From November 2013 through January 2014, the jet stream took on a remarkably extreme and persistent shape over North America and Europe. This global river of eastward-flowing winds high in the atmosphere dipped farther south than usual across the eastern U.S., allowing the notorious “polar vortex” of frigid air swirling over the Arctic to plunge southward, putting the eastern two thirds of the country into a deep freeze. Ice cover on the Great Lakes reached its second-greatest extent on record, and two crippling snow-and-ice storms shut down Atlanta for multiple days.