Maps show variability in UVI over New Zealand

I've been meaning to put this together for some time ... With the summer solstice less than three weeks away, now is as good a time as any.

Last week’s post really was a wake up call! I’d intended it to go out at 1 pm NZ time, but accidentally set it to 1 am. That meant I didn’t get around to making my last-minute edits. I was going to add that I’m even old enough to remember the days when wake-up calls weren’t really an option in Europe because of the language barrier.

I promised better news. At least this isn’t bad news …

A couple of weeks ago I showed the seasonal and diurnal changes in UVI at sites in northern and southern New Zealand. While those Madonna plots give a good overview at those two sites (and confirm that our UVI is not particularly high in a global context), it’s hard to get quantitative information for the entire country.

Daily maps like the one below are available on NIWA’s UV & Ozone web pages. This one shows the UVI forecast for 17 November, at 1:30 pm - close to the time of highest sun elevation for that day. Reading the dashed contour lines, you can see that on this day, the peak UVI ranged from 7 in the south to more than 10 in the north.

The map below is the forecast for last Sunday. By then, just two weeks later than the first map, the peak UVI had increased to more than 9 in the south, and more than 11 in the north.

This comes to you from my home in Alexandra, Central Otago. That’s just to the bottom left of the intersection of latitude 45 S and longitude 170 E (where the peak UVI those two days was about 7.6 and 9.4 respectively). Notice that we’re about 4 degrees of longitude to the west of Auckland, so overhead sun - and highest UVI - is about 15 minutes later. If you live outside New Zealand, lower resolution maps showing your current UVI can be found here.

Only the current forecast is publicly available at the NIWA website, so getting an annual overview is a bit more difficult. Here I’ve collected a few historical maps from the archive to help with that for your education and edification.

The four dates I selected are the summer and winter solstices (highest and lowest sun respectively), and the spring and autumn equinoxes. In particular, the 21st of March, June, and September this year; and December last year. All show the calculated clear-sky UVI over New Zealand at 12:30 NZST. In local time, that’s 12:30 pm in winter, and 1:30 pm in the summer daylight-saving period.

The north-south gradient is present in all, with highest UVI in the north. At the summer solstice, the peak exceeds 14 in the north and 10 in the south, while on the winter solstice it reaches 2 in the north and 1 in the south. The autumn and spring equinoxes have mid-range values which are very similar, but with slightly higher UVIs in autumn due to the lower ozone over New Zealand at that time of year (see the ozone time series here).

If you look closely, you can also see that the UVI is higher over mountainous regions. In unpolluted air - as over most of New Zealand - it increases by about 6 to 7 percent per kilometre of altitude. It’s not very obvious in these maps because the terrain model currently used has quite a low spatial resolution.

Although NIWA can include cloud effects in their UVI forecasts, those that are routinely made available to the public are only for clear-sky conditions. With New Zealand’s maritime climate, clouds are usually present at least somewhere over the country every day of the year. As you can see from the maps below, by comparing the cloud-free and cloud-affected forecasts for the summer solstice last year, those clouds can dramatically reduce the UVI.

I think it’s time for UVI forecasts to the public to include cloud effects. We showed years ago that it better represents reality because on average clear-sky forecasts will tend to overestimate the UVI. And cloud forecasts are improving all the time. The downside is that sometimes weather forecasts - including cloud forecasts - are wrong. It’s not a problem if it over-predicts cloud, but if it under-predicts cloud effects, the UV risk will be higher than forecast. Anywhere else in the world, we’d have to get everybody to promise not to sue, but in New Zealand it may not be required. We might just be OK with our ACC legislation. But it might be a good idea to read the fine print carefully.

If you don’t have the UVNZ or GlobalUV smartphone apps, you can get this sort of UV information from the NIWA web pages.

But, unlike the Smartphone Apps, it won’t
1. give the variation of UVI over the day, or
2. include forecast cloud effects, or
3. include associated behavioural messages tailored to your skin type and clothing.

I hope you all have a good supply of sunscreen, and are remembering to apply it if you’re going to be outdoors for more than a few minutes.