"Looking at the preliminary data so far, it looks as if the Antarctica ozone hole of 2008 in size and severity will be something in-between the record 2006 and the much weaker one in 2007," WMO ozone expert Geir Braathen told journalists.

"We expect an 'average' or 'normal' ozone hole," he said.

The hole in the layer over the Antarctic was discovered in the 1980s. It regularly tends to form in August before it fills again in mid-December, but the size it reaches is dependent on weather conditions.

Braathen said there is still more than enough chlorine and bromine in the atmosphere to cause complete ozone destruction in a certain height region (an altitude of 14-20 kilometres, or 9-12 miles).

Stratospheric ozone provides a natural protective filter against harmful ultra-violet rays from the sun, which can cause sunburn, cataracts and skin cancer and damage vegetation.

Its depletion is caused by extreme cold temperatures at high altitude and a particular type of pollution, from chemicals often used in refrigeration, some plastic foams, or aerosol sprays, which have accumulated in the atmosphere.

Most of the chemicals, chloroflurocarbons (CFCs), are being phased out under the 1987 Montreal Protocol, but they linger in the atmosphere for many years.

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