Meteorologists warn that a stretched polar vortex could bring severe winter weather to much of the United States.

There are strong upper-level winds rotating around the North Pole that normally keep all cold air contained, but in the winter, the winds weaken, and storms are able to send blasts of cold air directly from the Arctic south into the United States.

An unusually brutal winter storm is set to pummel more than 160 million Americans from Friday, as a stretched "polar vortex" sends a devastating blast of Arctic air, bringing heavy snows and freezing rains.

Here’s the wind chill chart. Notice the four shadings, indicating how long it will take for frostbite. When we get to wind chills of -35 degrees or colder, frostbite can occur in just 10 minutes. Even in the “not as cold” areas of lower Michigan we could get frostbite within 30 minutes.

As the U.S. prepares to take on another bout of cold weather, the term "polar vortex" is getting thrown around. But what is it?