The Antartic Ozone Hole
A hole in the ozone layer, covering an area larger than the Antarctic continent, was discovered in 1985 by Joe Farman and his colleagues on the British Antarctic Survey. They discovered a general thinning over the whole globe - a 3% decrease since 1969, but with greater depletions in middle and higher northern latitudes in winter. Every winter the ozone layer was thinning by up to 8% over Europe.
You can observe daily incoming data and view annual charts on the Antarctic's ozone layer and ozone hole on this NASA website.
The largest the hole has become so far was recorded on 24 September 2006 at 29.6 million km². The 2014 mean ozone hole size was 20.9 million km².
Why the Antarctic?
In some areas the ozone layer has deteriorated by 20%, but above the Antarctica, this can be up to 65%! Since about 90% of the chlorine in the atmosphere was emitted by industrialized nations, you would expect that any hole would be over one of those countries! However, ozone depleting chemicals (ODCs) are non-reactive which means that they can remain in the atmosphere for decades. It is only when they are hit by UV light in the stratosphere that they break apart and do their damage. By this stage they may be in totally different places from where they began. The extreme lower temperatures in the Antarctic can speed up the rate at which the CFCs are converted into chlorine.
Antarctica is the coldest place on earth - at times colder than Mars! During the winter time, the sun doesn’t rise over the continent, while temperatures can drop below minus 78 degrees centigrade for up to six months. This extreme cold is responsible for the formation of special ice clouds known as “polar stratospheric clouds” on the surface of which chlorine gas is created. Though chlorine is stable and does not react with ozone, it is easily broken down by UV light into chlorine radicals, which break down ozone.
When temperatures rise in the spring, ozone depleted air that had been concentrated over Antarctica moves over other countries in the southern hemisphere such as Australia, New Zealand, and South America.
At times there is also a smaller hole over the Arctic but the colder temperatures necessary for the formation of the polar stratospheric clouds may only last a month or two. However experts have found concentrations of chlorine there 50 times greater than expected.Read More: The Montreal Protocol