The Chicago skyline overlooks steam rising from Lake Michigan on a day when the temperature was -18 F and -43 F with the windchill. The polar vortex was blamed for the very cold temperatures.

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When the winter temperature abruptly plunges to a brutally cold low, your car is buried under ice-encrusted snow, and you're shivering despite wearing thermal underwear, a wool sweater and a down parka, it probably doesn't help much to hear the TV meteorologist cheerfully explain that the cause of your misery is a distortion of something called the polar vortex. That sublimely ominous term might sound to you like the name of a giant monster with freezing-cold breath, the sort you'd see battling Godzilla in a 1950s Japanese sci-fi movie. You'd say, "Huh?" if only your teeth would stop rattling.

Indeed, many people heard of the polar vortex for the first time in January 2014, when temperatures plunged to life-threatening lows -- in some places, 60 degrees Fahrenheit below zero (minus 51 degrees Celsius) when windchill was factored in -- across the Midwest and Northeast U.S. Conditions were so dire that even in hardy Minnesota, the state's governor closed the schools to protect children from exposure to the cold [source: O'Brien and Palmer].

But while the term wasn't familiar to most of us, the polar vortex really isn't anything new to scientists. The polar vortex actually is a seasonal atmospheric phenomenon, a system of strong, high-level winds -- called the jet stream -- surrounding an extremely cold pocket of Arctic air. And while it might sound like a malevolent force of nature, the polar vortex for the most part is a pretty good thing, because its winds usually form a boundary that keeps the cold air contained and prevents us from freezing [source: Duke].

The problem, though, is that once in a while, the polar vortex breaks down a little. The result is a big, powerful blast of Arctic air that can travel far south, causing the temperature to plunge in places accustomed to having mild winters [source: Duke]. In this article, we'll explain more about what the polar vortex is, and why it sometimes fluctuates.