What Is a Polar Vortex and Why Is It Making It So Cold Right Now?

Ice floes fill the Hudson River as the New Jersey waterfront is seen during sunset in New York City.
Ice floes fill the Hudson River in January, 2014—the cold snap in which the American public first became familiar with the term “polar vortex.” Afton Almaraz/Getty Images

As a frigid cold snap descends on the Midwest, threatening residents with temperatures as low as 40 degrees below zero—in some places, the low will be the coldest seen in decades—experts are warning that the conditions could turn deadly.

The cold, which will be most intense Tuesday night through Thursday, will spread to the East Coast and even seep into the South, where residents are less accustomed to dealing with freezing temperatures. By the end of the week, more than 87 million people are expected to have experienced temperatures at or below zero degrees.

For some in the Midwest, it will be the coldest temperatures they have ever experienced. Many will be at risk of frostbite (at minus-20 degrees it can occur in as little as half an hour) and hypothermia, and some cities could see wind gusts that lead to whiteout conditions on the roads. Wisconsin and Michigan have declared states of emergency.

Much of this can be blamed on the polar vortex, a large current of frigid air that, because of the Earth’s rotation, circulates, counterclockwise, in the upper atmosphere around the Arctic Circle. The polar jet stream—the strong winds that push the air—usually keeps it on a tight path, but every once in a while, the spin of the vortex kinks, the jet stream weakens, and a massive chunk of the vortex sags and swoops downward, blasting regions of the Northern hemisphere with Arctic air.

And then, there are cases when that distortion can become more extreme. As Eric Holthaus wrote for Slate in 2014:

Occasionally—about once every other year or so—the perturbation is so pronounced that the vortex breaks down completely. Scientists call these “sudden stratospheric warming” events, when a continent-sized chunk of rapidly sinking air (usually over Eurasia) quickly heats up and disrupts the polar circulation, sometimes completely disintegrating it. Statistically, that’s when the cold air floodgates can really open as the jet stream scrambles to contain the chaotic swirls of polar air that descend southward in the aftermath.

The result … isn’t necessarily a one-to-one link from the stratospheric disruption to a blast of cold at the surface, but more of a tilt in the odds favoring extremely cold weather. 

It’s a miserable scenario, but according to some meteorologists, the term “polar vortex” gets bandied about too much, even when it is technically involved in the punch of cold a region is facing. That’s because sometimes the cold air that blasts a region is not pushed down from the polar vortex itself but instead pulled down from the nearby air chilled by it.

In this case, we are looking at a true displacement of the polar vortex. According to the Associated Press, an unusually warm blast from the south dramatically heated the air 20 miles above the North Pole last month. That sudden stratospheric warming split the polar vortex, and it is one of the pieces that has calved from that warming that now bears down on the northern part of the country.

According to the AP, some scientists believe Americans should expect to see these types of cold snaps more often—a warming climate, they say, may mean more hot air to bust up the vortex, causing that cold air to swoop south and again make conditions dangerously cold.