Wind chill
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Wind chill is the apparent temperature felt on the exposed human (or animal) body due to the combination of air temperature and wind speed. Except at higher temperatures, where wind chill is reckoned not to exist, the wind chill temperature — or "wind chill factor" as it is more commonly called — is always lower than the air temperature, because the wind increases the rate at which moisture evaporates from the skin and carries heat away from the body (some meteorologists, however, disagree with this, on the grounds that the average wind speed is not calm at any locality on Earth, and propose the establishment of a benchmark wind-speed figure — typically in the range of 5 to 7½ miles per hour — so that any wind speed slower than that would result in the "wind chill factor" indeed being higher than the air temperature thus adjusted). The phase change of water (in sweat) from liquid to vapor requires that the molecules reach a higher energy state. That energy is acquired by absorbing heat from surrounding tissue by conduction. (See heat transfer)
Air movement increases the rate at which the temperature of an object reaches the temperature of the ambient air. Humans perceive or "feel" this increased rate of heat transfer as wind chill.
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. It is of great importance to the survival of humans and animals, and can even affect machinery and heating systems.
The official definition of wind chill in meteorology was originally based on measurements taken at a distance above the ground. The exact definition of wind chill has been controversial because it is a composite index, because animate and inanimate bodies behave differently, and because wind chill reports have a major impact on winter tourism.
The first wind chill formulae and tables were developed by the United States military during World War II, initially by Siple and Passel working in the Antarctic, and were made available by the National Weather Service by the 1970s. In 2001 the formulae were revised to reflect more accurate theories and testing than those done by the military. These formulae are designed specifically for the human body, or even more specifically for the human face. Wind chill also affects animals, and wet, inanimate objects, but different formulae apply to them.
It has generally been conceded that the original model for windchill was not necessarily the best possible for all purposes. The physical basis for the calculation of windchill is now the relationship between the temperature, volume and pressure of a fluid. Moving air reduces air pressure and increases the cooling effect. Still air can actually insulate, which is why wind chill was measured a number of metres above the ground rather than at ground level.
The new wind chill index used by the US and Canadian weather services is calculated from the following formula:
- <math>T_{wc}=35.74+0.6215 T_a-35.75 V^{0.16}+0.4275 T_a V^{0.16}<math>
where Twc is the wind chill temperature in °F, Ta is the air temperature, and V is the air speed in mph.
As the air temperature falls, the chilling effect of any wind that is present increases; that is to say, a 10-mph wind will lower the apparent temperature by a wider margin at an air temperature of -4 °F (-20 °C), than a wind of the same speed would if the air temperature was 14 °F (-10 °C). Winds of stronger than 40 mph are assumed to have no significant additional chilling effect beyond the effect felt at that velocity, and the wind-chill phenomenon is thought to be absent altogether at air temperatures above approximately 68 °F (20 °C).
The US and Canadian formulae are best suited to extremely cold climates. Other formulae such as the Steadman wind-chill index (developed by Australian environmental scientist Robert Steadman) have been developed for temperate climates, but are less well known. Some wind-chill indices also take humidity into account — and the wind-chill and heat index are sometimes collectively referred to by a single term, either "apparent temperature" or "relative outdoor temperature."