Weather Focus: The Wind Chill Factor

What is the Wind Chill?

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.

WIND CHILL INDEX CHART:

You can calculate the wind chill on your own using the following chart: