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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 windchill in meteorology was originally based on measurements taken at a distance above the ground. The exact definition of windchill has been controversial because it is a composite index, because animate and inanimate bodies behave differently, and because windchill 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 windchill 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:
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 AustraliaAustralia is the sixth-largest country in the world (geographically), the only one to occupy an entire continent, and the largest in the region of Australasia. Australia includes the island of Tasmania, which is an Australian State. Its neighbouring countn environmental scientist Robert Steadman) have been developed for temperate climates, but are less well known. Some wind-chill indices also take humidityHumidity is the amount of moisture in the air. It can be expressed as absolute humidity which is the mass of water in a specified volume or mass of air, or more commonly as relative humidity which is the absolute humidity divided by that absolute humidity into account.