By Lucy Sherriff
Published Wednesday 31st August 2005 11:00 GMT
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The hole in the Ozone layer over Antarctica this year is shaping up to be one of the biggest ever, satellite images from the European Space Agency (ESA) suggest.
The area of depletion, as the scientists call it, already covers ten million square kilometres, is still expanding, and won't reach its maximum until some time in September. Only the holes in 1996 and 2000 were larger at this stage in their development, ESA says.
The Ozone layer naturally fluctuates through the year, reaching its peak thickness in the summer months and fading away during the winter. Regional variations in weather systems play a part in the timing of the hole's appearance, ESA explains.
During the southern hemisphere winter, the prevailing winds in Antarctica, known as the polar vortex, keep the atmospheric mass above the Antarctic continent is isolated from exchanges with mid-latitude air.
This keeps the air mass above the continent cold, and in the cold and dark, clouds that contain chlorine can form in the polar stratosphere.
Once the spring returns, this chlorine, much of it originating from man-made pollutants like chlorofluorocarbons, disrupts the ozone layer.
The sunlight splits the chlorine into highly reactive ions that break ozone down into normal oxygen molecules. One molecule of chlorine can break down thousands of ozone molecules.
However, use of CFCs has decreased significantly since the 1987 Montreal Protocol, and there is evidence that the protective atmospheric layer is beginning to recover from the damage wrought. A full recovery is not expected before 2050. ®
