wind chill the cooling effect of wind and temperature combined, expressed in terms of the effect produced by a lower, windless temperature, also called wind chill factor, wind chill temperature, wind chill equivalent temperature, wind chill index, wind chill equivalent index, and wind chill temperature index. Wind chill is based on the rate of heat loss from exposed skin. Under windless conditions air provides an invisible blanket around the skin. As wind speed increases, this layer of heated air is carried away from the body at an accelerated rate, forcing the body either to work harder to generate more heat or cool down. If the actual air temperature is -5°F(-21°C) with a 20 mph (32 km/hr) wind, the wind chill temperature is -29°F(-34°C). Because wind chill is based the removal of heat from the human body, it does not reflect the increased rate of heat loss for inanimate objects such as automobile radiators under the same conditions but they also experience a faster heat loss with increasing winds.

The term wind chill was coined by the American geographer Paul A. Siple in his dissertation, Adaptation of the Explorer to the Climate of Antarctica, (1939). Subsequently, on the third Byrd Antarctic expedition, Siple and American geologist Charles Passel determined how quickly extreme conditions could produce frostbite on exposed skin. By 1945, Siple and Passel had published a set of numbers expressing heat loss as a function of temperature and wind speed.

A wind chill advisory is issued when the forecast projects a wind velocity of at least 10 mph (16 km/hr) producing a wind chill temperature of -15°F or lower for 3 hours or more. At these values wind chill is more of a nuisance than it is life threatening. A wind chill warning is issued when the forecasted wind chill temperature is -25°F or lower, which can be life threatening if the individual is not suitably dressed. Persons who go outside under such conditions may experience frostbite and other cold-related symptoms in a matter of minutes, even if properly clothed for normal winter conditions, and longer exposures may prove fatal.

Wind chill

Wind chill is the temperature sensed by humans as a result of air blowing over exposed skin. The temperature that humans actually feel, called the sensible temperature, can be quite different from the temperature measured in the same location with a thermometer. The reason for such differences is that the human body constantly gives off and absorbs heat in a variety of ways. For example, when a person perspires, evaporation of moisture from the skin removes heat from the body, and one feels cooler than the true temperature would indicate.

In still air, skin is normally covered with a thin layer of warm air that insulates the body and produces a sensible temperature somewhat higher than the air around it. When the wind begins to blow, that insulating layer is swept away, and body heat is lost to the surrounding atmosphere. An individual begins to feel colder than would be expected from a thermometer reading at the same location.

The faster the wind blows, the more rapidly heat is lost and the colder the temperature appears to be.

Wind chill charts or conversion tables relate the relationship among actual temperature, wind speed, and wind chill factor, to the temperature felt by a person at the given wind speed. According to standard conversion formulae, a wind speed of 4 mi/h (6 km/h) or less results in no observable change in temperature sensed. At a wind speed of 17 mi/h (30 km/h) and a temperature of 32°F (0°C), however, the perceived temperature is 7°F (−14°C).

Wind chill relationships are not linear. The colder the temperature, the more strongly the wind chill factor is felt. At a wind speed of 31 mi/h (50 km/h), for example, the perceived temperature at 32°F (0°C) is 7°F (−14°C), but at −40°F (−40°C), the perceived temperature is −112°F (−80°C).

See also Antarctica; Atmospheric lapse rate; Aviation physiology; Beaufort wind scale; Humidity; Space physiology

wind-chill factors A combination of cold and wind makes the human body feel cooler than the actual air temperature. The charts below give approximate equivalents for combinations of wind speed and temperature. In sub-zero temperatures, even moderate winds will significantly reduce effective temperatures: if human skin was exposed to winds of 48 km/h (30 mph) in a temperature of −34°C (−30°F) it would freeze solid in 30 seconds.

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