Chicago Weather: What Exactly Is The Polar Vortex?

CHICAGO (CBS) -- The Polar Vortex.

Chicago residents hear about it during extreme cold.

But what is it, exactly?

The short answer: It's a large expanse of swirling cold air that is stuck in polar regions, according to the National Oceanic and Atmospheric Administration. During winter, the polar vortex at the North Pole expands, sending cold air southward.

When the polar vortex becomes disrupted, it sends that air even farther south, creating extreme cold.

That's what happening this week. By Monday of next week, that cold air will retreat, bringing warmer temps back to Chicago.