Brrrr! Below are the temperatures that are a quarter of the continental U.S., according to The National Weather Service.

Here comes the polar vortex. But what is it?

The polar vortex – everyone's favorite wintertime whipping boy – is a large area of ​​cold air high in the atmosphere that normally spins over the North Pole (as its name suggests).

Sometimes, however, thanks to a meandering jet stream, some of the vortex can slosh down into North America, helping to tunnel unspeakably cold air down here where we all live.

And it's not exactly a new phenomenon either, Despite how hashtag-friendly it is:

The vortex has probably "existed in some form for the past 4.5 billion years," according to senior scientist Jeff Kiehl of the National Center for Atmospheric Research in Boulder, Colorado.

In addition, although it was understood by scientists for several decades and referred to in meteorological literature in the 1950s, it only entered the popular lexicon as a synonym for miserably cold winter weather five years ago.

The vortex is strongest during the winter and usually weakens or even disappears in the summer. Its position can determine what part of the USA the Arctic air will invade.

It can also divide into several parts, then get back together again, like the cop in Terminator 2.

Scientists report that the polar vortex appears to be more often in recent years. Some scientists – but not all – can be a connection between global warming and the wandering vortex: The theory is that when we warmth invades the Arctic, some of the cold that's supposed to stay up there – including the infamous polar vortex – instead sloshes down south to north america and europe.

"Warm temperatures in the Arctic cause the jet stream to take these wild swings, and when it swings farther south, that causes cold air to reach farther south," said Jennifer Francis, a climate scientist at Rutgers University, who published a study on the phenomenon last year.

In any event, don't fear the polar vortex. It's not like a tornado or hurricane; it's not something you can look up and see in the sky one day; there's no freakish spinning whirlwind of ice and snow roaring down from Canada.

It's just cold … very, very cold.

Contributing: The Associated Press

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