when did they change the wind chill formula
November 2001
title: Wind chill text: the formulas attempt to qualitatively predict the effect of wind on the temperature humans "perceive". Weather services in different countries use standards unique to their country or region; for example, the U.S. and Canadian weather services use a model accepted by the National Weather Service. That model has evolved over time. The first wind chill formulas and tables were developed by Paul Allman Siple and Charles F. Passel working in the Antarctic before the Second World War, and were made available by the National Weather Service by the 1970s. They were based on the cooling rate of a small plastic title: Wind chill text: four digit number with units of kilocalories/hour per square metre. Each individual calibrated the scale of numbers personally, through experience. The chart also provided general guidance to comfort and hazard through threshold values of the index, such as 1400, which was the threshold for frostbite. The original formula for the index was: where: In November 2001, Canada, the United States, and the United Kingdom implemented a new wind chill index developed by scientists and medical experts on the Joint Action Group for Temperature Indices (JAG/TI). It is determined by iterating a model of skin temperature under various wind speeds and title: Wind chill text: −20 °C and the wind speed increases to , the wind chill index falls to −33. The equivalent formula in US customary units is: Windchill temperature is defined only for temperatures at or below and wind speeds above . As the air temperature falls, the chilling effect of any wind that is present increases. For example, a wind will lower the apparent temperature by a wider margin at an air temperature of , than a wind of the same speed would if the air temperature were . The 2001 WCET is a steady state calculation (except for the time to title: Wind chill text: that experienced under the current ambient temperature and humidity. The formula is: where: The vapour pressure can be calculated from the temperature and relative humidity using the equation: Where: The Australian formula includes the important factor of humidity and is somewhat more involved than the simpler North American model. The North American formula was designed to be applied at low temperatures (as low as ) when humidity levels are also low. The hot weather version of the AT (1984) is used by the National Weather Service in the United States. In the United States, this simple version of the AT title: Wind chill text: wind. This led to equivalent temperatures that exaggerated the severity of the weather. Charles Eagan realized that people are rarely still and that even when it was calm, there was some air movement. He redefined the absence of wind to be an air speed of , which was about as low a wind speed as a cup anemometer could measure. This led to more realistic (warmer-sounding) values of equivalent temperature. Equivalent temperature was not universally used in North America until the 21st century. Until the 1970s, the coldest parts of Canada reported the original Wind Chill Index, a three or