Wind chill is the apparent temperature felt on the exposed human (or animal) body due to the combination of air temperature and wind speed. The wind chill temperature is always lower than the air temperature, even when the air is hotter than the body, because the wind increases the rate at which moisture evaporates from the skin and carries heat away from the body.

The concept 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 T_wc is the wind chill temperature in °F, T_a is the air temperature, and V is the air speed in mph.

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.

External Links

• National Science Digital Library - Wind Chill Temperature
• An Introduction to Wind Chill : A lesson plan on Wind Chill.