US polar vortex: why is it so cold?

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At least 20 deaths have been reported as icy cold winds caused by a polar vortex continue to batter the US.

The victims include “an elderly Illinois man who was found several hours after he fell trying to get into his home and a University of Iowa student found behind an academic hall several hours before dawn” on Wednesday, says Fox News.

In Chicago, temperatures have plummeted to -30C, forcing schools and businesses to close, as well as flight cancellations. And in Park Rapids, Minnesota, the mercury has dropped as low as -41C.

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President Donald Trump used the cold snap to mock the concept of global warming.

But scientists were quick to debunk his methodology.

So what is a polar vortex and why is it making the US so cold?

What is a polar vortex?

A polar vortex is “a large area of low pressure and cold air surrounding both of the Earth’s poles”, according to the US National Oceanic and Atmospheric Administration (NOAA).

The actual vortex is the counterclockwise flow of air that helps keep the colder air close to the poles. During winter in the northern hemisphere, the polar vortex repeatedly expands, sending cold air southward with the jet stream, bands of fast-moving air.

Warm air coming up from below then causes the jet stream to weaken and buckle, sending warmer air into Alaska and pushing cold winds down into the US Midwest and East Coast.

Is this current cold snap unusual?

Events of similar magnitude have occurred in the past. The last one “was back in 2004, and another one in the mid-1990s, and another one in the mid-1980s”, Greg Carbin, in charge of NOAA’s Weather Prediction Center Forecast Operations Branch, told news site NPR. “Every few decades we see these events occur, where that arctic air that normally resides at the pole starts to make its way south into parts of the US.”

But what is unusual is “the rapid swing from very cold to very warm”, Carbin added. “We’re going to see 60-degree changes in temperature across the Chicago area in about four days.”

Is climate change to blame?

Studies have “pointed to a recent increase in instances where the polar vortex has bulged down into heavily populated areas”, says The Guardian. Scientists are “gaining a better understanding of why this is happening, with many identifying climate change as an influence”, the newspaper continues.

“We aren’t entirely there yet but there’s more and more support for this concept,” said Jennifer Francis, senior scientist at the Massachusetts-based Woods Hole Research Center.

But “linking [this rise] directly to climate change is a very difficult task, and usually we wait until after these events occur to try to come up with attributions for these events”, adds NOAA’s Carbin.