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 so