Hole lot of rays coming our way

It might be a mini ozone hole, but, at 3000km long and 200km wide, there is nowhere to escape its effects if you are outside today, according to National Institute of Water and Atmospheric Research scientist Greg Bodeker.

Unless you are under heavy cloud, Dr Bodeker said UV levels throughout New Zealand would rise about 20 per cent as the ozone-light atmospheric system passed over this afternoon.

"It's a bit like a weather front. It will sweep over the whole country, and there isn't going to be one worse location than any other."

Heavy cloud will block the sun's rays, so the best place to be today may well be South Island's West Coast, where the MetService predict thunderstorms and heavy rain.

But for the rest of the country, forecasts show the weather as generally good.

Dr Bodeker said light cloud cover was no protection if the sun was still visible, as the clouds reflected the radiation and raised the UV index level.

He said the UV index reading was governed by how high the sun was in the sky. "The south of the South Island is normally about 4 at this time of year, and we expect an increase to 5.5."

Similarly, the north of the North Island was normally a 6.5, but would increase to 8, said Dr Bodeker. "At six-plus you want to start thinking about sunscreen and hats. This is nothing compared to summer, which gets readings of 12 to 13 but it's going to catch people out as we come out of winter."

Skifields were the worst places to be, and skiers needed to be careful, he said.