Antarctica is by far the coldest place on earth. This extreme cold helps explain why the so-called "ozone hole" forms in the stratosphere above the icy continent. There, 10 to 30 miles up, at temperatures below -100^oF, chlorine gas reacts with ozone. The result: almost 2/3 of the ozone above parts of Antarctica disappears during September and October each year.

Most of the ozone-destroying chlorine is believed to come from human sources. The good news is that the depletion of ozone in 1999 was no worse than during 1998. This may indicate that chlorine levels are no longer increasing in the stratosphere. This is certainly the first step to recovery of the ozone layer.