The development of the hole in the Southern Hemisphere is largely caused by chemical processes. The extremely cold surfaces of the ice particles in the polar stratospheric clouds facilitate the reaction of chlorine nitrate and hydrochloric acid to form nitric acid and chlorine gas. In early spring when sunlight returns to the Antarctic region, the chlorine gas produced in the polar clouds is converted in the presence of light to chlorine atoms, which destroys ozone molecules. The stratosphere over the Arctic is not as cold as it is over Antarctica, so fewer polar stratospheric clouds form. In the Northern Hemisphere, the lowest ozone values are believed to result more from the interaction of weather systems than from chemistry.
Stratospheric ozone has natural temporal and spatial variations that can be quite large.
Since the mid 1980s, though, there has been a downward trend in the amount of ozone in the Northern Hemisphere. Low levels of total ozone have also been measured in March and April in the Northern Hemisphere, and these low early spring levels are more likely related to atmospheric chemistry than to weather systems and atmospheric circulation. Since there's less than 7 hours of daylight at 60 degrees north latitude in late November, the presence of sunlight probably has minimal effects on where the lowest ozone concentrations are found. Even though the location of both low and high areas of ozone concentrations vary from season to season, the lowest values have often been found over northeastern Europe. Although the explanation for this is not completely understood, a combination of weather systems near the surface and the upper atmospheric air circulation are thought to play key roles.