Note about the cover:
The image on the cover is of the “ozone hole” over the Antarctic taken on September 17, 2001, the day that the hole was at its peak for this year. The hole, roughly the size of North America, is shown in blue. It is defined as the region with total ozone, measured as the thickness of the ozone layer in a vertical column, below 220 Dobson units. The areas in yellow are high in ozone. The ozone hole reached its all-time peak in 2000, when it was about 10 percent greater than the area shown on the cover. Image courtesy of NASA.