NASA explains the polar vortex we have recently lived through. The purple stuff is the coldest, in case you didn't catch that drift. Eric Fetzer provides some information in a visual explanation from NASA's Jet Propulsion Laboratory. What he does not mention in this 'movie,' as he describes it, is that brutal cold-snap seemed pretty harsh, but January 2014 ended on an almost exact average temperature for the continental United States. 

[Slate]