The concentration of ozone is most commonly measured in a unit called the Dobson Unit. “One Dobson Unit is the number of molecules of ozone that would be required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 0 degrees Celsius and a pressure of 1 atmosphere (the air pressure at the surface of the Earth).” The ozone layer around the Earth is 300 Dobson Units on average, or 3 millimeters thick.
The hole in the ozone layer is not actually a hole; rather, it’s an area over Antarctica that has a much lower concentration of ozone. The area outlined in white is 220 Dobson Units or less. This number is used as the baseline because numbers like that were not seen before 1979.
Image taken from this page.
“This map shows the ozone hole on October 4, 2004.”
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