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| {{About|the meteorological effect of "wind chill"|other uses|Wind chill (disambiguation)}}
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| [[Image:Wind chill.png|thumb|A chart of wind chill temperatures for given air temperatures and wind speeds]]
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| '''Wind-chill''' or '''windchill''', (popularly '''wind chill factor''') is the perceived decrease in air temperature felt by the body on exposed skin due to the flow of air.
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| Wind chill numbers are always lower than the air temperature for values where the formula is valid. When the [[apparent temperature]] is higher than the air temperature, the [[heat index]] is used instead.
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| == Explanation ==
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| The human body loses heat through [[convection]], [[evaporation]], [[Conduction (heat)|conduction]], and [[radiation]].<ref>{{cite web|url=http://books.google.com/books?id=q0QI19T_POkC&lpg=PA340&dq=%22evaporative%20cooling%22%20%22wind%20chill%22&pg=PA340#v=onepage&q&f=false |title=A Field Guide to the Atmosphere - Jay Pasachoff - Google Boeken |publisher=Books.google.com |date= |accessdate=2013-08-09}}</ref> The rate of heat loss by a surface through convection depends on the wind speed above that surface. As a surface heats the air around it, an insulating boundary layer of warm air forms against the surface. Moving air disrupts the boundary layer, allowing for new, cooler air to replace the warm air against the surface. The faster the wind speed, the more readily the surface cools.
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| The speed of cooling has different effects on inanimate objects and biological organisms. For inanimate objects, the effect of wind chill is to reduce any warmer objects to the ambient temperature more quickly. It cannot, however, reduce the temperature of these objects below the ambient temperature, no matter how great the wind velocity. For most biological organisms, the physiological response is to maintain surface temperature in an acceptable range so as to avoid adverse effects. Thus, 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 increasing the risk of adverse effects. {{cn|date=October 2013}}
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| A surface that is wet, such as a person wearing wet clothes, will lose heat quickly because the wet cloth will conduct heat away from the body more rapidly, and because the evaporating moisture carries away heat.{{cn|date=October 2013}} Conversely, humid air slows evaporation and makes a surface feel warmer, and this is incorporated into longer wind chill formulas. During warm months, this effect can be described in the [[heat index]] or [[humidex]].
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| == Alternative approaches ==
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| Many formulae exist for wind chill because, unlike temperature, there is no universally agreed standard for what the term should mean. All the formulae attempt to provide the ability to qualitatively predict the effect of wind on the temperature humans ''perceive''. Within different countries weather services use a standard for their country or region. U.S. and Canadian weather services use a model accepted by the [[National Weather Service]]. That model has evolved over time.
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| The first wind chill formulae 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. It was 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 and Passel as is generally believed.{{Citation needed|date=December 2009}} 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 were obviously exaggerations of the severity of the weather. Charles Eagan<ref>Eagan, C. (1964). Review of research on military problems in cold regions. C. Kolb and F. Holstrom eds. TDR-64-28. Arctic Aeromed. Lab. p 147–156.</ref> 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 {{convert|1.8|m/s|mph}}, 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.
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| === Original model ===
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| 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 meter. 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 change in the 1970s to the metric system units in Canada changed all the numbers, making it confusing and requiring that everyone recalibrate their personal wind chill scale. It should never have had units in the first place, as Siple later pointed out<ref>Paul Siple, quoted in: Cold Injury, 1958, Steven Horvath editor, Josiah Macy Foundation, p 216.</ref> for it was not a measure of human heat loss but simply a number proportional to human heat loss.
