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Wind chill or windchill (popularly wind chill factor) is the lowering of
body temperature Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
due to the passing-flow of lower-temperature air. Wind chill numbers are always lower than the air temperature for values where the formula is valid. When the
apparent temperature Apparent temperature, also known as feels like, is the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity and wind speed. The measure is most commonly applied to the perceived outdoo ...
is higher than the air temperature, the
heat index The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is als ...
is used instead.


Explanation

A surface loses heat through
conduction Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble, such as an orchestra. * Conductor (album), ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of structured f ...
,
evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
,
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
, and
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
. 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 surface cools.


Alternative approaches

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 The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...
. That model has evolved over time. The first wind chill formulas and tables were developed by
Paul Allman Siple Paul Allman Siple (December 18, 1908 – November 25, 1968) was an American Antarctic explorer and geographer who took part in six Antarctic expeditions, including the two Byrd expeditions of 1928–1930 and 1933–1935, representing the Boy Scouts ...
and
Charles F. Passel Charles Fay Passel (April 9, 1915 – December 27, 2002) was a polar scientist responsible along with Paul Siple for the development of the wind chill factor parameter. Biography Passel was born in Indianapolis, Indiana on April 9, 1915 and gr ...
working in the Antarctic before the
Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, and were made available by the
National Weather Service The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...
by the 1970s. 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. The so-called Windchill Index provided a pretty good indication of the severity of the weather. In the 1960s, wind chill began to be reported as a wind chill equivalent temperature (WCET), which is theoretically less useful. The author of this change is unknown, but it was not Siple or Passel as is generally believed. At first, it was defined as the temperature at which the windchill index would be the same in the complete absence of 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.


Original model

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 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 Frostbite is a skin injury that occurs when exposed to extreme low temperatures, causing the freezing of the skin or other tissues, commonly affecting the fingers, toes, nose, ears, cheeks and chin areas. Most often, frostbite occurs in the han ...
. The original formula for the index was: :WCI=\left(10\sqrt-v+10.5\right) \cdot \left(33-T_\mathrm\right) where: *''WCI'' = wind chill index, kg*cal/m2/h *''v'' = wind velocity, m/s *''T''a = 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 Temperature Indices (JAG/TI). 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. Heat transfer was calculated for a bare face in wind, facing the wind, while walking into it at . The model corrects the officially measured wind speed to the wind speed at face height, assuming the person is in an open field. The results of this model may be approximated, to within one degree, from the following formula: The standard wind chill formula for Environment Canada is: :T_\mathrm=13.12 + 0.6215 T_\mathrm-11.37 v^ + 0.3965 T_\mathrm v^ :where ''T''wc is the wind chill index, based on the Celsius temperature scale; ''T''a is the air temperature in degrees Celsius; and ''v'' is the wind speed at standard anemometer height, in kilometres per hour. When the temperature is and the wind speed is , the wind chill index is −24. If the temperature remains at −20 °C and the wind speed increases to , the wind chill index falls to −33. The equivalent formula in US customary units is: :T_\mathrm=35.74+0.6215 T_\mathrm-35.75 v^+0.4275 T_\mathrm v^\,\! :where ''T''wc is the wind chill index, based on the Fahrenheit scale; ''T''a 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 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 . File:Windchill effect en.svg, alt=Graph of degrees of wind chill for wind speed and air temperature, Celsius wind chill index File:WindChill Comparison.GIF, Comparison of old and new wind chill values at , alt=Graph comparing "old" and "new" wind chill values by wind speed at 15°C air temperature File:Windchill calculator.jpg, alt=Picture of a manual wind chill calculator, Wind chill calculator 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

The apparent temperature (AT), invented in the late 1970s, was designed to measure thermal sensation in indoor conditions. It was extended in the early 1980s to include the effect of sun and wind. The AT index used here is based on a mathematical model of an adult, walking outdoors, in the shade (Steadman 1994). The AT is defined as; the temperature, at the reference humidity level, producing the same amount of discomfort as that experienced under the current ambient temperature and humidity. The formula is: :\mathrm = T_\mathrm + 0.33e - 0.7v - 4.00 where: *''T''a = 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 Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...
using the equation: :e = \frac\mathrm \cdot 6.105 \cdot \exp Where: :''T''a = dry bulb temperature (°C) :RH = Relative humidity (%) :exp represents the
exponential function The exponential function is a mathematical function denoted by f(x)=\exp(x) or e^x (where the argument is written as an exponent). Unless otherwise specified, the term generally refers to the positive-valued function of a real variable, a ...
The Australian formula includes the important factor of
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...
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 is known as the
heat index The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is als ...
.


References


External links


National Center for Atmospheric Research
Table of wind chill temperatures in Celsius and Fahrenheit
Current map of global wind chill values

Wind chill calculator
at the US National Weather Service {{DEFAULTSORT:Wind Chill Atmospheric thermodynamics Meteorological indices Units of meteorology measurement