Ice has built up along Lake Michigan in Chicago as temperatures have plunged in recent days. A dip in the polar vortex is to blame.
We've mentioned the polar vortex several times in recent days.
But we're still getting asked this question:
Just what is the polar vortex, and why is it bringing super-cold temperatures to so much of the U.S.?
So here's another go at it.
On All Things Considered, Washington Post weather editor Jason Samenow described the polar vortex this way:
"We're talking about a huge sprawling area of circulating cold air originating from the North Pole. It's a low-pressure center, and typically during the winter months it resides up there. At times, some tentacles of it will slip southward and bring cold air outbreaks into the U.S., but this year, we're seeing a huge chunk of it, most of it descending into the U.S."
High-pressure systems over Alaska and Greenland, Samenow added, are "allowing the jet stream to dive south over the U.S. and also for this polar vortex to drop south with the jet stream."
For a more visual image of what's happening, though, we suggest an analogy offered by science writer Andrew Freedman, who spoke with Morning Edition's David Greene.
"This is air that is circulating the Arctic," Freedman said. "In the last couple of days, it's sort of become lopsided — sort of like a figure skater that has extended their arms and then tripped.
"You know, when a figure skater pulls their arms in, they spin tighter and tighter and faster and faster. But when they put their arms out, they are a little bit slower and a little bit more wobbly and more prone to fall or stop skating at the end of their routine.
"What's happening now is that a piece of it is down on the other side of the globe, but a piece of it kind of got lopsided and came down on top of us."
The next logical question is why the vortex has weakened so much that a big piece of it has spun down over the U.S.
Along with the effects that those high-pressure systems over Alaska and Greenland are having, there's the possibility that climate change is also a factor.
"More and more Arctic sea ice is melting during summer months. The more ice that melts, the more the Arctic Ocean warms. The ocean radiates much of that excess heat back to the atmosphere in winter, which disrupts the polar vortex. Data taken over the past decade indicate that when a lot of Arctic sea ice disappears in the summer, the vortex has a tendency to weaken over the subsequent winter."