What is the polar vortex, and how does it contribute to cold snaps?

The polar vortex is a regular feature of the arctic atmosphere. How much does it have to do with bitter cold spells elsewhere?

Bitter cold is expected across much of the U.S. this weekend. But what do headlines mean when they say a "polar vortex cold snap" is on the way?

The polar vortex is a regular feature of Earth's polar latitudes. It is a concentration of cold air and low pressure that is usually kept relatively close to the north and south poles.

It gets the "vortex" part of its name from the swirl of counter-clockwise air that extends as much as 30 miles up into the atmosphere.

The polar vortex does not, by itself, cause the severe cold spells we might see in a weather forecast.

But the polar vortex interacts closely with the jet stream, which is a shallower flow of winds that ranges from five to nine miles high.