A Closer Look at the Polar Vortex’s Dangerously Cold Winds

Temperatures will plummet to minus 26 degrees Fahrenheit in Chicago and minus 31 degrees in Madison, Wis., on Wednesday night — the worst cold to grip the Upper Midwest in a generation. Parts of Alaska, on the other hand, are unseasonably warm.

What is causing such extreme weather across the United States? The polar vortex, an ominous-sounding system of wind and cold air over the North Pole, broke apart this month. Here’s how it’s bringing dangerous cold nationwide.