The ozone hole occurs high in the stratosphere, about 18 miles high, and over the continent of Antarctica. It is not actually a hole, but the appearance of very low values of ozone in the stratosphere. Typically, the Antarctic ozone hole has its largest area in early September and lowest values in late September to early October.
The Antarctic ozone hole varies in size each year, from nearly zero in 1980 to an area larger than North America in 2000. The amount of ozone in the atmosphere now is routinely measured from instruments flying on satellites. The size of this year’s ozone hole reached a maximum in mid-September of almost 400,000 square miles, which is about six times larger than Wisconsin. It was a little larger than last year’s ozone hole and about the same as in 2009 and 2010. The amount of ozone likely is reaching its lowest values of the year this week.
The winter atmosphere above Antarctica is very cold. The cold temperatures result in a temperature gradient between the South Pole and the southern hemisphere middle latitudes. These temperature gradients lead to a belt of strong westerly stratospheric winds that encircle the South Pole region. These strong winds prevent the transport of warm equatorial air to the polar latitudes. These extremely cold temperatures inside the strong winds help to form unique types of clouds called Polar Stratospheric Clouds, or PSC.
PSCs begin to form during June, winter time at the South Pole. In the winter, chemical reactions on the surface of the particles composing PSCs result in chemical reactions that remove the chlorine from the atmospheric compounds. When the sun returns to the Antarctic stratosphere in the spring (our fall), sunlight splits the chlorine molecules into highly reactive chlorine atoms and ozone is rapidly depleted. The depletion is so rapid that it has been termed a “hole in the ozone layer.”