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 service changed to a more accurate wind chill index at the beginning of this month.
According the National Weather Service's Web site www.nws.noaa.gov/om/windchill, the wind chill temperature is the temperature that it feels like 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 F. and the wind is blowing at 15 mph, the wind chill is -19 degrees F. (See accompanying chart.) At this wind chill temperature, exposed skin can freeze in 30 minutes.
Confusion exists about how wind chill affects inanimate objects like a car radiator or exposed water pipe. Inanimate objects will not cool below the actual air temperature. For example, if the temperature outside is -5 degrees F and the wind chill temperature is -31 degrees F, the car's radiator still will not drop lower than -5 degrees F.
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 is supposed to tell people how warmly to dress, a crucial decision for those who spend long periods outdoors — whether at work or at play.
The former wind chill index was based on research conducted in the Antarctic in the 1940s. That study measured how long it takes 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.
But human skin freezes at different rates than water. Even different parts of the body — the hands and face, for example — freeze at different rates.
The National Weather Service says that the new index will provide a more accurate, understandable and useful formula for calculating the potential danger from the combination of wind and cold temperatures.
According the Weather Service's Web site, the new index will:
use calculated wind speed at an average height of five feet (typical height of an adult human face)
be based on readings from the national standard height of 33 feet (typical height of an anemometer)
be based on a human face model
incorporate modern heat-transfer theory
lower the calm wind threshold to 3 mph
use a consistent standard for skin tissue resistance
assume no impact from the sun
Alton Thygerson, professor of health sciences at Brigham Young University, is the National Safety Council's first aid and CPR author and technical consultant. For more information, the National Safety council First Aid Handbook by Thygerson is available in local bookstores.