If you’ve turned on the news or stepped outside lately, you’re familiar with the frigid weather that is the polar vortex. (You can see the scope of the vortex in the image above, captured by NOAA’s GOES-East satellite on January 6 as it was entering the U.S.) But just what is this weather phenomenon?

Polar vortexes are basically arctic hurricanes or cyclones. NASA defines them as “a whirling and persistent large area of low pressure, found typically over both North and South poles.” A winter phenomenon, vortexes develop as the sun sets over the pole and temperatures cool, and occur in the middle and upper troposphere and the stratosphere (roughly, between six to 31 miles above the Earth’s surface). In the Northern hemisphere, the vortexes move in a counterclockwise direction. Typically, they dip down over Canada, but according to NBC News, polar vortexes can move into the contiguous U.S. due to warm weather over Greenland or Alaska—which forces denser cold air south—or other weather patterns. But “this very well just may be one of those anomalies where it forces itself southward,” said Frank Giannasca, senior meteorologist at The Weather Channel.

Polar vortexes aren’t rare—in fact, arctic winds do sometimes dip down into the eastern U.S.—but the sheer size of the area affected is. This happens only once in a decade or even longer, according to NBC.

How Cold Is It?

So cold that even the polar bear at Chicago’s Lincoln Park Zoo is being kept indoors (she doesn’t eat seals, so she never developed the layer of blubber her wild brethren use to survive that kind of chill). It’s so cold that Memphis, Tennessee is 20 degrees colder than Anchorage, Alaska. Other areas of North America will experience temperatures of -50 or -60 degrees Fahrenheit, which some have compared to the temperature on Mars (though Boing Boing points out that it's a little more complicated than that).

But thankfully, it’s not going to stick around for long: NASA predicts that the vortex will head back to the pole toward the end of the week.