AILSA CHANG, HOST:
Thunder sleet, single-digit temperatures in Texas, record-setting deep freezes - much of the country is witnessing some bizarre and historic winter weather, but why? Well, it has to do in part with something called the polar vortex, a mass of cold air spinning over the Arctic and up into the stratosphere. It strengthens in the fall and normally stays stable over the North Pole, keeping cold air penned in over the Arctic. Dr. Michael Ventrice, a meteorological scientist at IBM, explains it like this.
MICHAEL VENTRICE: The polar vortex wants to sit over the North Pole. It feels like it's comfortable there. That's equilibrium.
CHANG: However...
VENTRICE: It can make weather very interesting when the polar vortex up in the stratosphere gets disrupted.
CHANG: He says that's what happened last month when temperatures at high altitudes above the North Pole spiked. Ventrice and other meteorologists were fascinated by that unusually intense sudden stratospheric warming event as it's called and by what might happen as cold air got pushed south into North America and Europe by an erratic jet stream.
VENTRICE: We can actually see blocking or high pressure setting up over the North Pole, essentially anomalous warmth over the Arctic region. And that's because all the Arctic air is getting squeezed out of the North Pole, and it's getting pushed down into the mid-latitudes where we live.
CHANG: But figuring out exactly how that push of cold air translates into regional winter storms is a challenge.
VENTRICE: Essentially, the only type of predictability we have at this point is understanding these windows of opportunity out the future, where there's going to be more cold air coming from the polar north, but we're not sure exactly how that cold air mass is going to, you know, come down across the northern arc, which kind of makes these predictions of snowstorms at that synoptic level a bit more difficult.
CHANG: He said one of the most stunning impacts he's seen was single-digit temperatures in Texas.
VENTRICE: It's just one of the most remarkable things I've seen in my lifetime. And I'm not sure if I'll ever see it again.
CHANG: Although he did point out that meteorologists aren't perfect at predicting (laughter) the future. Transcript provided by NPR, Copyright NPR.