How a winter 'wobble' brought freezing North Pole temperatures to the U.S.
JUDY WOODRUFF: We have heard the term polar vortex quite a bit during this deep freeze.
And here to help us understand the science behind it, I'm joined by Andrew Freedman. He's senior science writer at Climate Central, a news and research organization.
Andrew Freedman, thank you for being with us.
So, tell us, what is the polar vortex?
ANDREW FREEDMAN, Climate Central: So, the polar vortex actually is not really a new meteorological phenomenon.
It's just the national attention has been focused on it in the past couple of days. It's — you can think of it as this mass of very cold air that usually resides near the North Pole, that usually resides in the Arctic, and is surrounded by fast upper-level winds.
JUDY WOODRUFF: And so — so it usually sits there, but what happened? What caused it to move?
ANDREW FREEDMAN: Yes, so that's what is especially interesting.
So, usually these fast-moving upper-level winds kind of trap that air up there and keep it there, and that's relatively good for people down in the continental U.S. because it spares us from some of the brutal cold. But, occasionally, what can happen, those winds weaken, and it's sort of like when you have a spinning top. When it's spinning very quickly, it's pretty stable.
When it starts to slow down, it meanders and wobbles. And what we had was a wobble in the polar vortex that kind of pinched off this large chunk of it and slid it south with the aid of other weather systems and the jet stream that we're more familiar with.
JUDY WOODRUFF: So at the risk of getting really in the weeds here, what caused it to slow down, to start this wobble?
ANDREW FREEDMAN: Well, some of it is dynamics that are going on in the very high atmosphere, in the stratosphere. There's often waves that kind of go through the atmosphere from low to high and from high to low and that can help cause some of these wobbles.
There's also, you know, areas of high pressure called blocking highs that sort of act like stop signs in the atmosphere, preventing storms from kind of going through them. And we had a series of blocking highs over the past week or so over the North Pacific especially. They kind of rerouted the traffic in the atmosphere.
JUDY WOODRUFF: So, and is that what caused it to come so far south this time?
ANDREW FREEDMAN: Yes, that's pretty much an explanation of why the polar vortex, why such a big chunk of it came this far south.
But keep in mind, we haven't seen anything like this in a long time, in 20 to 30 years. A lot of the records are being broken from those periods. But we have seen this before. This is not an all-time cold event. We have had many — many worse outbreaks.
JUDY WOODRUFF: Any way to predict how often we're going to it happen in the future, when we're going to see the next one come?
ANDREW FREEDMAN: Well, we can predict it in advance in terms of a week or two perhaps.
But, you know, what we're sort of guaranteed is that we're going to continue to have cold outbreaks throughout the winter, but long-term climate change means that a lot of the cold outbreaks that we experience may not be quite as cold as they used to be. We're seeing less frequent extremely cold temperatures in many cold places.
Minneapolis has had much fewer days below about minus-10 degrees Fahrenheit, fewer nighttime lows that low, in the past decade vs. the 1970s, for example. So the long-term trend is up, but we have got these natural variations in weather patterns that we try to keep close track of.
JUDY WOODRUFF: And it sure does catch everybody's attention.
Andrew Freedman of Climate Central, we thank you.
ANDREW FREEDMAN: Thank you.