People who live somewhere where its sometimes windy and cold in the winter will often hear their weatherperson give out two temperatures: the actual temperature outside and a temperature based on the windchill temperature index.
A windchill temperature is how cold the air feels given current wind conditions.
While the fact that a gust of wind can really cool things down might seem obvious, actually calculating the windchill temperature index is quite complex.
Back in 1945 the scientists who came up with the original index did so based on how long it took plastic cylinders full of water to freeze under different conditions. The obvious problem with that method is that humans and animals don't have much in common with plastic cylinders.
In 2001, the United States' National Weather Service adopted an updated windchill temperature index which accounts for the way human bodies lose heat to their surroundings. That newer formula is much more accurate than the previous method, but the readings won't always be one hundred percent accurate.
Information for calculating the wind speed is collected from an anemometer that's at an average national height of thirty-three feet. An anemometer is a tool used for calculating wind speed (although it can be used for calculating the speed of any kind of gas).
The wind speed measurement is then calculated using the height of approximately five feet. This is because, in the United States, the average face is that high. They also chose to use the face, operating under the assumption that people will wrap up against the elements, because people will lose the majority of their heat there.
For those with faces above or below that heightor who refuse to dress for the weatherthe windchill can vary.
There are a few other factors at play. It doesn't take into account whether it's sunny or snowing outside. Contact with moistures also makes quite a difference in terms of heat loss. Still, it's far more accurate, which sure comes in handy when trying to avoid frostbite.
Sources and Further Reading:
Robert Henson: "Cold Rush," Weatherwise, Washington: Jan/Feb 2002. Vol. 55, Iss. 1; pg. 14, 6 pgs.
National Weather Service. Accessed January 24, 2005.