Quezi

This self-portrait was taken when the wind chill was -35 degrees Fahrenheit. Note the use of a balaclava to reduce the amount of skin exposed to the wind. LongitudeLatitude photo. CC-BY.

A day when there’s no wind outside and a temperature of 40 degrees Fahrenheit won’t seem all that uncomfortable — but add a 15 mph wind, and it’ll literally feel the same as if it were freezing.

This phenomenon is known as the wind chill factor, and anyone who spends much time outside in cooler climates know how much difference wind can make.

The cooling effect is caused by the wind literally blowing a person’s body heat away. Think of the body as something like the type of radiator used to heat a building. As hot water goes through the radiator, the radiator transfers the water’s heat to the air around it, and the water gets cooler and needs to be reheated. Place a fan to blow air through the radiator, and the water’s heat will be transferred more quickly to the surrounding air. Then think of the body’s blood as akin to the hot water, and you can see wind blowing across your body will remove heat from your blood and therefore your body more quickly (assuming it’s colder than body temperature outside).

Technically, the wind chill factor is used to determine how wind affects the perceived temperature of exposed skin. For people living in cold climates, the wind chill factor can be especially significant, as a person exposed to wind in the cold can suffer frostbite, the freezing of tissue, even when the thermometer temperature is well above zero Fahrenheit.

There is a formula for determining the wind chill factor, but it’s pretty complicated:

W = 35.74 + 0.6212T – 35.75(V^0.16) + 0.4275T(V^0.16)
where W in the wind chill, T is the thermometer reading in degrees Fahrenheit, and V is the wind velocity in miles per hour.

A much easier way is to consult a wind chill chart, such as one that can be found at the National Weather Service.

The easiest way to protect yourself from wind chill is to keep as much of your skin covered as possible. Even the small amount of air between clothing and the skin will have an insulating effect, and eliminating wind will eliminate the wind chill. Of course, zero degrees F will feel cold even with no wind, but the chance of frostbite will be greatly reduced. With very low temperatures and high wind, exposed tissue can freeze in as little as five minutes.