Wind chill - wikipedia Wind chill This article is about the meteorological effect of `` wind chill ''. For other uses, see Wind chill ( disambiguation ). The examples and perspective in this article deal primarily with the English - speaking world and do not represent a worldwide view of the subject. You may improve this article, discuss the issue on the talk page, or create a new article, as appropriate. ( January 2016 ) A chart of wind chill values for given air temperatures and wind speeds Wind - chill or windchill, ( popularly wind chill factor ) is the lowering of body 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 is higher than the air temperature, the heat index is used instead. Contents 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 Explanation 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 surface cools. 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 can not, 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 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. 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. That model has evolved over time. 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, and were made available by the National Weather Service 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 1.8 metres per second ( 6.5 km / h ), 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. The original formula for the index was : W C I = ( 10 v − v + 10.5 ) ⋅ ( 33 − T a ) ( displaystyle mathrm ( WCI ) = left ( 10 ( sqrt ( v ) ) - v + 10.5 \right ) cdot left ( 33 - T_ ( mathrm ( a ) ) \right ) ) where : WCI = wind chill index, kcal / m / h v = wind velocity, m / s T = 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 1.4 metres per second ( 5.0 km / h ). 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 w c = 13.12 + 0.6215 T a − 11.37 v + 0.16 + 0.3965 T a v + 0.16 ( displaystyle T_ ( mathrm ( wc ) ) = 13.12 + 0.6215 T_ ( mathrm ( a ) ) - 11.37 v ^ ( + 0.16 ) + 0.3965 T_ ( mathrm ( a ) ) v ^ ( + 0.16 ) ) where T is the wind chill index, based on the Celsius temperature scale ; T 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 is : T w c = 35.74 + 0.6215 T a − 35.75 v + 0.16 + 0.4275 T a v + 0.16 ( displaystyle T_ ( mathrm ( wc ) ) = 35.74 + 0.6215 T_ ( mathrm ( a ) ) - 35.75 v ^ ( + 0.16 ) + 0.4275 T_ ( mathrm ( a ) ) v ^ ( + 0.16 ) , ! ) where T is the wind chill index, based on the Fahrenheit scale ; T 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 ). Celsius Wind Chill index Comparison of old and new Wind Chill values at − 15 ° C ( 5 ° F ) Windchill calculator The method for calculating wind chill has been controversial, because experts 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. The exposure to wind depends on the surroundings and wind speeds can vary widely depending on exposure and obstructions to wind flow. 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. 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 : A T = T a + 0.33 e − 0.70 v − 4.00 ( displaystyle mathrm ( AT ) = T_ ( mathrm ( a ) ) + 0.33 e-0.70 v - 4.00 ) where : T = 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 : e = R H 100 ⋅ 6.105 ⋅ exp ⁡ ( 17.27 ⋅ T a 237.7 + T a ) ( displaystyle e = ( \frac ( mathrm ( RH ) ) ( 100 ) ) cdot 6.105 cdot exp ( left ( ( \frac ( 17.27 cdot T_ ( mathrm ( a ) ) ) ( 237.7 + T_ ( mathrm ( a ) ) ) ) \right ) ) ) Where : T = dry bulb temperature ( ° C ) RH = Relative humidity ( % ) exp represents the exponential function 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 Jump up ^ A Field Guide to the Atmosphere - Jay Pasachoff - Google Boeken. Books.google.com. Jump up ^ 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. Jump up ^ * Woodson, Wesley E. ( 1981 ). Human Factors Design Handbook, page 815. McGraw - Hill. ISBN 0 - 07 - 071765 - 6 Jump up ^ https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690003109_1969003109.pdf Jump up ^ `` Environment Canada - Weather and Meteorology - Canada 's Wind Chill Index ''. Ec.gc.ca. Retrieved 2013 - 08 - 09. Jump up ^ `` Meteorological Tables, Wind Chill. August, 2001 Press Release : ''. National Weather Service. Retrieved 14 January 2013. Jump up ^ `` Wind Chill ''. BBC Weather, Understanding weather. BBC. Archived from the original on 11 October 2010. Jump up ^ Osczevski, Randall ; Bluestein, Maurice ( 2005 ). `` The new wind chill equivalent temperature chart ''. Bulletin of the American Meteorological Society. 