THE "ozone hole" is nearly gone again. During the second half of November, it shrank from its maximum extent of 15 million square kilometres, about twice the size of the continent of Australia, to a tiny fraction of its former self. By now it will be virtually undetectable.
This annual wax and wane of the so-called "ozone hole" has been a regular feature of the high Antarctic atmosphere for over 20 years. In the southern spring, as the sun begins to peep over the horizon after the long polar night, temperatures high above the South Pole are at their lowest, having fallen to -80C. This creates conditions favourable for the formation of ice clouds, and the photochemical reaction of the sunlight on these clouds, combined with the presence of chlorofluorcarbons, or CFCs as we have come to know them, results in the widespread destruction of the ozone shield that protects the earth form harmful ultraviolet radiation.
The formation of a concentrated "hole" is facilitated by the fact that the high atmosphere, or stratosphere, around the South Pole is effectively isolated during the southern spring by a strong circumpolar vortex - a band of strong winds around the pole that prevent the polar air from mixing with the ozone-rich atmosphere at higher latitudes. The result is a roughly circular area over the pole with ozone concentrations well below normal values - the so-called "ozone hole".
This year's "ozone hole" made its first appearance as usual in late August, and for a period from mid-September until the end of October, nearly all the ozone in the layer between 14 and 21 kilometres above the ground had been destroyed. This corresponds to a depletion of 50 per cent or more of the total amount of ozone in the Antarctic atmosphere, and will have resulted, particularly later in the period as the sun climbed higher above the horizon, in a substantial increase in the amount of dangerous UV-B untraviolet radiation reaching the surface, with ecological consequences which remain as yet unknown. On a few occasions during the period, when the hole was "deepest", ozone values as low as 30 per cent of normal were observed.
During November and early December every year, however, temperatures high above the Antarctic begin to rise with the advance of spring, and ozone depletion becomes less severe. In addition, breaches appear in the circumpolar vortex, allowing incursions of ozone-rich air from higher latitudes. By Christmas, the polar stratosphere is back to normal.