DENVER (KDVR) – A major winter storm affecting millions of Americans is spreading rain, freezing rain and heavy snow across the country, with bone-chilling wind chills in some areas.

But what is the wind chill temperature, and what does it mean?

The National Weather Service says the wind chill temperature is a measure of how cold people and animals perceive it to be outside, even if the actual temperature is higher. The factors that determine wind chill are based on the rate of heat loss from skin exposed to windy, cold weather.

“As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature,” the NWS said. “Therefore, the wind makes it feel much colder. If the temperature is 0 degrees and the wind is blowing at 15 mph, the wind chill is minus 19 degrees. At this wind chill temperature, exposed skin can freeze in 30 minutes.”

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Credit: National Weather Service

This chart from the National Weather Service shows that it doesn’t have to be extremely cold — and the wind doesn’t have to blow hard — to give someone frostbite in as little as a 30 minutes. 

According to the NWS, the body parts most susceptible to frostbite include fingers, toes, ear lobes and the tip of the nose. If you believe you may have frostbite, seek immediate medical attention. The frostbitten extremities should also be warmed slowly, “using warm, not hot water,” according to the NWS.

The Associated Press contributed to this report.