A curious confluence of atmospheric events has produced the largest ozone hole ever measured over the Arctic.

A powerful polar vortex has trapped especially frigid air in the atmosphere above the North Pole, allowing high-altitude clouds to form in the stratosphere, where the ozone layer also sits. Within those clouds, chlorofluorocarbons and hydrochlorofluorocarbons already high in the atmosphere — gases used as refrigerants — react with ultraviolet rays from the sun to release chlorine and bromine atoms, which in turn react with and deplete the ozone.

Such conditions are more often seen over Antarctica, leading to a more frequent and much larger ozone hole in the Southern Hemisphere (SN: 12/14/16).

The ozone layer sits in the stratosphere, an atmospheric layer between about 10 and 50 kilometers above the ground, where it protects life on Earth from UV radiation from the sun. During the Southern Hemisphere’s spring, as much as 70 percent of the ozone can disappear; in some places, the ozone concentration drops to zero.

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During the Northern Hemisphere’s spring, the Arctic ozone layer also tends to thin. But the Arctic’s average winter temperatures are typically warmer than Antarctica’s, so it’s unusual for cold masses of air to be trapped around the pole for a month or longer and give the gases time to chip away at the ozone.

In April 2011, though, Arctic ozone thinned by about 40 percent, setting a new record (SN: 10/3/11). This year’s depletion, which currently covers less than 1 million square kilometers, has already surpassed that record, say researchers with the European Space Agency, based in Paris.

In the Arctic, the sun is just now starting to peek over the horizon following the end of the Northern Hemisphere’s winter, so the thinned ozone shield does not yet represent a strong threat to human health, researchers note.