Wind chill (often popularly called the wind chill factor) is the felt air temperature on exposed skin due to wind. It measures the effect of wind on air temperature. The wind chill temperature is usually lower than the air temperature, since the air temperature is usually lower than the human body temperature. In contrast, humidity on the skin can result in a higher felt air temperature, and the heat index or humidex is used instead.

Explanation

The human body loses heat through convection, evaporation, conduction, and radiation . The rate of heat loss by a surface through convection depends on the wind speed above that surface: the faster the wind speed, the more readily the surface cools. 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 such as frostbite and death.

Formulae and tables

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.

The first wind chill formulae and tables were developed by Paul Allman Siple and Charles 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. 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 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 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 for it was not a measure of human heat loss but simply a number proportional to human heat loss.

The original formula for the index was:

:$WCI=(10\backslash sqrt\{V\}-V+10.5)\; \backslash cdot\; (33-T\_\{a\})$ ::Where: :::$WCI$ = Wind chill index, kcal/m²/h :::$V$ = Wind velocity, m/s :::$T\_a$ = Air temperature, °C

The general public seems to have been put off by the strange sounding units, either the old ones or the newer watts per square metre, and developed a strong preference for equivalent temperature, a deceptive simplification that only seems to be easier to understand. Even in the cold areas of Canada, broadcast media began to switch to the newer method of reporting after metrification.

North American wind chill index

In November 2001 the National Weather Service implemented the new wind chill index, used by the U.S. and Canadian weather services, which is determined by iterating a model of skin temperature under various wind speeds and temperatures. The model used 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:

:$T\_\{wc\}=13.12\; +\; 0.6215\; T\_a-11.37\; V^\{+0.16\}\; +\; 0.3965\; T\_a\; V^\{+0.16\}\backslash ,\backslash !$

:where $T\_\{wc\}\backslash ,\backslash !$ is the wind chill index based on the Celsius scale, $T\_a\backslash ,\backslash !$ is the air temperature in °C, and $V\backslash ,\backslash !$ is the air speed in km/h measured at , standard anemometer height).

The equivalent formula in US customary units is:

:$T\_\{wc\}=35.74+0.6215\; T\_a-35.75\; V^\{+0.16\}+0.4275\; T\_a\; V^\{+0.16\}\backslash ,\backslash !$

:where $T\_\{wc\}\backslash ,\backslash !$ and $T\_a\backslash ,\backslash !$ are measured in °F, and $V\backslash ,\backslash !$ in mph.

Click here for a chart of windchills based on this formula.

Windchill Temperature is only defined 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 .

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 WCET's 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.

Australian Apparent Temperature

The Australian Bureau of Meteorology uses a different formula for wind chill. They also use the term "apparent temperature". The apparent temperature is defined as the temperature that, at a reference humidity level, produces the same amount of discomfort as that experienced under the current ambient temperature and humidity. This model was designed to model thermal sensation in indoor conditions, and extended to include the effects of wind and solar radiation. The full model was described by Robert Steadman in 1994, and is defined as follows:

:$T\_a=T\_d+0.348e-0.70V+0.70\; \backslash frac\{Q\}\{V+10\}-4.25\backslash ,$ ::Where: :::$T\_\{d\}$ = Dry-bulb air temperature, °C :::$e$ = Water vapour pressure, hPa :::$Q$ = Net solar radiation absorbed per unit area of body surface, W/m²

Water vapour pressure is defined:

:$e\; =\; \backslash frac\{R\}\{100\}\; 6.105\; \backslash exp\{\backslash left(\backslash frac\{17.27\; T\_\{d\}\}\{237.7\; +\; T\_\{d\}\}\backslash right)\}\backslash ,$

where $R$ is the Relative Humidity, expressed in percent. A simpler version of this formula, neglecting variations in solar radiation is often used:

:$T\_a=T\_d+0.348e-0.70V-4.00\backslash ,$

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. As these are qualitative models this is not necessarily a major failing. However it is worth noting the Australian model, or a variant, may be more appropriate to any given location or prevailing weather conditions.

Clothing, wet-cold and exposure duration

Wet clothing and the duration of a person's exposure to wind also affect the felt air temperature. The wind chill formulae do not take account of the variation of clothing worn and how wet or dry a person is. The typical assumption is that a person will be properly dressed and dry. Models generally also assume that thermodynamic equilibrium exists, something that might not be the case if a person quickly run outdoors and back indoors, for example.

Extraplanetary application

The wind chill formula for use on Earth are generally not applicable to weather conditions on other planets. For example, on Titan the average temperature is estimated at −179 °C while observed wind speeds in the atmosphere reach 430 km/h. This gives a wind chill of -315 °C, which is below absolute zero.

See also

Cold wave

Heat index

Humidex

References

External links

National Weather Service Wind Chill Temperature Index Table of wind chill temperatures

National Science Digital Library - wind chill Temperature

Environment Canada - Wind Chill

Weather Images Wind Chill Chart and Introduction

An Introduction to Wind Chill A lesson plan on wind chill

A Cold Wind Blowing An article on wind chill and tables in SI units

Wind Chill and Humidex Criticism about the use of wind chill and humidex

An article about wind chill from ''The Canadian Encyclopedia''

Category:Atmospheric thermodynamics Category:Weather

ca:Temperatura de sensació da:Kuldeindeks de:Windchill es:Temperatura de sensación fa:سرمایش باد fr:Refroidissement éolien it:Temperatura#Temperatura effettiva e temperatura percepita nl:Gevoelstemperatuur ja:体感温度 no:Vindavkjølingseffekt pl:Temperatura odczuwalna pt:Sensação térmica ru:Жёсткость погоды fi:Pakkasen purevuus sv:Vindavkylning zh:風寒指數

