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Over The Horizon

That Time of Year Again...

Veronika Meduna

This year's ozone hole above Antarctica has not only broken the record for size but the ozone levels have edged precariously close to the lowest ever measured. At the minimum, ozone levels dropped to 90 Dobson Units, only two units above the record low of 88DU measured in 1994.

Dr Greg Bodeker, a scientist with NIWA (the National Institute of Water and Atmospheric Research) at Lauder, Central Otago, says this year's ozone hole covered more than 27 million square kilometres at the end of September. This was 5% larger than the previously largest hole, measured in 1996. He says this year's hole developed quickly, at a rate similar to the 1996 event, but continued to grow for longer because the wind vortex above Antarctica was more stable than two years ago.

The spring vortex temporarily isolates the air above Antarctica from the surrounding atmosphere, preventing any exchanges with ozone-rich air. With temperatures of below -80oC in the air pool above the frozen continent, the formation of polar stratospheric clouds intensifies and ozone depletion increases. At Scott Base, NIWA scientist Dr Stephen Wood says ozone levels climbed up to 170DU after reaching a minimum during the last days of September.

"Scott Base is at the fringes of the vortex, so we can get levels from outside the hole in the mornings and inside later in the day."

During the hole's formation above Antarctica, the same vortex brings high ozone levels to New Zealand. Levels above New Zealand reach their annual high in October, dropping later in summer as the Antarctic hole disintegrates and the air mixes. However, the fact that this year's hole was the largest so far does not necessarily mean that New Zealand will see particularly low levels -- and consequently high UV radiation -- later this summer, says Dr Bodeker. While there is a link between the Antarctic hole and the conditions of the atmosphere above New Zealand, the size and level of depletion have no direct influence.

"Ozone is not the most important thing when it comes to UV radiation. The angle of the sun is the determining factor, cloud cover plays an important role, and ozone levels come third," he says.

Veronika Meduna is a freelance journalist with an interest in science stories.