The amount of ozone overhead should follow a regular seasonal pattern. The Antarctic ozone layer did so for the first 20 years of BAS measurements, thereafter clear deviations were observed. In every successive spring the ozone layer was weaker than before, and by 1984 it was clear that the Antarctic stratosphere was changing progressively. These gases are in widespread use in refrigeration, industrial solvents and fire control. If the provisions of the Montreal Protocol on Substances that Deplete the Ozone Layer of 1987 are strengthened and followed, there is a prospect that the Antarctic ozone hole will be repaired by 2100.
Ozone is destroyed in the Antarctic spring by chlorine formed during the sunless winter.
The chlorine is generated by an unusual reaction between stable molecules, on the surface of small stratospheric cloud particles which can only form in the intense cold of the polar winter. The stable molecules obtain their chlorine from CFCs which have previously been broken up in sunlit regions. An important need is to determine the amount and the biological effects of the increased ultraviolet flux which is anticipated under a thinner ozone layer. Total ozone amount during October at Halley. Data in yellow pre-date the discovery of the Ozone hole in 1985.