Ozone hole starts strong, fades quickly.

The ozone hole currently hovering over Antarctica has once again forced scientists to gulp down a strong dose of humility. Confounding expectations, the hole didn't even come close to record depths this month.

The lowest ozone concentrations above Antarctica this year measured 111 Dobson units, according to data collected by the Total Ozone Mapping Spectrometer (TOMS) on NASA's recently launched Earth Probe satellite. "That is nowhere close to a record," says Arlin J. Krueger of NASA's Goddard Space Flight Center in Greenbelt, Md.

The stratospheric ozone hole-a patch of sky marked by extremely low concentra- tions of ozone-has formed over Antarctica each August and September since the late 1970s. During these months, springtime sunlight returns to the cold polar skies and powers chemical reactions in which chlorine and bromine pollution destroy ozone.

As atmospheric chlorine and bromine have grown more abundant over the last 2 decades, the ozone hole has gradually worsened. In 1993, a TOMS device on a Russian satellite measured an all-time low of 85 Dobson units. In 1995, that instrument was no longer working, but balloon-borne instruments measured near-record values below 100 Dobson units in the atmosphere above the South Pole.

David J. Hofmann of the National Oceanic and Atmospheric Administration in Boulder, Colo., predicted earlier this year that ozone amounts in 1996 would drop below those of 1995.

"He blew it," says Krueger.

Hofmann based his prediction on the status of upper atmosphere winds above the equator. When the winds shift toward the west-as they did this year-more air tends to blow toward the poles. This influx of air would bring more pollution and exacerbate Antarctic ozone loss, he suggested in the Sept. 12 Nature.

The prediction showed promise when the 1996 hole first formed. At that time, the area of ozone depletion was growing extremely rapidly. Then, in the first week of September, the situation began to change. Stratospheric pressure pat- terns buffeted the giant vortex of winds that normally isolates the Antarctic atmosphere. With this vortex pushed off center and weakened, the process of ozone destruction slowed.

The inability to forecast the ozone hole is nothing new. Scientists failed to anticipate the hole when it first appeared in the 1970s and have since had little success in predicting its annual variability. Still, atmospheric chemists are confident that pollution controls will allow the ozone layer to recuperate after the year 2000.