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THE ULTRAVIOLET INDEX (UVI)
What is it?
The Ultraviolet Index (UVI) is an international, scientific measure of the level of ultraviolet radiation (UVR) at the Earth’s surface. It was developed in the early 1990’s as part of an international effort to raise public awareness of the risks of sun exposure. The UVI is based on a scale starting at 0 and reaching as high as 25 in some parts of the world. The higher the number, the greater the risk of skin and eye damage from UVR. The body needs UVB to make Vitamin D. When the UVI is low, that is less than 2, there is insufficient UVB for optimum Vitamin D production, except possibly for individuals with very fair skin.
Note: People need to protect themselves from the sun when the UVI is 3 or above.
The UVI in weather forecasting
The UVI information is gathered by the National Institute of Water and Atmospheric Research (NIWA), who provide daily forecasts to MetService. Between October and March MetService sends daily data to all media as part of their weather forecasts. The UVI level is usually expressed as a maximum for the day, in clear sky conditions (that is, no clouds). Temperature is not a good indicator of the need for sun protection. Even when the temperature is relatively low, the UVI can be high or extreme. Using the UVI to be better protected The UVI has three levels of information. It forecasts the level of UV Radiation, categorises it in relation to risk, then recommends appropriate sun protection behaviours for that level. For example: 
UVI Levels in New Zealand
New Zealand has large seasonal and latitudinal variations in UVI levels. During the summer, the UVI can reach 13 in the north of the country, while in the south during winter it can be as low as 1. In many countries close to the equator, summertime values can range up to 20. During the European summer the Index is generally not more than about 8, but can be higher, especially at beach resorts.
During the daylight saving months, New Zealand has 40% higher peak UVR levels than places at similar latitudes in the northern hemisphere(1). On most days between October and March, the UVI reaches 6 or above, and regularly reaches 10 or above. In winter, the UVI is often high enough to require protection especially at high altitudes and/or in snow. On ski-fields, UVI values can be about 20-30% more than at sea level(2).
NIWA developed the following UVI Table to illustrate peak UVI levels (taken at solar noon) throughout the year at five New Zealand centres.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 10 8 7 4 2 1 2 2 3 6 8 9 9 8 6 3 1 1 1 2 2 5 7 8 8 7 5 2 1 1 1 1 2 4 7 8 8 7 5 2 1 1 1 1 2 4 6 8 Invercargill 7 6 4 2 1 0 0 1 2 3 5 6
Mean UVI (including clouds)
Peak UVI (cloudless)
For more information, see NIWA's report A Climatology of UVI For New Zealand.
Variations in UVR levels on a typical summer's day in New Zealand
The following graph shows the variation in UVR levels on a tyhpical summer's day in New Zealand. It illustrates how UVR peaks at around solar noon (1.00 pm to 1.30 pm during the daylight saving months).
 More information on the UVI:
References
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