The Heat Is Online

The polar vortex: an explainer

Feeling a Chill? Blame the Polar Vortex. And Global Warming.

The New York Times, Dec. 15, 2016

On Thursday, temperatures on the East Coast are expected to plummet, and some people — fellow journalists and weather broadcasters, we’re looking at you — may start talking about a “polar vortex.”

We thought you might want to know what the polar vortex is, and what it’s not.

(And we wanted to pre-empt the inevitable chatter about climate change that usually crops up when the thermometer drops — “It’s bone-shakingly cold, how could the Earth be warming?” We’ll tell you how.)

First, the polar vortex always exists. That catchy, extreme-sounding phrase is another term for the polar jet streams, which are caused by low-pressure and cold air, encircling both poles. They swirl from west to east, centered around the poles.

When a dip in the polar vortex comes to our part of the world, it’s usually the result of a change in pressure, which disturbs the swirl, and can push the frosty Arctic air south, according to Faye Barthold, a meteorologist with the National Weather Service. And the Arctic jet stream (or polar vortex) can sometimes dip far enough south that it allows the cold air to travel down to places that do not normally have Arctic conditions, like wherever you live.

When there is this dip in the jet stream that brings cold to the East, there’s usually a countervailing loop that takes warm air into Alaska or the Arctic. In the last few years, a frozen East has often coincided with a warm Alaska.

But this phenomenon is not exclusive to North America — the polar vortex can freeze Europe and Asia, too.

When the Arctic jet stream moves back north, it sort of forces the cold air to travel back that way, too, Ms. Barthold said, and that is why the frigid temperatures brought by the polar vortex are usually short-lived.

But for it to bring record-setting cold temperatures, it usually has to come to us straight from the Arctic, as opposed to dipping somewhere else in the country, and then moving to wherever you are. That’s because as the cold air stays at lower latitudes, it warms up, gaining heat from adjacent warmer air.

These super cold temperatures may generally be brought our way by the polar vortex, but cold winter weather is not always a result of Arctic air moving right into our midst.

When these cold snaps come, you may hear other people asking,” If global warming is supposed to be warming the globe, then why is it so cold?”

Well, for starters, there is a difference between weather and climate. Climate refers to the long-term averages and trends in atmospheric conditions over large areas, while weather deals with short-term variations, which is what happens when the polar vortex visits your hometown.

And of course, an Arctic blast can still occur in a warmer world. The air that comes down from the North Pole might not be as cold, Ms. Barthold said, but it would still be the product of the same phenomenon.

Some studies suggest that climate change could actually make these frigid waves of Arctic air more common, a result of shrinking sea ice. However, other scientists remain skeptical of this theory.

And the earth is definitely warming: Temperature records show that, by the end of last year, the earth’s surface had warmed by about 1.8 degrees Fahrenheit since the 19th century. But even though the earth’s surface is warming, scientists say that winter will still exist.

And even if parts of the United States are experiencing unusually cold temperatures, it represents such a small portion of the earth’s surface — about 2 percent — that it does not mean much in terms of average global temperatures.

So, if, for instance, a senator (perhaps James M. Inhofe, Republican of Oklahoma) brandishes a snowball on the floor of the Senate to dispute the validity of climate science when a chill wind blows through Washington, you will know that the unseasonably cold temperatures he is talking about do not mean that global warming is not happening.

It is.

http://www.nytimes.com/2016/12/15/science/feeling-a-chill-blame-the-polar-vortex-and-global-warming.html