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You're probably familiar with the infamous ozone hole over Antarctica,Watch Flesh Clerk Who Shoves You Up And Down Online caused by damaging and now-illegal chemicals.

But during March and April this year there was a notable zone of depleted ozone — which protects life from the sun's ultraviolet radiation — over the Arctic, too. It closed last week, though it wasn't nearly as robust as the annual Antarctic ozone hole, and it certainly wasn't a big health threat to humanity. (Though it’s always wise to diligently use sun protection, regardless of the ozone layer's condition.)

"It’s unusual but not unexpected," Paul Newman, the chief scientist in the Earth Sciences Division at NASA's Goddard Space Flight Center, said of the recent Arctic ozone hole.

"It’s unusual in that we only have events like this about once per decade," Newman added.

There are two other years on record, 1997 and 2011, when there were similar ozone depletions over the Arctic, explained Antje Innes, a senior scientist at the European Union's Copernicus Atmosphere Monitoring Service. But recent measurements suggest the ozone levels were even lower this year, she said.

Importantly, these recent ozone numbers aren't nearly as low as they are in Antarctica's slowly healing, big ozone hole, which opens up in August through October each year.

"These two are really different animals," said NASA's Newman. "This [Arctic ozone hole] is not comparable to the Antarctic ozone hole."

"If this was happening over the Antarctic we would be shouting for joy," he added, referencing the still significantly lower ozone numbers over Antarctica caused by decades of releasing ozone-depleting gases, like chlorofluorocarbons, or CFCs, into the atmosphere. (Fortunately, scientists discovered the Antarctic ozone hole in 1985, and CFCs were soon banned.)

For reference, on Oct. 12, 2018, ozone levels plummeted to 104 Dobson units (the measurement of ozone in the atmosphere) over Antarctica versus 205 Dobson units over the Arctic on Mar. 12, 2020. (Typically, ozone levels don't drop below 240 Dobson units in the Arctic during March.)

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Why did the Arctic ozone hole form this year?

A major reason for the 2020 Arctic ozone hole is the polar vortex — a whirling jet of air that forms over the Arctic (and the Antarctic) during winter — was "incredibly strong and persistent" this year, said Innes.

So frigid Arctic air stayed mostly locked in the Arctic, rather than wobbling around and spilling notoriously cold air into places like the U.S. This stable, profoundly cold air (below -78 degrees Celsius, or -108 degrees Fahrenheit) allowed lofty clouds to form high up in the atmosphere where ozone lives, in a place called the stratosphere. These clouds create an ideal environment for the human-made chemical CFCs to react with sunlight, which then makes chlorine, the chemical that ultimately destroys ozone.

(In contrast, these high stratospheric clouds frequently form over Antarctica, which allow CFCs to regularly deplete the ozone there, notably in September and October.)

What's more, this robust polar vortex prevented ozone-rich air from other regions from washing into the Arctic, which kept ozone levels depleted. "There was little mixing this year," explained Newman. He likened the atmospheric situation to dropping a dollop of red paint into a bucket of white paint. The red paint, signifying the air in the polar vortex, will stay put unless it's mixed up. "It will just sit there unless the atmosphere is stirring," he said.

But, last week the polar vortex split apart, explained Innes. This allowed ozone-rich air from the mid-latitudes to flow into the Arctic, mostly putting an end to the region's depleted ozone levels. The Arctic ozone hole is essentially no more.

Importantly, thanks to a strong international climate treaty (the Montreal Protocol) that banned ozone-damaging CFCs in the late 1980s, this recent Arctic ozone hole wasn't nearly as bad as it could have been.

"Without the Montreal Protocol, it would have been much, much worse," said Newman, noting the atmosphere would have inevitably been saturated with more ozone-killing chemicals.

As this northern ozone hole vanishes, one big question still remains: Why was the polar vortex so robust this year?

"Why the polar vortex was so strong and lasted so long this year we don't know yet," said Innes. "Scientists will surely research what triggered this."