Bitter cold is expected across much of the U.S. this weekend. But what do headlines mean when they say a "polar vortex cold snap" is on the way?
The polar vortex is a regular feature of Earth's polar latitudes. It is a concentration of cold air and low pressure that is usually kept relatively close to the north and south poles.
It gets the "vortex" part of its name from the swirl of counter-clockwise air that extends as much as 30 miles up into the atmosphere.
The polar vortex does not, by itself, cause the severe cold spells we might see in a weather forecast.
But the polar vortex interacts closely with the jet stream, which is a shallower flow of winds that ranges from five to nine miles high.
When the polar vortex is stable, it encourages the winds of the jet stream to stay stable — and keeps cold air wrapped up tight in the Arctic.
But when the Arctic polar vortex goes through a weaker cycle, the jet stream may also weaken and meander. This can cause the jet stream to sweep further south and bring cold air from the Arctic with it. That's what sometimes leads to unusually cold temperatures in the mid-latitudes — such as the continental U.S.
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The exact interactions between the polar vortex and the polar jet stream aren't fully understood. Experts aren't sure why the polar vortex sometimes weakens, or what effects global warming may have on the balance of these air currents.
Some scientists who study the atmosphere point out we don't have a long record of data to go on.
"We’ve only been observing [the stratosphere] directly since the 1950s," said NOAA stratosphere expert Amy Butler. "That’s not very long to understand what kind of natural variability the polar vortex might be capable of."
And models deliver conflicting results about the future of the polar vortex, too, depending on variables like ocean temperatures and sea ice levels.
One thing is more certain: Unusual cold brought down from the Arctic will probably keep chilling us as these natural cycles continue. The National Weather Service recommends keeping an eye on winter forecasts and staying prepared for seasonal cold temperatures.
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