What is a 'polar vortex' and why is it so dangerous?
January 07, 2014  04:07
As of Monday afternoon, the Midwest was battling uncommonly frigid weather that threatened to break some all-time-low temperature records. January is typically cold, but not this cold, with several Midwestern cities such as Chicago gripped by daytime temperatures well below zero.

So what's happening?

A huge mass of arctic air has slipped away from the North Pole, settled over the U.S. and made everything really, really cold, meteorologists told The Times. You may know the phenomenon by a phrase you've seen in headlines lately: the ominous-sounding "polar vortex."

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