The amount of Antarctic ozone varies from day to day by as much as 30 per cent, as a result of changing weather, so it is advisable to consider monthly averages. These are found to be least during October-December, the southern spring. The reason is that then there is both sufficient cloud from the previous winter and enough solar radiation (as summer approaches) for the ozone-destruction reaction described in Note 1.E. Later, the ozone layer refills, as increasing summer warmth evaporates the ice-crystal clouds and increasing radiation creates more ozone in the southern hemisphere.
The annual polar depletion of ozone was first appreciated in 1985, and subsequently confirmed by re-examination of disregarded measurements since 1971. Since then, there has been a growing enlargement of the extent and duration of the hole (Figure 1.5) with corresponding alarm about increased UV concentrations at ground level. Observations in Antarctica in October 1993 showed a total absence of ozone at 14-19 km, for the first time.
A similar but less dramatic annual thinning of the ozone layer has been found more recently over the North Pole too, with an accompanying increase of springtime UV in Canada, for instance. The difference between conditions at the two poles is due to the lower temperatures in the south (Chapter 16) and the greater intensity of a vortex of upper winds around the South Pole in winter (Chapter 12). The vortex excludes ozone coming from the equator.
There has been some evidence of a two-year fluctuation of the thinning of the ozone layer, which might be related to the Quasi-Biennial Oscillation (Chapter 12).
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