What’s a polar vortex?

Today, the National Weather Service warned residents of Bismarck, North Dakota, to expect a high of 10 degrees Fahrenheit and windchill of minus 35 degrees Fahrenheit. In those conditions, frostbite can occur in 30 minutes on exposed skin. The forecast for Bismarck on Saturday includes an overnight low of minus 22 degrees Fahrenheit. At that temperature, vodka freezes solid.

The “polar vortex” has come for the Upper Midwest. And it’s not stopping there.

Over the course of the week, a mass of freezing Arctic air will creep down from the upper Midwest, into the South and East to cover most of the Eastern United States. Across much of this region, the Washington Post reports, temperatures are expected to drop 20 to 30 degrees below normal. “Through Thursday, 75 percent of the Lower 48 will have experienced a temperature below freezing, including Texas, the Deep South and the Pacific Northwest,” the Post’s Capital Weather Gang writes.

In short: It’s going to be cold. Here’s why.

The term “polar vortex” was popularized in 2014 when a particularly brutal January cold snap sent the country into a deep freeze. Babbitt, Minnesota, reached a low of minus 37 degrees. The low of 4 degrees in New York City broke a 118-year record.

But while the name sounds dramatic, the polar vortex isn’t anything out of the ordinary or all that new.

When meteorologists talk about the “polar vortex,” they’re talking about the mass of cold, low-pressure air that consistently hovers over the Arctic. It’s called a “vortex” because it spins counter-clockwise like a hurricane does.

“It ALWAYS exists near the poles,” the National Weather Service emphasizes on its website.

Usually this mass of cold polar air remains, well, over the North Pole. That’s because the polar vortex is usually strong and stays compact and centered over the Arctic regions.

But sometimes the vortex weakens.

A weak vortex means colder weather down here in the mid-latitudes. When it’s weaker, the vortex is less stable, and lobes of it can start to creep away from the Arctic region.

The weak vortex influences the path of the jet stream, an area of fast-moving air high in the atmosphere. A weak vortex can cause the jet stream to wobble southward, bringing the cold polar air down with it.

The vortex weakens, in part, because of warming temperatures in the stratosphere, as the Weather Underground explains. (There is evidence that with climate change, these polar vortex intrusions into the more southern latitudes will become more common.) And the shape of its southward intrusion is influenced by high-pressure systems.

In 2014 “the polar vortex suddenly weakened, and a huge high-pressure system formed over Greenland,” NASA’s Jet Propulsion Lab explains on its website. “The high-pressure system blocked the escape of all that cold air in the jet stream, and allowed part of the polar vortex to break off and move southward.”

Again, this isn’t all that unusual. “Many times during winter in the northern hemisphere, the polar vortex will expand, sending cold air southward with the jet stream,” the NWS explains. “This occurs fairly regularly during wintertime.” The 2014 polar vortex was notable because it recurred throughout the winter months and into early spring, setting record low temperatures through March.

It’s important to note that the polar vortex isn’t a storm system in itself. The vortex exists in the stratosphere (around 10-plus miles up), well above the troposphere where we experience our weather. Storms occur much lower in the troposphere. So it isn’t necessarily the case that the vortex will bring snowstorms — just the cold air. So bundle up.