# Explainer: How Wind Chills are Calculated by the National Weather Service

With ferocious winds and frigid temperatures forecasted in the Chicago area week, residents will see plenty of coverage of wind chills, a measurement dedicated to informing individuals how cold it feels outside.

Wind chills in Chicago could potentially drop to 20-to-30 degrees below zero at times during the weekend, with the wind expected to remain gusty through at least Saturday as a winter storm impacts the region.

So how exactly is wind chill calculated? Are there specific guidelines for how it is measured?

According to the National Weather Service, this is the formula used to calculate wind chill:

35.74 + 0.6215T – 35.75 (V^0.16) + 0.4275T(V^0.16) = Wind chill

In this equation, the “T” variable represents the air temperature, while the “V” variable represents the wind speed.

For example, an air temperature of 10 degrees, and a wind speed of 25 miles per hour will create a wind chill of approximately minus-11 degrees Fahrenheit.

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According to officials, the formula takes into account a measurement of the wind speed as measured at the average height of a human face, which is five feet. It also takes into account that a typical anemometer, a device used to measure wind speed, is placed 33 feet in the air.

The formula also accounts for heat transfer theory and skin tissue resistance, according to the NWS.

Finally, the formula assumes that measurements are taken at night, with no sunlight influencing the “feels-like” temperature.

In order for wind chill to be calculated, the air temperature must be at or below 40 degrees Fahrenheit.

Winds are expected to remain gusty through at least Saturday, but frigid air temperatures will likely stick around after the Christmas holiday, with highs rebounding to more seasonal levels by the middle of the coming week.