Much of the country had to bundle up this week owing to some unusually cold weather. Even in the Deep South, residents struggled with temperatures in the low 20s.
With the big chill comes the revival of an ominous phrase: "the polar vortex."
The sinister-sounding label has been hard to escape on TV news. The Today Show warned of the vortex in its promo spots. Some cautioned that the phenomenon might already put the squeeze on holiday shopping.
Even The Tonight Show's Jimmy Fallon poked fun at the hype.
"It's a phenomenon that signals the return of colder temperatures across North America," Fallon said. "... Or as it used to be called, the month of November."
Fallon's joke makes a point that Jeff Masters, chief meteorologist at Weather Underground, wants people understand.
"This is just a regular old cold front," Masters says. "The polar vortex has been around forever. It's just the media happened to notice it last year, and it's really not a very scientifically accurate thing to talk about."
He says the recent popularity of the phrase is misleading. The polar vortex is a constant flow of arctic air circling in the upper atmosphere above the North and South poles. The cold is usually corralled up there — but sometimes little bits of the arctic air escape.
"It's just the ordinary sort of weather you expect in winter," Masters says. "Every now and then you get a big trough of low pressure. It dips down from the pole and it allows arctic air to seep southwards."
That's not to say the polar vortex wasn't involved in this bout of unseasonably cold weather. Masters says Typhoon Nuri, which hit Alaska last week, pushed one of those troughs of arctic air south across the Eastern U.S.
Such temperature shifts serve a purpose, says Steven Nelson of the National Weather Service.
"These cold intrusions, cold fronts, are really restoring the balance in the temperature and moisture across the Earth's surface," he says.
A good chunk of the South is expected to get another wave of unseasonably cold weather starting Monday. Just don't call it the polar vortex.
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