Wind Chill - original model

Description

Wind-chill or windchill, (popularly wind chill factor) is the perceived decrease in air temperature felt by the body on exposed skin due to the flow of air.

Wind chill numbers are always lower than the air temperature for values where the formula is valid. When the apparent temperature is higher than the air temperature, the heat index is used instead.

A surface loses heat through conduction, convection, and radiation. The rate of convection depends on the difference in temperature between the surface and its surroundings. As convection from a warm surface heats the air around it, an insulating boundary layer of warm air forms against the surface. Moving air disrupts this boundary layer, or epiclimate, allowing for cooler air to replace the warm air against the surface. The faster the wind speed, the more readily the surface cools.

The effect of wind chill is to increase the rate of heat loss and reduce any warmer objects to the ambient temperature more quickly. It cannot, however, reduce the temperature of these objects below the ambient temperature, no matter how great the wind velocity. For most biological organisms, the physiological response is to generate more heat in order to maintain a surface temperature in an acceptable range. The attempt to maintain a given surface temperature in an environment of faster heat loss results in both the perception of lower temperatures and an actual greater heat loss. In other words, the air 'feels’ colder than it is because of the chilling effect of the wind on the skin. In extreme conditions this will increase the risk of adverse effects such as frostbite.

Original model

Equivalent temperature was not universally used in North America until the 21st century. Until the 1970s, the coldest parts of Canada reported the original Wind Chill Index, a three or four digit number with units of kilocalories/hour per square metre. Each individual calibrated the scale of numbers personally, through experience. The chart also provided general guidance to comfort and hazard through threshold values of the index, such as 1400, which was the threshold for frostbite.

The original formula for the index was as shown here.

Related formulas

Variables

WCIwindchill index (°C) (dimensionless)
Vwind velocity (m/s) (dimensionless)
Taair temperature (°C) (dimensionless)