What Is Wind Chill?

Wind chill

An index describing how cold it feels to a person or animal outdoors, based on the rate of heat loss from exposed skin caused by the combined effect of cold air and wind.

Why wind makes you feel colder

Your body warms a thin layer of air next to your skin. In still air that layer stays in place and acts as a small insulating blanket. Wind blows it away and brings colder air in contact with the skin, so the body loses heat faster. The air temperature has not changed; the rate at which you lose heat has.

This is the same idea that explains why metal feels colder than wood at the same temperature, covered in Why Metal Feels Colder Than Wood: the feeling reflects the rate of heat flow, not just the temperature of the surroundings.

What the National Weather Service index says

The National Weather Service (NWS) describes wind chill as how cold people and animals feel when outside. Its examples show the scale of the effect: with air at 0 °F and a 15 mph wind, the wind chill is −19 °F. The index applies to people and animals, and the NWS stresses that it does not change the temperature of inanimate objects, which cool only faster toward the air temperature, never below it.

The NWS also ties wind chill to risk. At a wind chill of −19 °F, it says exposed skin can freeze in 30 minutes. Note that frostbite requires the actual air temperature to be below freezing; wind chill speeds the danger up but does not create it in above-freezing air.

The formula

The NWS Wind Chill Temperature index for Fahrenheit and mph is:

Wind chill = 35.74 + 0.6215 T − 35.75 V^0.16 + 0.4275 T V^0.16

where T is the air temperature in °F and V is the wind speed in mph. The formula is defined only for air temperatures at or below 50 °F and wind speeds above 3 mph, so it does not give a meaningful answer for warm or calm conditions.

Worked example: 20 °F with a 15 mph wind

Take air at 20 °F with a 15 mph wind. First, 15^0.16 is about 1.54. Then:

35.74 + 0.6215 × 20 − 35.75 × 1.54 + 0.4275 × 20 × 1.54 = 35.74 + 12.43 − 55.06 + 13.17 = about 6 °F

So 20 °F air with a 15 mph wind feels like roughly 6 °F to exposed skin. Repeat the calculation at 0 °F and the same wind and you get about −19 °F, matching the NWS example.

To think in Celsius, convert the inputs first and the result afterwards using the formulas in Celsius vs Fahrenheit.

What the index assumes

The NWS index is built on a model of a human face and heat transfer theory. It calculates the wind speed at about five feet above the ground, the height of an average adult face, from measurements made at the standard anemometer height of 33 feet. It assumes no effect from sunshine, as on a clear night. Humidity was left out after testing showed it changed results by less than one degree.

Because it is built for exposed skin, it is a poor guide to what happens to pipes, car engines, water bowls or houseplants. Those objects cool toward the air temperature and no lower. Wind can make them reach that temperature sooner, which matters for a pipe about to freeze but does not change where the cooling ends.