2013 Ozone Hole over Antarctica stretched 8.1 million Square Miles; NASA

The ozone hole near the South Pole was smaller this year due to warm air in the months of September and October, NASA said.

The ozone layer above the earth serves as a protective shield against harmful ultraviolet radiation. In recent years, this layer has been getting thinner due to the presence of chlorine and bromine that eat away ozone molecules.

This year the hole was slightly smaller than the previous years. Researchers say that the local weather was responsible for this shrinkage. Ozone holes form during the Antarctic spring (August and September). NASA said that in 2013, the average size of the ozone hole was 8.1 million square miles. In comparison, the hole had reached over 10 million square miles in 2006.

"There was a lot of Antarctic ozone depletion in 2013, but because of above average temperatures in the Antarctic lower stratosphere, the ozone hole was a bit below average compared to ozone holes observed since 1990," Paul Newman, an atmospheric scientist and ozone expert at NASA's Goddard Space Flight Center in Greenbelt, Md., said in a news release.

Since the 1970s, researchers at NASA and the National Oceanic and Atmospheric Administration (NOAA) have been monitoring the size of the ozone layer.

Many countries have reduced emission of certain chemicals following the 1987 Montreal Protocol. Now, the ozone hole depends largely on meteorological factors rather than ozone-depleting chemicals.

According to the National Ozone and Atmospheric Administration, the levels of chlorine at the Poles were highest during the beginning of the century but are now showing signs of decline.

"We cannot say that this represents recovery, but it is certainly good news to see this year on the higher side of the average ozone range," said NOAA's Bryan Johnson, with the Earth System Research Laboratory (ESRL) in Boulder, Colo., according to a press release.