The polar vortex swept parts of the US Midwest and East Coast this week, forcing millions of Americans to contend with subzero temperatures.

The polar vortex is an area of circulating cold air that rings the planet’s North and South poles. Sometimes that vortex destabilizes, sending surges of Arctic air south.

Windchill temperatures were as cold as minus 60 degrees in Minnesota and minus 45 in Chicago Thursday, far below average for this time of year.

So far, 21 deaths have been linked to the cold snap.

If Arctic ice continues to melt, polar-vortex events could get more frequent.

For the first time since 2014, the polar vortex descended on North America.

More than 84 million Americans in the Midwest and parts of New England experienced subzero temperatures, CNN reported. Wind-chill advisories in the Midwest and Northeast impacted 140 million Americans.

So far, 21 deaths have been linked to the recent weather, according to Reuters. Fatalities have been reported in Illinois, Indiana, Iowa, Wisconsin, Ohio, New York, Minnesota, and Michigan.

Windchill temperatures were as cold as minus 50 degrees Fahrenheit in parts of Minnesota and Wisconsin on Thursday. Some areas in Minnesota and the Dakotas faced temperatures 50 degrees below average for this time of year. Des Moines, Iowa saw a record-breaking low of minus 20 degrees Fahrenheit yesterday, according to the BBC, while Rockford, Illinois also set a record at minus 31 degrees Fahrenheit.

The term polar vortex describes the mass of low-pressure cold air that circulates in the stratosphere above the Arctic and Antarctic regions. Sometimes the circulation of the polar vortex weakens during the winter, causing surges of frigid air to splinter off and drift south.

The freezing air is carried by the jet stream, a current of wind that extends around the hemisphere and divides the air masses in the polar region from those farther south.

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Foto: How the jet stream impacts the polar vortex.sourceBI Graphics/NOAA

This polar air mass has always been present, but scientists first dubbed it the “polar vortex” in 2014, when a similar (though less severe) cold snap hit a majority of the continental US.

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This story has been updated to reflect the latest numbers.

Kevin Loria contributed to a past version of this story.