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| The original formula for the index was:
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| :<math>WCI=(10\sqrt{V}-V+10.5) \cdot (33-T_{\rm a})</math><ref>*Woodson, Wesley E. (1981). ''Human Factors Design Handbook'',page 815. McGraw-Hill. ISBN 0-07-071765-6</ref><ref>http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690003109_1969003109.pdf</ref>
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| ::Where:
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| :::<math>WCI</math> = Wind chill index, kcal/m<sup>2</sup>/h
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| :::<math>V</math> = Wind velocity, m/s
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| :::<math>T_{\rm a}</math> = Air temperature, °C
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| === North American and United Kingdom wind chill index ===
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| In November 2001 Canada, U.S. and U.K. implemented a new wind chill index developed by scientists and medical experts on the Joint Action Group for Temperature Indices (JAG/TI).<ref>{{cite web|url=http://www.ec.gc.ca/meteo-weather/default.asp?lang=n&n=5FBF816A-1#wc6 |title=Environment Canada - Weather and Meteorology - Canada’s Wind Chill Index |publisher=Ec.gc.ca |date= |accessdate=2013-08-09}}</ref><ref name=NWSpress>{{cite web|title=Meteorological Tables , Wind Chill. August, 2001 Press Release:|url=http://www.erh.noaa.gov/er/iln/tables.htm|publisher=National Weather Seervice|accessdate=14 January 2013}}</ref><ref name=BBCchill>{{cite web|title=Wind Chill|url=http://www.bbc.co.uk/weather/features/understanding/wind_chill.shtml|archiveurl=http://web.archive.org/web/20101011123948/http://www.bbc.co.uk/weather/features/understanding/wind_chill.shtml|archivedate=11 October 2010|work=BBC Weather, Understanding weather|publisher=BBC}}</ref> 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 {{convert|1.4|m/s|mph}}. The model corrects the officially measured wind speed to the wind speed at face height, assuming the person is in an open field.<ref>Osczevski, Randall and Maurice Bluestein. The New Wind Chill Equivalent Temperature Chart. Bulletin of the American Meteorological Society, Oct. 2005, p. 1453–1458.</ref> The results of this model may be approximated, to within one degree, from the following formula:
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| The standard Wind Chill formula for Environment Canada is:
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| :<math>T_{\rm wc}=13.12 + 0.6215 T_{\rm a}-11.37 V^{+0.16} + 0.3965 T_{\rm a} V^{+0.16}\,\!</math>
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| :<small>where <math>T_{\rm wc}\,\!</math> is the wind chill index, based on the Celsius temperature scale, <math>T_{\rm a}\,\!</math> is the air temperature in degrees Celsius (°C), and <math>V\,\!</math> is the wind speed at 10 metres ([[Anemometer#Instrument location|standard anemometer height]]), in kilometres per hour (km/h).<ref>{{cite web|url=http://climate.weatheroffice.gc.ca/prods_servs/normals_documentation_e.html |title=Calculation of the 1971 to 2000 Climate Normals for Canada |publisher=Climate.weatheroffice.gc.ca |date=2013-07-10 |accessdate=2013-08-09}}</ref></small>
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| The equivalent formula in [[US customary units]] is:<ref name=noaa/>
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| :<math>T_{\rm wc}=35.74+0.6215 T_{\rm a}-35.75 V^{+0.16}+0.4275 T_{\rm a} V^{+0.16}\,\!</math>
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| :<small>where <math>T_{\rm wc}\,\!</math> is the wind chill index, based on the Fahrenheit scale, <math>T_{\rm a}\,\!</math> is the air temperature, measured in °F, and <math>V\,\!</math> is the wind speed, in mph.</small><ref>{{cite web|url=http://www.weather.gov/os/windchill/index.shtml |title=A chart of windchills based on this formula |publisher=Weather.gov |date=2009-12-17 |accessdate=2013-08-09}}</ref>
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| Windchill temperature is defined only for temperatures at or below {{convert|10|°C|°F}} and wind speeds above {{convert|4.8|km/h|mph}}.<ref name=noaa>{{cite web|url=http://www.weather.gov/os/windchill/index.shtml |title=NWS Wind Chill Index |publisher=Weather.gov |date=2009-12-17 |accessdate=2013-08-09}}</ref>
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| As the air temperature falls, the chilling effect of any wind that is present increases. For example, a {{convert|16|km/h|mph|abbr=on}} wind will lower the apparent temperature by a wider margin at an air temperature of {{convert|-20|°C|°F}}, than a wind of the same speed would if the air temperature were {{convert|-10|°C|°F}}.
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| {{Gallery
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| |align=center
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| |File:Windchill effect en.svg|
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| alt1=none
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| |Celsius Wind Chill index
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| |File:WindChill Comparison.GIF|
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| alt2=none
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| |Comparison of old and new Wind Chill values at {{convert|-15|°C|°F}}
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| |File:Windchill calculator.jpg|
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| alt3=none
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| |Windchill calculator
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| }}
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| The method for calculating wind chill has been controversial because experts{{Who|date=March 2009}} 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 WCET's would be a few degrees more severe.
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| The 2001 WCET is a steady state calculation (except for the time to frostbite estimates).<ref>Tikuisis, P., and R. J. Osczevski (2002)Facial Cooling During Cold Air Exposure. Bull. Amer. Meteor. Soc. July 2003, p. 927–934</ref> 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.
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| The exposure to wind depends on the surroundings and wind speeds can vary widely depending on exposure and obstructions to wind flow.
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| The 2007 McMillan Coefficient worked to simplify wind chill. This formula calculates wind chill by subtracting the wind speed in mph from the Fahrenheit temperature.
