(AP) — It’s not the heat, it’s the humidity, goes the old saying.

For the tens of millions of Americans currently trapped in the deep freeze: It’s not the cold, it’s the wind.

Air temperatures plunging into the negative teens, twenties and even thirties Sunday into Monday are bad enough. But add wind speeds of even a few miles per hour, and what’s already deeply unpleasant becomes downright dangerous.

Severely low wind chills are a serious threat to the body. Emergency room doctor Douglas Brunette from Hennepin County Medical Center in Minneapolis says he’s “seen frostbite occur through clothing.” He says it’s important to have clothes “made for the elements.”

Winds are blowing arctic air across large swaths of the U.S. — with temperatures suppressed by what meteorologists call a “polar vortex.”