Monitoring UV

Dr Richard McKenzie, a senior scientist for the National Institute of Water and Atmospheric Research (NIWA), has been conducting research into ultraviolet radiation for a number of years. He leads NIWA’s research programme in measuring the variability of natural UV radiation in New Zealand, to understand the causes of its variability and to monitor long-term trends.

Monitoring and learning more about UV radiation is important because it has an impact on our climate and even more directly on human health. Richard is involved in research projects to learn more about these impacts.

Since December 1989, detailed measurements across the whole UV spectrum have been taken at the NIWA centre for atmospheric research at Lauder in Central Otago. A high precision UV spectrometer that is regularly checked against international standards takes many measurements every day. Other instruments – called broadband spectrometers – take measurements of particular ranges of the UV spectrum.

Richard and other researchers have found that the intensity of UV falling on New Zealand varies greatly with the seasons. The UV Index (UVI) – an indication of the amount of sunburning UV (mainly UVB) – regularly reaches 13 in the summer and reduces to about 1 in the winter. This variation is due to a number of factors that Richard measures directly or calculates from the data he and other scientists collect around the world.

UV intensity is greater in the summer because the Sun rises higher in the sky during the day and so the Sun’s radiation is more direct. This more direct sunlight is also the reason why tropical areas near the Earth’s equator receive more intense UV than latitudes further from the equator.

Richard estimates that Lauder receives approximately 40% more radiation in summer than places of similar latitude in the northern hemisphere during their summer.

There are three main reasons for this:

• The sky over Lauder is very clear and does not contain as many aerosols (tiny solid and liquid particles suspended in the air) that would scatter or absorb UV radiation. This accounts for about 20% of the higher radiation.

• The amount of UV-absorbing ozone in the upper atmosphere over Lauder is measured to be less than in the northern hemisphere. This accounts for about 10% of the higher radiation that reaches the Earth’s surface.

• The Earth orbits the Sun in a slightly elliptical orbit and is slightly closer to the Sun during a southern hemisphere summer, giving a 7% greater radiation intensity than during a northern hemisphere summer.

• These effects combine to give Lauder a summer UVI greater than at Boulder, in Colorado USA, even though Boulder is 5 degrees closer to the equator and is 1.3 kilometres higher above sea level.

Recently, Richard has collaborated with Martin Allen from the University of Canterbury to compare the UV exposure on the Mt Hutt skifield with the UV exposure at sea level at the same time in the city of Christchurch. Mt Hutt is 90 kilometres west of Christchurch and is 2 kilometres higher. The peak UVI was found to be higher at Mt Hutt by about 20–30%. This was due to less air to scatter the radiation, enhanced reflection of UV off fresh snow and a lower amount of ozone in the lower atmosphere above Mt Hutt. However, it is not clear whether the benefit in increased production of vitamin D from greater UV exposure while skiing would outweigh the increased risk of skin cancer from that same UV exposure.