Wind chill
Its values are always lower than the air temperature in the range where the formula is valid.
A surface loses heat through conduction, evaporation, convection, and radiation.
All the formulas attempt to qualitatively predict the effect of wind on the temperature humans perceive.
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,[3] and were made available by the National Weather Service by the 1970s.
[3] They were based on the cooling rate of a small plastic bottle as its contents turned to ice while suspended in the wind on the expedition hut roof, at the same level as the anemometer.
[3] The so-called Windchill Index provided a pretty good indication of the severity of the weather.
The author of this change is unknown, but it was not Siple or Passel as is generally believed.
[citation needed] At first, it was defined as the temperature at which the windchill index would be the same in the complete absence of wind.
Charles Eagan[4] realized that people are rarely still and that even when it is calm, there is some air movement.
Until the 1970s, the coldest parts of Canada reported the original Wind Chill Index, a three- or four-digit number with units of kilocalories/hour per square metre.
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).
[7][8][9] It is determined by iterating a model of skin temperature under various wind speeds and temperatures using standard engineering correlations of wind speed and heat transfer rate.
where Twc is the wind chill index, based on the Celsius temperature scale; Ta is the air temperature in degrees Celsius; and v is the wind speed at 10 m (33 ft) standard anemometer height, in kilometres per hour.
where Twc is the wind chill index, based on the Fahrenheit scale; Ta is the air temperature in degrees Fahrenheit; and v is the wind speed in miles per hour.
[12] As the air temperature falls, the chilling effect of any wind that is present increases.
[14] There are significant time-dependent aspects to wind chill because cooling is most rapid at the start of any exposure, when the skin is still warm.
The apparent temperature (AT), invented in the late 1970s, was designed to measure thermal sensation in indoor conditions.
The AT index used here is based on a mathematical model of an adult, walking outdoors, in the shade (Steadman 1994).
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 hot-weather version of the AT (1984) is used by the National Weather Service in the United States.
