Wind Chill Temperature Index

Wind Chill - what it is and what it is not. The wind chill is the chilling effect of the wind in combination with a low temperature. Humans do not sense the temperature of the air directly. When we feel that it is cold, we are actually sensing the temperature of our skin. Because our skin temperature is lower when it is windy (we lose heat from our skin faster than our body can warm it), we feel that it is colder when there is wind. This sensation is what the wind chill index attempts to quantify. It must be noted that although the wind chill index is expressed on a temperature scale (the Celsius scale in Canada), it is not a temperature: it only expresses a human sensation.

A bit of history. Calculating Wind Chill - Siple Passel. Up until 2001 wind chill was calculated as a cooling rate by using the 'Siple Passel' equation. Wind chill was typically expressed as a 'wind chill factor' which was a number in the thousands (e.g. 1800) based on a cooling rate in watts per square meter. Most Canadian media at the time were using equivalent temperatures in order to express wind chill however people living in cold regions, such as the Arctic, had often reported that this temperature equivalence was not correct. Research undertaken by Environment Canada in the late 90's indicated that there was widespread misunderstanding amongst the general public about wind chill.

The new wind chill index. During the fall of 2000, a special group, called the Joint Action Group on Temperature Indices (JAG/TI), was formed to evaluate the existing wind chill formula and make necessary changes to improve it. This group consisted of Environment Canada's Meteorological Service of Canada, several American government agencies, the university community and the International Society of Biometeorology. In February 2001 the JAG/TI agreed on a new wind chill formula. This new formula is now used in both Canada and the United States as a consequence of which there is now a consistent wind chill formula across North America. It is;

Wind Chill equation

where;
is the Wind Chill index based on the Celsius scale,
is the air temperature in ░C, and
is the air speed measured in kph (kilometers per hour). This is typically measured at 10 meters and this height should be assumed unless otherwise stated.

The new wind chill index - when it works and when it doesn't. The above equation has been derived for temperatures between +5░C and -50░C and for wind speeds in excess of 3 kph.

Environment Canada's wind chill web site.

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