Polar vortex: What is it, and when is it going away?

http://usnews.nbcnews.com/_news/2014/01/06/22201007-polar-vortex-what-is-it-and-when-is-it-going-away?lite

By Erik Ortiz
Jan. 6, 2014

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Twenty-six states are under wind-chill warnings or watches, and the polar vortex is to blame, breaking loose from its North Pole confines and plunging the country in a deep, dangerous freeze.

"Think of this as a polar hurricane," said NBC News' Al Roker.

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Basically an arctic cyclone, it ordinarily spins counterclockwise around the north and south poles.

While it tends to dip over northeastern Canada, it’s catching everyone’s attention because it has moved southward over such a large population — as many as 140 million Americans are feeling the freeze.

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There’s a variety of reasons why a chunk of cold air over Canada would break off our way.

Chiefly, warmer air builds up over areas such as Greenland or Alaska, and that air forces the colder, denser air southward.

Also, weather patterns can create the right conditions for the polar vortex to point south.

But in this case, “this very well just may be one of those anomalies where it forces itself southward,” Giannasca said.

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Through the course of a winter, the arctic air can get displaced southward, typically into the eastern U.S. But it is uncommon for such cold air to cover such a large part of the country, happening maybe once a decade or longer.

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