2007 ozone hole 'smaller than usual'( The ozone hole over Antarctica has s...)

The ozone hole over Antarctica has shrunk 30 percent as compared to last year's record size. According to measurements made by ESAs Envisat satellite, this years ozone loss peaked at 27.7 million tonnes, compared to the 2006 record ozone loss of 40 million tonnes.

Ozone loss is derived by measuring the area and the depth of the ozone hole. The area of this years ozone hole where the ozone measures less than 220 Dobson Units is 24.7 million sq km, roughly the size of North America, and the minimum value of the ozone layer is around 120 Dobson Units.

A Dobson Unit is a unit of measurement that describes the thickness of the ozone layer in a column directly above the location being measured. For instance, if an ozone column of 300 Dobson Units is compressed to 0 C and 1 atmosphere (the pressure at the Earths surface) and spread out evenly over the area, it would form a slab of ozone approximately 3mm thick.

Scientists say this years smaller hole a thinning in the ozone layer over the South Pole is due to natural variations in temperature and atmospheric dynamics (illustrated in the time series to the right) and is not indicative of a long-term trend.

"Although the hole is somewhat smaller than usual, we cannot conclude from this that the ozone layer is recovering already, Ronald van der A, a senior project scientist at Royal Dutch Meteorological Institute (KNMI), said.

"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."

During the southern hemisphere winter, the atmospheric mass above the Antarctic continent is kept cut off from exchanges with mid-latitude air by prevailing winds known as the polar vortex. This leads to very low temperatures, and in the cold and continuous darkness of this season, polar stratospheric clouds are formed t
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