Scientists say they have new insight into the recovery of the ozone layer.
The BBC reports scientists have been puzzled why the hole, which forms each year over Antarctica, has been changing considerably in size from year to year.
Now NASA researchers say it is the weather that is primarily driving this variability - not the ozone-destroying chemicals in the upper atmosphere.
The findings were presented at the American Geophysical Union Fall meeting in San Francisco.
Twenty years ago the international Montreal Protocol came into force to phase out substances such as CFCs.
These chemicals break down in the high atmosphere and release chlorine, which, in a reaction driven by the Sun, goes on to destroy ozone gas. As a result, each year, the ozone layer over the southern polar region experiences a deep thinning.
After the ban, this hole stopped getting bigger.
However, the BBC reports there have not yet been signs of a full recovery and ultraviolet rays from the Sun are still streaming through.
And in some years, such as 2006 and 2011, the hole appeared to be very large, while in others, such as 2012, it looked small.
Now scientists from NASA believe that the weather plays a much more significant role in the complex system than had previously been suggested.
