Ozone Hole

Since the mid-1970s, scientists have observed a seasonal thinning (depletion) of ozone in the upper atmosphere (stratosphere) above Antartica. This phenomenon, which is commonly called the ozone hole, occurs in September and October of each year, the beginning of the Antarctic spring. Up to 70% of the ozone above Antarctica is lost during these two months.

Winter conditions in Antarctica include extreme cold (coldest location on Earth) and total darkness. When sunlight appears in the Spring, it triggers the chemical reactions that lead to ozone depletion. By the end of November, weather conditions are such that the ozone-depletion reactions stop. Then the ozone hole disappears as air from nonpolar areas flows into the polar region, replenishing the depleted ozone levels.

[source: Chemical Principles by H. Stephen Stoker]