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Rights: The University of Waikato
Published 29 July 2008

Dr Richard McKenzie, Senior Research Scientist at NIWA, Lauder, outlines three main reasons why UV levels in New Zealand during summer are higher than at an equivalent latitude in the northern hemisphere during its summer.


The UV intensities are greater in the southern hemisphere for three main reasons. Firstly – and surprisingly importantly – the Earth orbits the Sun in an ellipse, not a circle. The closest point of approach of the Sun to the Earth occurs in December/January, which is the southern hemisphere summer. The furthest away is in June/July, which is the northern hemisphere summer. That factor alone means that the southern hemisphere summer will have 7 percent more UV radiation than the northern hemisphere summer. The second one is ozone. During the summer months, there is a lot less ozone in the southern hemisphere than in the northern hemisphere. Because ozone is generated at the equator, it gets transported to higher latitudes, and that transport is more efficient in the southern hemisphere, and that factor is about a 10 percent effect. The remaining 20 percent is due to the cleaner air we have here. And so there are those three factors which lead to the higher UV radiation intensities here, compared with corresponding latitudes in the northern hemisphere.