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| === Australian Apparent Temperature ===
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| The Australian [[Bureau of Meteorology (Australia)|Bureau of Meteorology]] uses a different formula for cooler temperatures.<ref>{{cite web|url=http://www.bom.gov.au/info/thermal_stress/#wc |title=Thermal Comfort observations |work=Bureau Of Meteorology, Australia |publisher=Bom.gov.au |date=2010-02-05 |accessdate=2013-08-09}}</ref>
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| The formula<ref>{{cite web|url=http://www.bom.gov.au/info/thermal_stress/#atapproximation |title=The formula for the apparent temperature |work= Bureau Of Meteorology, Australia |publisher=Bom.gov.au |date=2010-02-05 |accessdate=2013-08-09}}</ref> is:
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| :<math>AT = T_{\rm a} + 0.33e - 0.70ws - 4.00</math>
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| Where:
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| :<math>T_{\rm a} =</math> Dry bulb temperature (°C)
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| :<math>e =</math> Water vapour pressure (hPa)
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| :<math>ws =</math> Wind speed (m/s) at an elevation of 10 meters
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| The vapour pressure can be calculated from the temperature and relative humidity using the equation:
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| :<math>e = \frac{rh}{100} \cdot 6.105 \cdot \exp {\left(\frac{17.27 \cdot T_{\rm a}}{237.7 + T_{\rm a}}\right)}</math>
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| Where:
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| :<math>T_{\rm a} =</math> Dry bulb temperature (°C)
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| :<math>rh =</math> Relative humidity [%]
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| :<math>\exp</math> represents the [[exponential function]]
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| The Australian formula includes the important factor of [[humidity]] and is somewhat more involved than the simpler North American model. However humidity can be a significant factor. The North American formula is designed mainly on the basis that it is expected to be applied at low temperatures, when humidity levels are also low, and also for much colder temperatures (as low as −50 °F). As these are qualitative models this is not necessarily a major failing.
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| A more exhaustive model (which factors in wind speed) 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 off the island during summer months. It utilized three specialized thermometers.<ref>{{cite web|url=http://www.bom.gov.au/info/thermal_stress/#wbgt |title=About the WBGT and Apparent Temperature |work= Bureau Of Meteorology, Australia |publisher=Bom.gov.au |date=2010-02-05 |accessdate=2013-08-09}}</ref> This research is what the Australian Apparent Temperature formula was derived from.
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| ==See also==
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| *[[Cold wave]]
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| *[[Heat index]]
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| *[[Humidex]]
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| *[[Wind chill warning]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| {{commonscat}}
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| * [http://www.eol.ucar.edu/homes/rilling/wc_table.html National Center for Atmospheric Research] Table of wind chill temperatures in Celsius and Fahrenheit
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| * [http://www.nws.noaa.gov/om/windchill/index.shtml National Weather Service Wind Chill Temperature Index] Table of wind chill temperatures in Fahrenheit with frostbite times
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| * [http://www.nsdl.arm.gov/Library/glossary.shtml#Wind_chill_temperature National Science Digital Library - wind chill Temperature]
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| * [http://www.msc.ec.gc.ca/education/windchill/index_e.cfm Environment Canada - Wind Chill]
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| * [http://www.weatherimages.org/data/windchill.html Weather Images Wind Chill Chart and Introduction]
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| * [http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?An_Introduction_To_Wind_Chill An Introduction to Wind Chill] A lesson plan on wind chill
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| * [http://www.tc.gc.ca/civilaviation/systemsafety/newsletters/tp202/1-03/V029.htm A Cold Wind Blowing] An article on wind chill and tables in SI units
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| * [http://ptaff.ca/humidex/?lang=en_CA Wind Chill and Humidex] Criticism about the use of wind chill and humidex
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| * [http://www.thecanadianencyclopedia.com/index.cfm?PgNm=TCE&Params=A1ARTA0010320 An article about wind chill from ''The Canadian Encyclopedia'']
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| * {{cite news|url=http://www.nytimes.com/2004/02/10/science/beyond-brrr-the-elusive-science-of-cold.html |title=Beyond Brrr: The Elusive Science of Cold |last=Gorman |first=James |date=February 10, 2004 |work=[[The New York Times]]}}
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| {{Meteorological variables}}
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| {{DEFAULTSORT:Wind Chill}}
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| [[Category:Atmospheric thermodynamics]]
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| [[Category:Weather]]
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| {{Link GA|de}}
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| {{Link GA|sv}}
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| [[it:Temperatura#Temperatura effettiva e temperatura percepita]]
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