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Loss of heat from exposure of skin to wind. Heat loss is proportional to the speed of the wind. Thus, skin exposed to a wind velocity of 20 mph (32 km/hr) when the temperature is 0°F (−17.8°C) is cooled at the same rate as in still air at −46°F (−43.3°C). Similarly, when the temperature is 20°F (−6.7°C) and the wind is 10, 20, or 35 mph (16.1, 32.2, or 56.3 km/hr), the equivalent skin temperature is −4°, −18°, or −28°F (−20°, −27.8°, or −33.3°C), respectively.
The windchill factor is calculated for dry skin; skin that is wet from any cause and exposed to wind loses heat at a much higher rate. Wind blowing over wet skin can cause frostbite, even on a comfortably warm day as judged by the thermometer.
See also: factor