Ozone hole

The hole in Earth’s protective stratospheric ozone layer above Antarctica was the smallest since 1988 at the time of year it typically reaches its greatest expanse.

The hole reached its peak on Sept. 11, covering about 7.6 million square miles before beginning its annual retreat. The average peak since 1991 has been about 10 million square miles.

Researchers say the limited size was mainly due to abnormally unstable and warmer air swirling high above Antarctica in the southern polar vortex.