore, the wind makes it feel much colder. If the temperature is 0 degrees Fahrenheit and the wind is blowing at 15 mph, the wind chill is -19 degrees Fahrenheit. At this wind chill temperature, exposed skin can freeze in 30 minutes.
Confusion still exists about how wind chill affects inanimate objects (e.g., car’s radiator, exposed water pipe). The inanimate object will not cool below the actual air temperature. For example, if the temperature outside is -5 degrees Fahrenheit and the wind chill temperature is -31 degrees Fahrenheit, the car’s radiator will not drop lower than -5 degrees Fahrenheit.
A detailed description of the new wind chill index including the new wind chill chart can be found at: www.nws.noaa.gov/om/windchill.
We are pleased to have Dr. Alton Thygerson as the guest author of our Featured Article section. Dr Thygerson is a professor in the Department of Health Sciences at Brigham Young University and is the medical writer for our ECSI products. You can find out more about Dr. Thygerson at http://www.byu.edu/health/faculty/thygerson.html.