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.

The Vortex Forms ... The polar vortex spins to life every winter 10 miles above the ground, seen here on Dec. 22. Typically, it has a single center, surrounded by a wall of powerful winds. But sometimes it weakens — shifting, and even breaking into pieces. … Splits Apart … On Jan. 3, 2019, the polar vortex split into parts. It's a pattern that is happening more frequently and is often followed by severe cold and winter storms, said Judah Cohen, a climatologist at Atmospheric and Environmental Research, a weather risk-assessment firm. … Then the Cold Arrives The split takes two weeks to affect the weather: It warps the polar jet stream, bringing freezing arctic air south and warm air north. Parts of the Midwest were expected on Tuesday night to have a wind chill of minus 60 degrees.

Dr. Cohen said extreme weather could continue for six to eight weeks after the split.

The effects of the polar vortex could become more frequent and severe. Scientists looking at links between climate change and the polar vortex believe that the rapidly warming Arctic could bring about more intense periods of cold snaps and storms, even as winters become shorter and warmer.