Chill factor may refer to:

Wind chill factor, a meteorological measurement

Chill Factor may refer to:

in film:

''Chill Factor'' (film), 1999 American action film starring Cuba Gooding Jr. and Skeet Ulrich

''A Cold Night's Death'', a 1973 American made-for-TV film, also called ''The Chill Factor''

in other uses:

''Chill Factor'' (Deathlands novel), a 1992 science fiction novel by Laurence James (James Axler)

Chill Factore (styled Chill Factor^e), an indoor ski slope in Manchester, England

name	Clint Mansell
background	non_performing_personnel
birth name	Clinton Darryl Mansell
birth date	January 07, 1963
origin	Coventry, England
genre	ClassicalContemporary classicalElectronicAlternative
occupation	Musician, Composer
years active	1981–present
associated acts	Pop Will Eat Itself
website	clintmansell.com
notable instruments	}}

Clinton Darryl "Clint" Mansell, (born 7 January 1963) is an English musician, composer, and former lead singer and guitarist of the band Pop Will Eat Itself.

After the disbanding of Pop Will Eat Itself in 1996, Mansell was introduced to film scoring when his friend, director Darren Aronofsky, hired him to score his debut film, ''π''. Mansell then wrote the score for the next Aronofsky film, ''Requiem for a Dream'', which has been well received. Its main composition "Lux Æterna" has become extremely popular, appearing in a wide variety of advertisements and film trailers.

Mansell's composition for ''The Fountain'' was nominated for Best Original Score at the 64th Annual Golden Globe Awards. His other notable film scores include ''Moon'', ''Smokin' Aces'', ''The Wrestler'', and ''Black Swan''.

Career

Pop career

Mansell was the lead singer and guitarist of the British band Pop Will Eat Itself. When PWEI signed to Nothing Records, Mansell became friends with Trent Reznor, and he also performed back-up vocals on the 1999 album ''The Fragile'' by Nine Inch Nails.

Soundtrack composer

Mansell's score for Darren Aronofsky's ''π'' was followed up by his score for Aronofsky's next film, ''Requiem for a Dream'', which became a cult hit. Since their first collaboration in 1996, Mansell has composed the music to every Aronofsky film.

Other notable achievements include the theme for the film ''The Hole'', the music for the pilot episode of ''CSI: NY'', and the score for Aronofsky's later films ''The Fountain'', which was nominated for Best Original Score in the 64th Annual Golden Globe Awards, and ''The Wrestler''. Mansell has also contributed the score to HBO's Voyeur. The song was also used as the base theme for the song "Throw It Up" by Lil Jon. His most recent work has been on the soundtrack for ''Black Swan'' in 2010.

Mansell has garnered a cult following for his soundtrack work, so much that ''Smokin' Aces'' director Joe Carnahan admits to receiving "blatant threats" when the soundtrack was released without much of Mansell's score for the film.

Film trailers

The trailer for ''The Lord of the Rings: The Two Towers'' contained a rearranged version of the track "Lux Æterna", using a full orchestra and choir. The piece, named "Requiem for a Tower", was recorded especially for the trailer. The orchestration was arranged by Simone Benyacar, Dan Nielsen, and Veigar Margeirsson.

"Lux Æterna" has since become popular, with both the original and the "Requiem for a Tower" version having appeared in a wide variety of advertisements and trailers, including the trailer for the Red Sox–Yankees games in the 2007 Major League Baseball season, and trailers for the films ''Zathura'', ''The Da Vinci Code'', ''Sunshine'', ''Babylon A.D.'' and the TV series ''Lost'' and ''Top Gear''. It was also used on Sky Sports News and as the theme for ''Soccer Saturday'' from 2007-2009. In 2006, the theme was used in EuroSport LIVE trailers. The theme was also used by ''America's Got Talent'' as an introduction of the judges before being changed slightly. The new variant is now a regular piece on the show.

The piece "Death Is the Road to Awe" from the score for ''The Fountain'' was featured in a trailer for the 2007 film ''I Am Legend'', and the trailer for the film ''The Mist'', as well as the trailer for the film ''Frost/Nixon'', and toward the end of 2007 The Final Cut trailer for ''Blade Runner''.

Scoring credits

''Pi'', 1998

''Requiem for a Dream'', 2000

''World Traveler'', 2001

''The Hole'', 2001

''Knockaround Guys'', 2001

''Rain'', 2001

''Abandon'', 2002

''Murder by Numbers'', 2002

''Sonny'', 2002

''The Hire: Ticker'', 2002

''11:14'', 2003

''Suspect Zero'', 2004

''Sahara'', 2005

''Doom'', 2005

''The Fountain'', 2006

''Trust the Man'', 2006

''Smokin' Aces'', 2006

''Wind Chill'', 2007

''In The Wall'', 2007

''Definitely, Maybe'', 2008

''The Wrestler'', 2008

''Blood: The Last Vampire, 2009

''Moon'', 2009

''L'affaire Farewell'', 2009

''The Rebound'', 2010

''Black Swan'', 2010

''Last Night'', 2010

''Faster'', 2010

''United'', 2011

''Mass Effect 3'', 2012 (TBA)

References

External links

Mansell at Yahoo Movies

Mansell at Rotten Tomatoes

Interview focusing on Pop Will Eat Itself

The Wrestler Score review

Two and a Half Questions with Clint Mansell on Headphone Commute (2009)

Category:1963 births Category:British indie rock musicians Category:English film score composers Category:Living people Category:People from Coventry

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