A relatively warm Antarctic winter in 2005 kept the thinning of the protective ozone layer over Antarctica, known as the ozone 'hole,' slightly smaller than in 2004. The ozone hole is not technically a 'hole' where no ozone is present, but is actually a region of exceptionally depleted ozone in the stratosphere over the Antarctic that happens at the beginning of Southern Hemisphere spring (August–October). The average concentration of ozone in the atmosphere is about 300 Dobson Units; any area where the concentration drops below 220 Dobson Units is considered part of the ozone hole. Each year the 'hole' expands over Antarctica, sometimes reaching populated areas of South America and exposing them to ultraviolet rays normally absorbed by ozone. This data was acquired by the Ozone Monitoring Instrument on NASA’s Aura satellite, NASA's newest tool to study this annual phenonmenon. On September 15, 2005, ozone thinning over Antarctica reached its maximum extent for the year at 24.2 million square kilometers (9.4 million square miles). The largest maximum area on record was 29.2 million square kilometers, in 2000.
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