In 2006, the ozone layer took a real beating, and a hole formed that was of truly epic proportions. It was a record-breaking hole, caused by some 40 million tonnes of the protective layer going AWOL.
After that, the hole recorded in 2007 is something of a flop. Weather conditions conspired to keep us and our cancer-prone skin safe, seeing off just 27.7 million tonnes of ozone and exposing an area of 24.7 million square kilometres to the full* wrath of the sun.
The loss of ozone is calculated by combining the area of the hole with the depth of the ozone layer. The depth is measured in Dobson units, which describes the thickness of the layer directly over the location being measured. It gets classed as a hole when the thickness falls below 220 Dobson units.
But keep the champagne on ice. Scientists are not sure that they can conclude much at all from the size of this year's hole. Local weather conditions are hugely important in determining how much ozone is lost during the polar spring, when the layer naturally thins anyway.
"Although the hole is somewhat smaller than usual, we cannot conclude from this that the ozone layer is recovering already," noted Ronald van der A, a senior project scientist at Royal Dutch Meteorological Institute (KNMI). KNMI uses data from Envisat's Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instrument to generate daily global ozone analyses and nine-day ozone forecasts.
"This year's ozone hole was less centred on the South Pole as in other years, which allowed it to mix with warmer air, reducing the growth of the hole because ozone is depleted at temperatures less than -78 degrees Celsius," he added.
The conditions for the hole are set up during the Antarctic winter. In this cold season, a weather pattern known as the polar vortex keeps the atmospheric mass above the Antarctic continent isolated from exchanges with warmer mid-latitude air. This keeps the air mass above the continent cold, and in the cold and dark, clouds that contain chlorine (much of it originating from man-made pollutants like chlorofluorocarbons) can form in the polar stratosphere.
Once the spring returns, this chlorine disrupts the ozone layer causing the ozone to unravel like a sock in urgent need of darning. ®