Wind Chill
Wind chill refers to the cooling effect of any combination of temperature and wind, expressed as the loss of body heat in watts per square meter of skin surface.
The body has a very thin layer of still air immediately adjacent to it called the boundary layer that helps to insulate the body from heat loss. As wind speed increases, the thickness of the boundary layer diminishes, and the rate of sensible heat loss from the body increases.
The danger from wind chill is measured by the Wind Chill Equivalent Temperature (WET) air temperature index. WET attempts to approximate the sensible heat loss from exposed skin caused by the combined effect of low air temperature and wind. For example, an air temperature of 0 degrees Celsius with 10 m/s wind corresponds to a WET of -26 degrees Celsius, while an air temperature of -20 degrees Celsius with 10 m/s wind corresponds to a WET of -40 degrees Celsius. To put that in perspective, exposed flesh freezes at -31 degrees Celsius. Only when the wind is essentially calm will the actual air temperature be the same as the wind chill equivalent temperature.