"The public should be aware that the UV radiation over New Zealand, particularly in the north of the country, will be particularly high over the next few days under clear sky conditions", said National Institute of Water and Atmospheric Research (NIWA) scientist Paul Johnston.
At this time of year, the sun is highest in the sky, so the path length of UV rays though the atmosphere is shortest. Consequently, the UV blocking from the ozone layer is at a minimum. Currently, ozone amounts as measured by satellite are also lower than normal, especially in the North.
Over the next few days UV index values exceeding 15 may be possible in the Auckland region. The peak UV intensities will occur around 1:30 p.m. when the sun is not obscured by clouds. These new values are somewhat larger than the values predicted by NIWA earlier this month, which were based on monthly means rather than day-to-day variability in ozone. UV index values of 10 or more may be considered extreme. A UV index of 15 is 50% more intense than an index of 10 and, in terms of the old "time to burn" (still used by TVNZ), it corresponds to a "burn time" of less than 10 minutes. However, the concept of burn time is poorly defined, and depends on many other factors (such as skin type), so this conversion should be considered as only an approximate guide.
The UV index was originally devised in Canada and is defined so that the maximum UV value experienced in the south of that country (Toronto, latitude 43.6 °N – comparable to Christchurch) is approximately 10.