A 'polar vortex' is creating record subzero temperatures in the US – but why?

Some states are seeing temperatures colder than the Arctic.

By Sarah Gibbens
Published 31 Jan 2019, 09:39 GMT
A person walks along Chicago's lakeshore on Wednesday where Arctic air from the polar vortex has ...
A person walks along Chicago's lakeshore on Wednesday where Arctic air from the polar vortex has led to -20 degree Fahrenheit temperatures.
Photograph by Kiichiro Sato, Ap

If reading winter forecasts doesn't send a chill down your spine, stepping outside will. Last night the UK hit low for this winter, with Braemar in Aberdeenshire plunging to -11 C. But if you think it's cold in the UK right now, think again. The United States has it much worse.

This week Chicago's temperature has hit -31C with some regions of the United States experiencing a mind-blowing wind chill of -59 C  thanks to something called the polar vortex. Accuweather predicts the Midwestern states will experience the extreme temperatures until later this week and cautions against stepping outside with any exposed skin. In these record-shattering temperatures, frostbite can occur after only a few minutes.

A blast of Arctic air

What is the polar vortex, the doomsday-sounding weather pattern blamed for these frigid conditions?

The term refers to a swirling mass of Arctic air that hovers around the north pole all year. Its swirling eddies spin counter clockwise, and during winter months in the Northern Hemisphere, these eddies grow longer and dip farther south.

When the vortex grows, these cold swirls of air are transported south by the polar jet stream (also called the polar front). Moving from west to east, the polar jet stream hovers farther north in the summer and farther south in the winter, altered by changing seasonal sunlight. Along with its subtropical jet stream counterpart in the south, these two fronts play a determining role in seasonal weather changes.

Intermittent blasts of cold polar vortex air happen when the vortex becomes less stable, circulating in waves rather than a tight circle. According to the National Oceanic and Atmospheric Administration, denser cold air in the north and warm air currents moving south can all cause the vortex to become unstable. The more unstable the vortex becomes, the more likely it is that parts of North America, Europe, and Asia feel stray blasts of Arctic weather.

Will climate change make the vortex more unstable?

Scientists are only recently beginning to understand how a climate changed by warming temperatures may lead to colder winters.

One study published last March in the journal Nature Communications found a link between warmer Arctic air and colder U.S. winters, particularly in the northeastern part of the country.

In a press release about the research, study author Jennifer Francis said, “Warm temperatures in the Arctic cause the jet stream to take these wild swings, and when it swings farther south, that causes cold air to reach farther south. These swings tend to hang around for awhile, so the weather we have in the eastern United States, whether it’s cold or warm, tends to stay with us longer.”

One theory for why this might occur focuses on the stability of jet streams. These westerly winds are propelled by the difference between cold air in the north and warm air in the tropics. Without this strong difference, jet streams could become weaker, concluded a paper published last October.

Though global temperatures are rising, climate change could lead to more erratic, extreme weather conditions. In parts of the Midwest and Northeast experiencing these temperature drops, the risk of hypothermia makes an unstable vortex potentially deadly.

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