What is the polar vortex that has brought bitter cold to the southern U.S.?

The polar vortex is bringing sub-zero temperatures to some of the southernmost parts of the US, with snowfall expected in parts of Texas and Oklahoma, while communities in Louisiana and Florida could face bitter cold. The polar vortex of extremely cold air usually circles around the North Pole, but it sometimes plunges south into the US, Europe and Asia, causing drastic changes in weather across…