86 ( 10 ) : 1453 -- 1458. Bibcode : 2005BAMS... 86.1453 O. doi : 10.1175 / BAMS - 86 - 10 - 1453. Jump up ^ `` Calculation of the 1971 to 2000 Climate Normals for Canada ''. Climate.weatheroffice.gc.ca. 2013 - 07 - 10. Archived from the original on 2013 - 06 - 27. Retrieved 2013 - 08 - 09. ^ Jump up to : `` NWS Wind Chill Index ''. Weather.gov. 2009 - 12 - 17. Archived from the original on 2011 - 09 - 18. Retrieved 2013 - 08 - 09. Jump up ^ `` A chart of windchills based on this formula ''. Weather.gov. 2009 - 12 - 17. Retrieved 2017 - 04 - 13. Jump up ^ Tikuisis, Peter ; Osczevski, Randall J. ( 2003 ). `` Facial Cooling During Cold Air Exposure ''. Bulletin of the American Meteorological Society. 84 ( 7 ) : 927 -- 933. Bibcode : 2003BAMS... 84... 927T. doi : 10.1175 / BAMS - 84 - 7 - 927. Jump up ^ `` About the WBGT and Apparent Temperature ''. Bureau Of Meteorology, Australia. Bom.gov.au. 2010 - 02 - 05. Retrieved 2013 - 08 - 09. Jump up ^ `` Thermal Comfort observations ''. Bureau Of Meteorology, Australia. Bom.gov.au. 2010 - 02 - 05. Retrieved 2013 - 08 - 09. Jump up ^ `` The formula for the apparent temperature ''. Bureau Of Meteorology, Australia. Bom.gov.au. 2010 - 02 - 05. Retrieved 2013 - 08 - 09. External links Wikimedia Commons has media related to Wind chill. Look up wind chill in Wiktionary, the free dictionary. National Center for Atmospheric Research Table of wind chill temperatures in Celsius and Fahrenheit Wind chill calculator National Weather Service Wind Chill Temperature Index Table of wind chill temperatures in Fahrenheit with frostbite times Real - time map of global wind chill values Meteorological data and variables General Adiabatic processes Advection Buoyancy Lapse rate Lightning Surface solar radiation Surface weather analysis Visibility Vorticity Wind Wind shear Condensation Cloud Cloud condensation nuclei ( CCN ) Fog Convective condensation level ( CCL ) Lifted condensation level ( LCL ) Precipitation Water vapor Convection Convective available potential energy ( CAPE ) Convective inhibition ( CIN ) Convective instability Convective momentum transport Convective temperature ( T ) Equilibrium level ( EL ) Free convective layer ( FCL ) Helicity K Index Level of free convection ( LFC ) Lifted index ( LI ) Maximum parcel level ( MPL ) Bulk Richardson number ( BRN ) Temperature Dew point ( T ) Dew point depression Dry - bulb temperature Equivalent temperature ( T ) Forest fire weather index Haines Index Heat index Humidex Humidity Relative humidity ( RH ) Mixing ratio Potential temperature ( θ ) Equivalent potential temperature ( θ ) Sea surface temperature ( SST ) Thermodynamic temperature Vapor pressure Virtual temperature Wet - bulb temperature Wet - bulb potential temperature Wind chill Pressure Atmospheric pressure Baroclinity Barotropicity Pressure gradient Pressure - gradient force ( PGF ) Retrieved from `` https://en.wikipedia.org/w/index.php?title=Wind_chill&oldid=821841727 '' Categories : Atmospheric thermodynamics Meteorological indices Units of meteorology measurement Hidden categories : Articles with limited geographic scope from January 2016 Anglophone - centric All articles with unsourced statements Articles with unsourced statements from December 2009 All articles with specifically marked weasel - worded phrases Articles with specifically marked weasel - worded phrases from March 2009 Articles with unsourced statements from January 2018 Talk Contents About Wikipedia Català Dansk Deutsch Español فارسی Français Gaeilge Hrvatski Ilokano Bahasa Indonesia Italiano Nederlands Norsk Norsk nynorsk Polski Português Română Русский Simple English Srpskohrvatski / српскохрватски Suomi Svenska Українська 中文 Edit links This page was last edited on 22 January 2018, at 23 : 08. 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Question: when did they change the wind chill formula
Answer: November 2001