When temperatures are low, wind chill becomes a threat to human comfort and may even cause death. Clearly, how it affects you depends on other variables such as the type of clothing worn, amount of exposed flesh, amount of solar radiation, and your general physical condition at the time.

On November 1, 2001, the National Weather Service implemented a new Wind Chill Temperature index for the 2001/2002 winter season, designed to more accurately calculate how cold air feels on human skin. The chart replaces the old one (see above) used since 1973 to measure how the weather 'feels' when the cooling effect of wind is factored into temperatures. The National Weather Service also developed a guide to explain wind chill and the development of the new Wind Chill Temperature Index. Click on the image at right to view or download the guide.

One of the biggest differences between the new index and the old one is that now, wind chill values will reflect what's happening five feet off the ground -- the height at which the average person's face sits.

The old wind chill index factored in wind and temperatures located 33 feet above ground, the typical height of an anemometer.

The new index also takes into account heat loss by the human body, skin's resistance to the cold and scientific and technological advances.

The science behind the previous index dates back to 1945, when two explorers determined heat loss from water as it froze in a plastic container suspended from a tall pole in the Antarctic. While their findings were eventually applied to people, humans were never involved in their research. Human volunteers took part in testing of the new index.

According to the new index, the wind chill would measure 16 degrees below zero on a day when thermometers read 0 degrees and the winds blew at 10 mph. According to the previous index, that wind chill would have measured -22 degrees.

For the first time, the wind chill chart includes a frostbite indicator, which shows how long a person's skin can be exposed to the air, given wind speed and temperature, before it will become frostbitten, according to the National Oceanic and Atmospheric Administration.

Following is a wind chill temperature comparision chart (old vs. new):

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