Desperately cold weather is now gripping the Midwest and Northern Plains of the United States, as well as interior Canada. The culprit is a familiar one: the polar vortex.

A large area of low pressure and extremely cold air usually swirls over the Arctic, with strong counter-clockwise winds that trap the cold around the Pole. But disturbances in the jet stream and the intrusion of warmer mid-latitude air masses can disturb this polar vortex and make it unstable, sending Arctic air south into middle latitudes.

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