A Dobson unit is a unit of measurement that describes the thickness of the ozone layer in a column directly above the location being measured. Before Antarctica's springtime ozone hole begins forming, the normal Dobson unit reading is around 275. The ozone hole is defined geographically as the area wherein the total ozone amount is less than 220 Dobson Units. The ozone hole begins shrinking as Antarctica warms up and the ring of high-altitude winds above the continent, which had been keeping out air from elsewhere, begins weakening. When this happens, ozone-rich air from the north begins filling the hole and by early December the ozone hole has filled in.
Stratospheric ozone blocks harmful ultraviolet radiation from reaching the earth.
Even though the Antarctic ozone hole is not over populated areas and even though ozone returns to normal there each December, scientists were concerned when they discovered the hole in the 1980s because of fears that if the hole continued to grow, it could help reduce the amounts of ozone over other parts of the Earth. International treaties have reduced the amounts of ozone-depleting chemicals humans are adding to the air. The amounts of such chemicals in the lower atmosphere are decreasing. But it will take decades for lower-level air to reach the stratosphere, which means the Antarctic ozone hole is expected to continue for another couple of decades.