AskDefine | Define windchill

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Noun

  1. The still-air temperature equivalent to a given combination of temperature and wind speed - as far as its cooling effect on exposed flesh is concerned
This page is for the meteorological effect of "wind chill." For the film, see Wind Chill.
' Wind chill is the apparent temperature felt on exposed skin, which is a function of the air temperature and wind speed. The wind chill temperature (often popularly called the wind chill factor) is always lower than the air temperature, except at higher temperatures where wind chill is considered less important. In cases where the apparent temperature is higher than the air temperature, the heat index is used instead.

Explanation

There is a thermal boundary layer surrounding the skin which may be several millimetres thick. This boundary layer acts as an insulator. When it is cold and the wind is blowing, the air feels colder than it does when it is calm because the wind blows away the boundary layer. In a perfect calm, if free convection could be suppressed (as it is in microgravity), the boundary layer would be infinitely thick. Add a wind, and the only still air that remains would be the air in the immediate vicinity of some surface, like the skin. The stronger the wind, the thinner the layer. Because the outer layers of still air are blown off more easily than the ones closer to the skin, when it is nearly calm, a small increase in wind speed causes a much greater thinning of the boundary layer thickness than the same increase in wind speed when the wind is already strong.
Convective heat loss is really conduction through an insulating boundary layer. The insulation of the boundary layer depends on its thickness. When there is wind, the thermal resistance of the boundary layer is smaller, the heat loss is higher, and the temperature of the skin is closer to the air temperature. Humans do not sense the temperature of the air but the temperature of the skin. Because skin temperature is closer to the air temperature when it is windy, the wind causes it to feel colder.
A wind-chill factor of 25° F (- 4° C) will not freeze water if the air temperature is 35° F (2° C). Water changes state according to the temperature of the body of the water. In this case, the water and air temperature are about the same — too high to freeze water.

Significance

The concept of wind chill is of particular significance in very cold climates such as the Arctic and Antarctic, at high altitude, at high speeds, or in very high winds. In much of North America in winter, wind chill is forecast and reported by news media. To some degree, people make decisions as to how they will dress for outdoor activity, or whether they will take part in outdoor activity based on the wind chill. This has a potential economic impact on ski operators and other outdoor recreation areas, and to merchants. Schools use the wind chill forecast to decide whether to let students outside for recess or lunch in cold weather. Heart patients pay attention to the wind chill, to estimate the stress the weather might place on their circulatory systems to avoid problems. The military modifies its training exercises when wind chill reaches dangerous levels. It is of great importance to the survival of humans and animals and can even affect some machinery and heating systems.

Formulae and tables

The first wind chill formulae and tables were developed by Paul Allen 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 4 mph (1.78 m/s), 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=\tfrac
The general public seems to have been put off by the strange sounding units, either the old ones or the newer Watts per sq. 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.

New wind chill index

In 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 3 mph (1.37 m/s). 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_=13.12 + 0.6215 T_a-11.37 V^ + 0.3965 T_a V^\,\!
where T_\,\! is the wind chill index based on the Celsius scale, T_a\,\! is the air temperature in °C, and V\,\! is the air speed in km/h measured at 10 m (33 ft, standard anemometer height).
The equivalent formula in US customary units is:
T_=35.74+0.6215 T_a-35.75 V^+0.4275 T_a V^\,\!
where T_\,\! and T_a\,\! are measured in °F, and V\,\! in mph. These equations cannot be used at wind speeds below about 2 mph or 3 km/h or temperatures below -50°C or above 5°C.
As the air temperature falls, the chilling effect of any wind that is present increases. For example, a 16 km/h (10 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).
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.

See also

windchill in Catalan: Temperatura de sensació
windchill in Danish: Kuldeindeks
windchill in German: Windchill
windchill in Spanish: Temperatura de sensación
windchill in Persian: سرمایش باد
windchill in French: Refroidissement éolien
windchill in Italian: Temperatura#Temperatura_effettiva_e_temperatura_percepita
windchill in Dutch: Gevoelstemperatuur
windchill in Japanese: 体感温度
windchill in Norwegian: Vindavkjølingseffekt
windchill in Polish: Temperatura odczuwalna
windchill in Portuguese: Sensação térmica
windchill in Russian: Жёсткость погоды
windchill in Finnish: Pakkasen purevuus
windchill in Swedish: Vindavkylning
windchill in Chinese: 風寒指數
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