Hawaii's heightened UV risk

If you'll excuse the pun ...

Last week I talked about the effects of clouds on UV. They are - of course - site dependent. At Lauder, like most mid-latitude sites, the frequency of cloudy conditions exceeds that for cloud-free conditions, so skewing the distribution of UVI to lower values. At sunnier sites, the measured distribution is more symmetric and closer to the calculated one.

An interesting example is the high-altitude Mauna Loa Observatory in Hawaii (altitude 3,400 m, latitude 19.5 N), where one of our (NIWA) UV spectrometers has been operating since the 1990s. The frequency distribution of all high-sun UVI measurements taken at the site over a 24 year period is shown below. Note the extended x-axis compared with the data I showed for Lauder last time. The most common value is close to UVI = 14, and the legend text in the plot tells us that highest we’ve ever measured there is UVI = 21.2.

When I first produced this plot, that legend gave the maximum as 26.9. That’s about twice as high as ever occurs in New Zealand and would have been a world record high, which would be a bit surprising. And being so much larger than any other data point is statistically improbable (some may say an outlier 😊). More on that later …

We do expect to see the strongest UV at tropical sites, because it’s only there that the sun can be directly overhead, therefore minimizing the path length of its rays through the atmosphere. Also, ozone values are typically quite low in the tropics. The lowest recorded at this site is 199 DU (which easily undercuts the threshold of 220 DU taken to define the edges of the Antarctic ozone hole). The high altitude is important too, because unpolluted air, the UVI increases by 6 or 7 percent per kilometre. The UVI at the observatory (altitude 3.4 km) is therefore about 20 percent more than at nearby sea-level sites like Hilo.

At this particular site, UV levels can be even higher than usual because of clouds that form below the observatory. These clouds increase the reflectivity of the underlying surface, which leads to an increase in UV because some of that reflected light subsequently gets scattered back down towards the surface from air molecules above the measuring site. You can see the effect by comparing the above plot with the one I showed last time for Lauder. At Lauder, there was a good match between measured and calculated values for UVI values larger than the peak value. By contrast, at Mauna Loa Observatory, there’s a distinct offset, with the red curve displaced to the right. That’s because my model calculation assumed low surface reflectance whereas under these conditions, a high reflectance would have been more appropriate.

But is that peak UVI of 26.9 plausible? If true, it would be the largest ever verified. It occurred at noon on June 30, 2022, when the sun was close to being directly overhead. At the time, the ozone amount of about 280 DU, which isn’t particularly low. I checked the calculated values using the on-line calculator tool that my colleague Sasha Madronich developed. For low surface reflectance, the calculated clear-sky UVI is ‘only’ about 14.5, and for high surface reflectance, which might apply if there’s underlying cloud, it increases to about 18. To get from there to 26.9 would requires a further cloud enhancement of about 50 percent. That’s not impossible, but it stretches the bounds of plausibility.

It’s also suspicious that this was the last measurement of the day. After further investigation I found that that an instrument problem had arisen, which is why subsequent scans for that day were aborted. I’ve now flagged it as a bad data point.

So, it looks like the altiplano region of the South American Andes still holds the record, as expected, because of its higher altitude, and because overhead sun in southern hemisphere occurs when the Earth-Sun separation is smaller. That record is about UVI = 25.8, nearly twice as high as occurs in New Zealand, and three times as high as in the British Isles.

Incidentally, the time series of data from Hawaii currently ends in November 2022 after a lava flow (pictured here) from the volcanic eruption of Mauna Loa severed the power supply to the NOAA observatory where the instrument is located. Hopefully it will back on-line again soon.

In the meantime, take special care to protect yourself against UV damage if you’re up there. Or if you’re outdoors anywhere at high altitudes for that matter. Especially if there’s cloud above or below.

I really should publish this stuff, but I can’t resist the instant gratification of putting it out here.