Wind-chill or windchill, (popularly wind chill factor) is the lowering
of body temperature due to the passing-flow of lower-temperature air.
1 Explanation 2 Alternative approaches
2.1 Original model 2.2 North American and United Kingdom wind chill index 2.3 Australian Apparent Temperature
3 References 4 External links
A surface loses heat through conduction, convection, and radiation.
The rate of convection depends on both the difference in temperature
between the surface and the fluid surrounding it and the velocity of
that fluid with respect to the surface. As convection from a warm
surface heats the air around it, an insulating boundary layer of warm
air forms against the surface. Moving air disrupts this boundary
layer, or epiclimate, allowing for cooler air to replace the warm air
against the surface. The faster the wind speed, the more readily the
The effect of wind chill is to increase the rate of heat loss and
reduce any warmer objects to the ambient temperature more quickly. Dry
air cannot, however, reduce the temperature of these objects below the
ambient temperature, no matter how great the wind velocity.[citation
needed] For most biological organisms, the physiological response is
to generate more heat in order to maintain a surface temperature in an
acceptable range. The attempt to maintain a given surface temperature
in an environment of faster heat loss results in both the perception
of lower temperatures and an actual greater heat loss. In other words,
the air 'feels' colder than it is because of the chilling effect of
the wind on the skin. In extreme conditions this will increase the
risk of adverse effects such as frostbite.
Many formulas exist for wind chill because, unlike temperature, wind
chill has no universally agreed upon standard definition or
measurement. All 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
The first wind chill formulas and tables were developed by Paul Allman
Charles F. Passel working in the Antarctic before the Second
World War, and were made available by the
National Weather Service
W C I
− v + 10.5
displaystyle mathrm WCI =left(10 sqrt v -v+10.5right)cdot left(33-T_ mathrm a right)
WCI = wind chill index, kcal/m2/h v = wind velocity, m/s Ta = air temperature, °C
North American and United Kingdom wind chill index
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
= 13.12 + 0.6215
displaystyle T_ mathrm wc =13.12+0.6215T_ mathrm a -11.37v^ +0.16 +0.3965T_ mathrm a v^ +0.16
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.
When the temperature is −20 °C (−4 °F) and the wind
speed is 5 km/h (3.1 mph), the wind chill index is −24. If
the temperature remains at −20 °C and the wind speed increases
to 30 km/h (19 mph), the wind chill index falls to −33.
The equivalent formula in
US customary units
= 35.74 + 0.6215
displaystyle T_ mathrm wc =35.74+0.6215T_ mathrm a -35.75v^ +0.16 +0.4275T_ mathrm a v^ +0.16 ,!
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.
Windchill temperature is defined only for temperatures at or below 10 °C (50 °F) and wind speeds above 4.8 kilometres per hour (3.0 mph). As the air temperature falls, the chilling effect of any wind that is present increases. For example, a 16 km/h (9.9 mph) wind will lower the apparent temperature by a wider margin at an air temperature of −20 °C (−4 °F), than a wind of the same speed would if the air temperature were −10 °C (14 °F).
Comparison of old and new
The method for calculating wind chill has been controversial, because experts[who?] disagree on whether it should be based on whole body cooling either while naked or while wearing appropriate clothing, or if it should be based instead on local cooling of the most exposed skin, such as the face. The internal thermal resistance is also a point of contention. It varies widely from person to person. Had the average value for the subjects been used, calculated WCETs would be a few degrees more severe. The 2001 WCET is a steady state calculation (except for the time to frostbite estimates). 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. Australian Apparent Temperature A more exhaustive model was developed for the US Navy stationed in Parris Island in South Carolina. It was developed with a consideration for heat stroke due to the high humidity of the island during summer months. It utilized three specialized thermometers. This research led to the Australian Bureau of Meteorology establishing a different formula, the Australian Apparent Temperature, for cooler temperatures. The formula is:
+ 0.33 e − 0.70 v − 4.00
displaystyle mathrm AT =T_ mathrm a +0.33e-0.70v-4.00
Ta = dry bulb temperature (°C) e = water vapour pressure (hPa) v = wind speed (m/s) at an elevation of 10 m
The vapour pressure can be calculated from the temperature and relative humidity using the equation:
⋅ 6.105 ⋅ exp
displaystyle e= frac mathrm RH 100 cdot 6.105cdot exp left( frac 17.27cdot T_ mathrm a 237.7+T_ mathrm a right)
Ta = dry bulb temperature (°C)
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 −46 °C or −50 °F), when humidity levels are also low. References
^ Vincent J. Schaefer; John A. Day; Jay Pasachoff (1998). A Field
Guide to the Atmosphere. Houghton Mifflin Harcourt.
^ Eagan, C. (1964). Review of research on military problems in cold
regions. C. Kolb and F. Holstrom eds. TDR-64-28. Arctic Aeromed. Lab.
^ *Woodson, Wesley E. (1981). Human Factors Design Handbook, page 815.
McGraw-Hill. ISBN 0-07-071765-6
equation 55, page 6-113
^ "Environment Canada - Weather and Meteorology - Canada's
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