Windchill Factor
Scientific Equations Measure How Wind Makes Cold Feel Colder
Because of Wind Chill We Feel Colder
In countries where the winters are cold, it's important to pay attention to the weather forecast. The predicted temperatures don't tell the whole story. Wind on exposed flesh makes humans feel colder than the actual temperature would suggest.
This phenomenon is very easy to understand. Even on warm days, a breeze feels cooling. That welcome summer breeze can become a deadly cold amplifier in winter.
The Siple and Passel Index of Windchill and the New Wind Chill Index
Until 2001, the Canadian and U.S. weather agencies (Environment Canada and the NWS, or National Weather Service) used an index devised in 1945 to calculate windchill. This is the Siple-Passel Index.
In the late 1990s, the Canadian and American authorities decided it would be appropriate to revisit the windchill formula. A working group from Canada and the U.S.A. investigated how cold wind really feels, and decided the old formula needed updating.
In 2001, a new index was clinically tested with (brave) human volunteers. The result of all the work was a new formula, now in use.
The formula in U.S. units (degrees F and m.p.h.) is:
Windchill=35.74 + 0.6215T - 35.75(V^0.16) + 0.4275T(V^0.16)
and in metric units (degrees C and km/h) is:
Windchill=13.12 + 0.6215T - 11.37(V^0.16) + 0.3695T(V^0.16)
where T = air temperature and V = windspeed.
The difference between the old and new formulae is well illustrated by a graph. The attached picture shows the old windchill value in pink and the new one in yellow. The temperature in the illustration is a constant 5 degrees F. The horizontal axis shows windspeed, the vertical axis the perceived temperature - how cold it feels when the windchill factor is applied. There is a dramatic difference in the shape of the two curves.
The scientists believe the new index gives a more reliable indication of how windchill feels.
Windchill, Frostbite and Hypothermia
The windchill index reflects how cold the wind feels on exposed skin. This in turn can predict the probability of frostbite, a dangerous condition where the flesh becomes numb and freezes.
To guard against frostbite and hypothermia (dangerous loss of body heat), it is important to pay attention to the windchill factor. Even though the thermometer may be reading close to zero degrees F, frostbite is likely after 30 minutes if the windspeed is about 15 mph. The temperature will feel 20 degrees F colder than the thermometer reads.
The standard advice is to limit exposure time, wear enough clothing to cover all the exposed areas, and add a windproof layer. A hat, mitts or gloves on the hands, and a scarf to cover the face are necessary. Fingers, toes, noses and ears are easy frostbite targets if unprotected.
Old and New Wind Chill Calculations
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