As winter melts into spring across Earth's southern hemisphere, the Sun's return triggers the onset of another event: the opening of the ozone hole. Having begun its slow yawn in mid-August, this year's hole already looks like it will be a big one.

Forecasting the path that the ozone hole will take is tricky business. The concentration of pollutants such as chlorofluorocarbons (CFCs) represents one key factor in ozone destruction, but weather conditions impact the hole as well. For instance, last year's record-setting hole, which gaped over an area three times larger than the United States, opened on the heels of an especially chilly Antarctic winter. (Such low air temperatures encourage the formation of icy clouds in the upper atmosphere known as polar stratospheric clouds, which foster the chemical reactions that turn harmless chlorine compounds into ozone eradicators.) Yet after growing to such gargantuan proportions, the 2000 ozone hole mysteriously disappeared a month earlier than usual.

"Right now it's too early in the season to say anything definitive about how [the 2001] hole will come out," atmospheric physicist Paul Newman of NASA's Goddard Space Flight Center observes. "But we can say that it will be a big ozone hole, like it has been the last few years." Indeed, Newman notes, "it will certainly exceed 25 million square kilometers in size." Researchers may achieve a better understanding of ozone hole evolution once NASA's ozone-mapping QuikTOMS spacecraft, scheduled to launch on Friday, is up and running.