| The "ozone hole" occurs over Antarctica in the very early spring when weather
conditions cause the stratospheric ozone layer in
this region to become very thin (very little ozone). An atmospheric circulation pattern known
as the "circumpolar vortex" builds over Antarctica in the winter, due to the very cold air mass
that forms there. This vortex prevents stratospheric air from higher latitudes, which contains
lots of ozone, from penetrating into the atmosphere over Antarctica. At the same time, the long
winter night deprives the air in the vortex of sunlight, which is necessary to produce
stratospheric ozone. The
result is that very low levels of ozone occur during September and October. The term "ozone hole"
has become more popular
since it was discovered that some man-made gases called "chloroflurocarbons" can interact with and
destroy
ozone. This has caused the ozone levels over Antarctica, which were already low at this time of year,
to virtually disappear in some years. This situation was considered by scientists to be more of an "early warning signal"
than a present danger, since the protective layer of ozone over Antarctica at this time doesn't do
very much, as there is little or no sunlight (and thus ultraviolet radiation from the sun) to cause harm.
A global thinning of the ozone layer, however, has been observed since the 1970's, and this could
be harmful to some forms of life. An international treaty to stop the production of chloroflurocarbons
has since led to a reversal of ozone depletion, and ozone levels are expected to return to normal
in the coming decades.
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