Wind chill factor

With the premature cold snap across Chicago and much of the rest of the country, the media weathermen are again talking about wind chill factor and its impact on cars and other non-human things.

I had always believed that wind chill only referred to evaporation of moisture from exposed skin, and had no effect on machinery and metal and such, so I did some research years back. The World Book Encyclopedia (1996 edition) says wind chill factor is "the estimate of the relationship between wind speed and temperature, with respect to the physical effects of the wind on living things."

The New Encyclopedia Brittanica says wind chill can be "expressed by a formula K=(10.45+10 (square root of v-v)) (33-t) where K is the heat in kilogram calories lost by one square meter of SKIN in one hour..."

Furthermore, the physics teachers at the high school where I taught all agreed, stating further that "the average temperature of the air is as cold as an object can become."

The Wikipedia entry agrees, stating "Wind chill is the apparent temperature felt on exposed skin, which is a function of the air temperature and wind speed." It adds later, "A wind-chill factor of 25° F (−4 °C) will not freeze water if the air temperature is 35° F (2 °C). Water changes state according to the temperature of the body of the water. In this case, the water and air temperature are about the same—too high to freeze water."

NOAA's National Weather Service in 2001 implemented an updated Windchill Temperature (WCT) index based on a human face model, incorporating modern heat transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days), lowered the calm wind threshold to 3 mph, and used a consistent standard for skin tissue resistance. (Click to enlarge.)




So use of wind chill by TV/weather reporters is useful to let people know they should put on more clothes and take other precautions in extremely cold weather, but should not be used by the reporters (or received and interpreted by listeners) as hype regarding the effect of the cold on car engines, etc.