A record-breaking cold wave has swept through the US Midwest, with 22 states hitting sub-zero temperatures. Among cities, Chicago dropped to a low of -30°C, slightly above the city’s lowest-ever reading of -32°C from January 1985. Minneapolis recorded 32°C and Sioux Falls (South Dakota) -31°C.
The extreme cold has been caused by a blast of Arctic air, which in turn is a result of what is known as a “polar vortex” event.
What is a polar vortex?
Essentially a low-pressure area, it is a wide expanse of swirling cold air surrounding both polar regions. The counter-clockwise flow of air helps keep the colder air near the poles. “Polar vortexes are not something new. The term ‘polar vortex’ has only recently been popularised, bringing attention to a weather feature that has always been present… However, when we feel extremely cold air from the Arctic regions at Earth’s surface, it is sometimes associated with the polar vortex,” the US National Oceanic and Atmospheric Administration (NOAA) explains on its website.
So, when does the polar vortex cause extreme cold?
In winter, the polar vortex sometimes becomes less stable and expands. “Many times during winter in the northern hemisphere, the [north] polar vortex will expand, sending cold air southward with the jet stream,” NOAA explains. This is called a polar vortex event, defined by NOAA’s SciJinks as the “breaking off” of a part of the vortex. “Normally, when the vortex is strong and healthy, it helps keep a current of air known as the jet stream traveling around the globe in a pretty circular path. This current keeps the cold air up north and the warm air down south,” NOAA says. “But without that strong low-pressure system, the jet stream doesn’t have much to keep it in line. It becomes wavy and rambling. Put a couple of areas of high-pressure systems in its way, and all of a sudden you have a river of cold air being pushed down south along with the rest of the polar vortex system.”
Is all cold weather the result of a polar vortex event?
No. Though the polar vortex is always “hanging out” up North, it takes pretty “unusual conditions” for it to “weaken” for it to migrate far south, NOAA explains. It is not confined to the US either. “Portions of Europe and Asia also experience cold surges connected to the polar vortex. By itself, the only danger to humans is the magnitude of how cold temperatures will get when the polar vortex expands, sending Arctic air southward into areas that are not typically that cold,” it states.