Earth's lifetime, natural processes have regulated the balance of ozone in the stratosphere. Scientists are finding that ozone levels change periodically as part of regular natural cycles such as seasons, periods of solar activity, and changes in wind direction. (Roan 1989) Concentrations are also affected by isolated events that inject materials into the stratosphere, such as volcanic eruptions. Polar regions reflect the greatest changes in ozone concentrations, especially the South Pole. The topography of Antarctica is such that a stagnant whirlpool of extremely cold stratospheric air forms over the region during the long polar night. (Roan 1989) The air stays within this polar vortex all winter, becoming cold enough to allow the formation of polar stratospheric clouds. Polar stratospheric clouds speed up the natural process of ozone destruction by providing ice crystal surfaces on which the destructive reactions take place. (Rowland 1989) After the long polar winter, ozone within this extremely cold vortex is very vulnerable to the arrival of sunlight. As spring arrives, major ozone losses occur. In the southern hemisphere, the area of most severe ozone depletion is localized above Antarctica and is generally referred to as the ozone hole. The hole appears in the southern spring, following the continent's coldest season and polar night. Ozone depletion over the Arctic is not as well defined as in Antarctica. The rugged topography results in an air circulation pattern that is quite different from that of the South Pole, but expeditions have shown that the atmospheric chemistry of the two Polar Regions is very similar. (Roan 1989) In the Northern Hemisphere, the polar vortex is not as strong. It can break up and reform several times during the course of winter. One air mass after another enters the polar darkness and soon emerges back into low sunshine. Thus, a bit of ozone is lost from each parcel of air, rather than a large amount ...
