A polar vortex is blasting cold air across the northern half of the U.S. this week — and by Christmas, it’ll be warm enough to barbecue in Washington.

The National Weather Service warned residents of Bismarck, North Dakota, to expect a high of 2 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.

What is the Polar Vortex?

Simply, the Polar Vortex is a large area of cold air and low pressure that sprawls across Earth’s poles. The scary-sounding “vortex” monicker comes from the fact that it’s cyclonic: It rotates (counter-clockwise in the Northern Hemisphere) in a flow of air that keeps cooler air near the poles.

It’s also fairly high up in the lower/middle stratosphere, but its effects stretch down into the upper part of the troposphere, the first layer of Earth’s atmosphere where our planet’s weather takes place.

The polar vortex is really dependent on large-scale temperature variations in Earth’s atmosphere between the poles and the equator as the year progresses. As such, the polar vortex is confined to polar latitudes in summertime, but “dips” in the winter as the northern hemisphere’s average temperatures drop, bringing colder air southward.

It’s also not even slightly a new phenomenon. Though its extent ebbs and flows, the vortex has always existed over the north pole, and counterparts have even been observed on other planets in our solar system.

Though the cold it brings can factor into winter weather, for the most part the polar vortex is not in itself a weather maker, being quite high in the atmosphere. Winter weather is more shaped by the position of the jet stream, along which low pressure systems move.

In practice, the northern jet stream is the boundary between cooler air from the north and warmer air from the south, though the jet stream is not as high in the atmosphere as the polar vortex.

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