A polar vortex dipping down from Siberia is bringing a cold front with frigid temperatures to millions Americans. See maps of the arctic blast.

With the Northern Hemisphere well into the winter months, it may seem like temperatures are cold enough already. However, a deep freeze, thanks to the polar vortex dipping down from Siberia ...

Outbreaks from the Arctic will persist throughout the Northern Hemisphere in the upcoming decades ... can trigger an outbreak ...

But what is it? Definition: The polar vortex is a flow of wind aloft that is always present above the cold, North and South Pole areas. In the Northern Hemisphere this results in a counter ...