Thinner ozone brings fiery sun to spring

Sara Crawford of Hamilton dodges the sun on Takapuna Beach yesterday. Picture / Martin Sykes

Ultraviolet levels in the north of the country will still be at summer-time levels this morning as a layer of thin ozone slowly moves off the country.

As much of the country basked in the spring sun, the filament of depleted ozone moved slowly over New Zealand yesterday, bringing record UV levels for this time of the year.

It is expected to move off the country this afternoon.

The rare phenomenon - which occurs less than once a year on average - saw summer-time UV levels over the country and prompted warnings to "slip, slop, slap and wrap".

The National Institute of Water and Atmospheric Research sent a hydrogen balloon 32km into the atmosphere above Central Otago to sample the 3000km-long and 200km-wide patch yesterday morning. Preliminary results showed the filament had a reading of about 290 Dobson units - about 20 per cent less ozone than normal for this time of year.

The UV index reading was 6, slightly higher and more dangerous than the predicted reading of 5.5; it is normally about 4 for September.

"The previous lowest ozone in Central Otago was 299 Dobson units, and [yesterday's] is below that. The UV would have also been a record high," said Niwa scientist Dr Greg Bodeker.

He said the filament acted like a low-altitude weather pattern.

"They are predictable days in advance but certainly not weeks in advance."

The phenomenon happens when air patterns cause a tongue of wispy ozone-depleted air to break off from over Antarctica, where the ozone layer is always thin.

They are most likely to occur in summer, pushing ultraviolet levels to the extreme.

"Summer UV levels are usually high anyway, about 11 or 12. When a filament comes in summer it has relatively less effect on the ozone layer because it is already thinner, but it will be 13 or 14, a very high level," Dr Bodeker said.

"They are not becoming more frequent and the jury's still out on how climate change will affect them. We have no idea what they expect to do in the future."

Skifields were the worst places to be yesterday because of the reflection from the snow, as well as the high altitude. Mike Smith, spokesman for skifields Turoa and Whakapapa, said warnings had gone out about the increased UV danger.

"We had it on the day's snow report and had give-away sachets of sunblock in the lift lines in the morning. All the lifties were reminding people that it's pretty extreme today."

He said about 4000 people had enjoyed the spring conditions yesterday and most seemed to be aware of the extra risk.