![]() Peak UVI in New Zealand is about 40% more than at similar latitudes in the Northern hemisphere summer. Reflection: UVR is reflected by different surfaces fresh snow can reflect as much as 80% of UVR, sand can reflect about 15%, and sea foam about 25%.Variations from day to day can exceed 10%. In NZ, ozone levels vary seasonally, with a maximum in spring of around 400 DU and a minimum in autumn of around 250 DU. Ozone: ozone absorbs some UVR that would otherwise reach the Earth's surface. Annual mean ozone amount is 300 DU.In polluted areas, the gradient can be much greater. ![]() Altitude: the thinner atmosphere at higher altitudes means less UVR is absorbed. In clean air, UVI increases by about 5% per cent per km increase in altitude.And with scattered cloud, it can be even higher if the sun is unobscured by clouds, because of reflection from cloud edges. Cloud cover: UVI is highest on a clear day, but even with cloud cover, UVI can be high.Latitude: the closer to equatorial regions, the higher the UVI.This means that UVR levels vary with the time of the day and the time of year. The angle of the sun: the higher the sun in the sky, the higher the UVI.
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