Wind chill is a measure of how cold we feel as a result of the combined effects of cold temperature and wind. On a calm day, our bodies insulate us somewhat from the outside temperature by warming up a thin layer of air close to our skin, known as the boundary layer. When the wind blows, it takes this protective layer away-exposing our skin to the outside air. It takes energy for our bodies to warm up a new layer, and if each one keeps getting blown away, our skin temperature will drop, and we will feel colder. Wind also makes you feel colder by evaporating any moisture on your skin. Evaporation draws even more heat away from your body.

Wind chill is subjective, describing the way we feel as a result of the combined cooling effect of temperature and wind. Since the feeling can't be measured using an instrument scientists have developed a mathematical formula for calculating wind chill that relates air temperature and wind speed to the cooling sensation we feel on our skin.

The original wind chill formula was derived from experiments conducted in 1939 by Antarctic explorers, Paul Siple and Charles Passel. These hardy scientists measured how long it took for water to freeze in a small plastic cylinder when it was placed outside in the wind.

There is a considerable difference between a human body and a plastic cylinder filled with water, most notably the fact that people produce body heat. So a new wind chill index was developed in 2001. The new index is based on the loss of heat from the faces - the part of the body that is most exposed to severe winter weather - of volunteers who were exposed to a variety of temperatures and wind speeds inside a refrigerated wind tunnel.

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