Quick Dips

Burn, Baby, Burn

While New Zealanders are increasingly aware of the dangers of staying outside in summer without sun protection, they may not know that broken cloud cover actually increase UV exposure by more than 10%. This, and other information related to UV exposure, can be found in a Web-based UV information service operated by the National Institute of Water and Atmospheric Research (http://www.niwa.cri.nz/lauder).

The UV Index provides a measure which is proportional to the intensity of sunburning UV radiation. This differs slightly from the old "time to burn" measurement which, according to NIWA scientists Kip Marks and Richard McKenzie, was poorly defined and out-of-step with international practice.

"It clearly depends on skin type -- dark-skinned people can safely withstand much longer periods in the sun," they say. A UV Index value greater than 10 is considered "extreme"; values of 12 or more can occur when the sun is high in summer in northern New Zealand.

NIWA can map the UV Index over the country, taking into account spatial variations in ozone and in surface pressure. The maps show distinct increases in UV over high ground, particularly where snow is present. Mapping "meteograms" for the country's cities, where UV and other weather conditions are mapped over time, shows that UV is more intense in the north and that the time of maximum UV occurs slightly later for the more western cities, between 1-2 pm.

"The sunburning UV radiation received on the New Zealand summer day is about 50% more than in Europe at similar latitudes,"McKenzie points out. "Averaged over a year, tropical locations (where high solar elevations occur year-round) receive more UV than in New Zealand. However, the populations indigenous to these regions are typically darker-skinned and furthermore the climate is less comfortable for long periods of exposure to the sun. Fair-skinned people living in New Zealand are therefore at risk of overexposure to UV, as evidenced by the fact that our melanoma rates are among the highest in the world."