Above the Arctic hovers a counter-clockwise spinning mass of cold air that grows and shrinks with the changing seasons—the polar vortex.

When this vortex dips toward the equator, it brings with it blasts of freezing air.  

The record-breaking cold front heading into the continental U.S. is the result of an unusually far south polar vortex says Bob Ovarec, a lead weather forecaster with the National Weather Service. 

"As that system is reforming to the south, it’s helping push all the cold air south and eastward, so a lot of parts of the country are going to see some dramatic temperature changes," says Ovarec.

In extreme cold, frostbite can attack exposed skin in minutes, and prolonged exposure leads to hypothermia. At least 21 people