BOULDER, Colo. (UPI) -- NASA says its monitoring of ozone holes in the atmosphere over the North and South poles found a significant hole remaining over Antarctica.

The Antarctic ozone hole reached its annual peak Sept. 12, stretching more than 10.05 million square miles, the ninth-largest on record, NASA said in a release Thursday.

NASA and the National Oceanographic and Atmospheric Administration use balloon-borne instruments, ground instruments and satellites to monitor the ozone layer, which helps protect Earth's surface from harmful ultraviolet radiation.

The "upper part of the atmosphere over the South Pole was colder than average this season and that cold air is one of the key ingredients for ozone destruction," James Butler, director of NOAA's Global Monitoring Division in Boulder, Colo., said.

Ozone-depleting chemicals that remain in the atmosphere are also a key factor in the finding, scientists said.

"Even though it was relatively large, the size of this year's ozone hole was within the range we'd expect given the levels of manmade, ozone-depleting chemicals that continue to persist," said Paul Newman, chief atmospheric scientist at NASA's Goddard Space Flight Center.