Editing Wind chill (section)

=== 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).<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 |access-date=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 Service|access-date=14 January 2013}}</ref><ref name=BBCchill>{{cite web|title=Wind Chill|url=http://www.bbc.co.uk/weather/features/understanding/wind_chill.shtml|archive-url=https://web.archive.org/web/20101011123948/http://www.bbc.co.uk/weather/features/understanding/wind_chill.shtml|archive-date=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|km/h+mph|abbr=on}}. The model corrects the officially measured wind speed to the wind speed at face height, assuming the person is in an open field.<ref>{{cite journal|last1=Osczevski|first1=Randall|last2=Bluestein|first2=Maurice|title=The new wind chill equivalent temperature chart|journal=Bulletin of the American Meteorological Society|date=2005|volume=86|issue=10|pages=1453–1458|doi=10.1175/BAMS-86-10-1453|doi-broken-date=2 December 2024 |bibcode=2005BAMS...86.1453O|doi-access=free}}</ref> The results of this model may be approximated, to within one degree, from the following formulas.

The standard wind chill formula for [[Environment Canada]] is:<ref name=wChill/>

<math display=block>T_\mathrm{wc}=13.12 + 0.6215 T_\mathrm{a}-11.37 v^{+0.16} + 0.3965 T_\mathrm{a} v^{+0.16},</math>

where ''T''<sub>wc</sub> is the wind chill index, based on the Celsius temperature scale; ''T''<sub>a</sub> is the air temperature in degrees Celsius; and ''v'' is the wind speed at {{cvt|10|m|ft}} [[Anemometer#Instrument location|standard anemometer height]], in kilometres per hour.<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 |access-date=2013-08-09 |url-status=dead |archive-url=https://web.archive.org/web/20130627223738/http://climate.weatheroffice.gc.ca/prods_servs/normals_documentation_e.html |archive-date=2013-06-27 }}</ref>

When the temperature is {{convert|-20|°C|°F}} and the wind speed is {{convert|5|km/h|mph|0|abbr=on}}, the wind chill index is −24. If the temperature remains at −20&nbsp;°C and the wind speed increases to {{convert|30|km/h|mph|abbr=on}}, the wind chill index falls to −33.

The equivalent formula in [[US customary units]] is:<ref name=noaa/><ref name=wChill/>

<math display=block>T_\mathrm{wc}=35.74+0.6215 T_\mathrm{a}-35.75 v^{+0.16}+0.4275 T_\mathrm{a} v^{+0.16},</math>

where ''T''<sub>wc</sub> is the wind chill index, based on the Fahrenheit scale; ''T''<sub>a</sub> is the air temperature in degrees Fahrenheit; and ''v'' is the wind speed in miles per hour.<ref>{{cite web|url=http://www.nws.noaa.gov/om/cold/wind_chill.shtml |title=A chart of windchills based on this formula |publisher=Weather.gov |date=2009-12-17 |access-date=2017-04-13}}</ref>

Windchill temperature is defined only for temperatures at or below {{convert|10|°C|°F}} and wind speeds above {{convert|4.8|km/h|mph|abbr=on}}.<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 |access-date=2013-08-09 |url-status=dead |archive-url=https://web.archive.org/web/20110918010232/http://www.weather.gov/os/windchill/index.shtml |archive-date=2011-09-18 }}</ref>

As the air temperature falls, the chilling effect of any wind that is present increases. For example, a {{convert|16|km/h|mph|0|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}}.

<gallery mode="packed" heights="160px">
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 {{convert|-15|°C|°F}}|alt=Graph comparing "old" and "new" wind chill values by wind speed at 15&nbsp;°C air temperature
File:Windchill calculator.jpg|alt=Picture of a manual wind chill calculator|Wind chill calculator
</gallery>

The 2001 WCET is a steady-state calculation (except for the time-to-frostbite estimates).<ref>{{cite journal|last1=Tikuisis|first1=Peter|last2=Osczevski|first2=Randall J.|title=Facial Cooling During Cold Air Exposure|journal=Bulletin of the American Meteorological Society|date=2003|volume=84|issue=7|pages=927–933|doi=10.1175/BAMS-84-7-927|doi-broken-date=1 November 2024 |bibcode=2003BAMS...84..927T}}</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.