The Wind Chill Factor map shows the forecast wind chill. color contoured every 10 to 20 degrees F. Wind chill is the apparent temperature
felt on exposed skin, which is a function of the air temperature and wind speed. The wind chill temperature (often popularly called the wind chill factor) is always lower than the air temperature, except at higher temperatures where wind chill is considered less important. In cases where the apparent temperature is higher than the air temperature, the heat index is used instead.
There is a thermal boundary layer surrounding the skin which may be several millimetres thick. This boundary layer acts as an insulator. When it is cold and the wind is blowing, the air feels colder than it does when it is calm because the wind blows away the boundary layer. In a perfect calm, if free convection could be suppressed (as it is in microgravity), the boundary layer would be infinitely thick. Add a wind, and the only still air that remains would be the air in the immediate vicinity of some surface, like the skin. The stronger the wind, the thinner the layer. Because the outer layers of still air are blown off more easily than the ones closer to the skin, when it is nearly calm, a small increase in wind speed causes a much greater thinning of the boundary layer thickness than the same increase in wind speed when the wind is already strong.
Convective heat loss is really conduction through an insulating boundary layer. The insulation of the boundary layer depends on its thickness. When there is wind, the thermal resistance of the boundary layer is smaller, the heat loss is higher, and the temperature of the skin is closer to the air temperature. Humans do not sense the temperature of the air but the temperature of the skin. Because skin temperature is closer to the air temperature when it is windy, the wind causes it to feel colder.
A wind-chill factor of 25°F (- 4°C) will not freeze water if the air temperature is 35°F (2°C). Water changes state according to the temperature of the body of the water. In this case, the water and air temperature are about the same — too high to freeze water.
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. In much of North America in winter, wind chill is forecast and reported by news media. To some degree, people make decisions as to how they will dress for outdoor activity, or whether they will take part in outdoor activity based on the wind chill. This has a potential economic impact on ski operators and other outdoor recreation areas, and to merchants. Schools use the wind chill forecast to decide whether to let students outside for recess or lunch in cold weather. Heart patients pay attention to the wind chill, to estimate the stress the weather might place on their circulatory systems to avoid problems. The military modifies its training exercises when wind chill reaches dangerous levels. It is of great importance to the survival of humans and animals and can even affect some machinery and heating systems.