As parts of Otago finally experience sunny days in the last month of summer, Central Otago reporter JonoEdwards asks atmospheric scientist Ben Liley about those pesky UV (ultraviolet) rays.
There have been many cold days in the region so far this summer. Does this impact ultraviolet levels?
Not directly. Air temperature, or even the perceived warmth of the sun, is no guide to the UV intensity. The most important factor is how high in the sky the sun is. If you stand on level ground and your shadow is shorter than you are, the UVI (ultraviolet index) is more than 6 in New Zealand conditions. If cloud is thick enough that you don’t have a shadow, UVI will be low, but thin cloud may have little effect. Cloud can even enhance UV, when the sun is not obscured. Ozone is the next most important, and New Zealand has its lowest ozone levels in autumn.
We are starting to get more sunny days. What is the main advice you have for people going out in the sun?
Don’t get sunburned. Follow the advice in news media about covering up when the sun is high. There is also a free cellphone app (UV2Day) that gives expected UVI values anywhere for New Zealand. From both, you will see that it is better to spend your outdoor time at the ends of the day in summer. You may have heard about the importance of vitamin D from sunlight, but that is a concern for winter, when UVI is low (0-3). Well before you burn, or even tan noticeably, your skin has stopped making much vitamin D and the initial products are broken down by more UV.
New Zealand has the highest rates of melanoma in the world. Is this related to high UV levels?
Yes, to a degree. We have UVI levels that are extreme on the standard scale developed for light skin, but then 90% of the world’s population experiences UVI more than 10 at some time of year, and for some of them, all year. On the Altiplano in South America, 30million people have UVI values as high as 20. The big differences are that peoples who have evolved for high UV climates have dark skin, and in the heat of the tropics there is strong incentive to stay in the shade when the sun is high.
Our high rates of melanoma are the combined effect of several factors. Rates are much higher for Pakeha (especially Celtic) than for Maori, Pasifika, or Asian skin types. Our climate is temperate enough to encourage basking in the sun, not hot enough to alert us to the danger. On top of both, we do have higher UV intensity than at similar northern latitudes.
How do New Zealand’s UV levels compare to those at a similar latitude in the northern hemisphere? Why?
New Zealand experiences peak UV levels about 40% higher than at equivalent latitudes in the northern hemisphere, as measured in both Europe and the United States. The North Island and northern South Island are opposite Spain and Portugal, which have higher solar intensity and higher temperatures but somewhat lower peak UV.
We can explain about half the hemispheric difference from several factors. Our atmosphere is cleaner, and stratospheric ozone is somewhat less in summer and autumn. Earth’s orbit that takes us 3% closer to the sun in our summer, increasing the seasonal difference. The remaining difference is the subject of ongoing research on clouds and aerosols (atmospheric dust).
Are UV levels here related to the hole in the ozone layer?
When the Antarctic ozone hole appears each spring, New Zealand actually has more ozone above than the annual average, so UV intensity is reduced. When the hole breaks up in summer, ozone-depleted air from the Antarctic reduces ozone at mid-latitudes by a few percent, amplifying the annual cycle which has its minimum in autumn.
Have UV levels in New Zealand changed from year to year?
The UV levels in New Zealand vary substantially with cloud cover and ozone variation, but there has been no large change in the average intensity because human measures to stop ozone destruction (Montreal Protocol and Amendments) were effectively introduced and enforced.
Locally, UV intensity can be enhanced by reflection from the ground, especially from snow but also surf, concrete, sand, and some other surfaces.
Could you briefly talk about the history of UV research at the National Institute of Water and Atmospheric Research’s Lauder station?
Niwa’s research station at Lauder has been measuring UV spectra routinely since 1989, monitoring the influence of ozone variation but also exploring the effect of clouds, aerosol, elevation, ground reflectivity, and trace gases. We have collaborated extensively with health agencies to help understand New Zealand’s high rates of melanoma, and to help reduce that rate by providing information about what makes our light climate different.
