Peak UV in Antarctica exceeds that in San Diego
Not new news, but now put into better historical context ...
Who would have believed that the peak UV in icy Antarctica exceeds that at sub-tropical San Diego? I for one would have been very skeptical.
The UV received at the Earth’s surface is strongly dependent on the solar elevation angle. When the sun is low in the sky, the increased path-length of sunlight through the atmosphere means that absorption by ozone and other scattering losses are much greater. The remaining energy in the beam is also spread over a larger area. For example, if the sun is only 30 degrees above the horizon, then the UVI (a measure of sunburning energy arriving at a horizontal surface) is less than 20 percent of what it would be for overhead sun. The sun never gets very high in the sky in Antarctica (does it?), so even with the severe Antarctic ozone depletion in spring, surely the sun elevations are so low that high UV wouldn’t be a problem.
But, seeing is believing. It certainly can be. Admittedly, we see these high values exceeding UVI = 14, only on the Palmer Peninsular - latitude (64S) in what’s playfully known as the ‘banana belt’ of Antarctica. At that balmy latitude, the sun elevation can be close to 45 degrees in the middle of ‘summer’. Much higher than further south at Scott Base (78S) , where the peak is less than UVI = 7.5, or the South Pole itself (90S), where it never exceeds UVI = 4.1. More on that next week.
I was a minor player in a recent paper about this high UV at Palmer Station compared with San Diego. My colleague, Germar Bernhard, who lives in San Diego, led the study. We showed that since ozone depletion began, the peak UVI ever measured on a given day of the year at Palmer Station has more than doubled. It now comfortably exceeds that in San Diego. Normally, the UVI decreases rapidly with latitude because the maximum sun elevation angle decreases. But at Palmer Station - which is twice as far from the equator as San Diego (32N) - the peak UVI observed can be greater by about 40 percent.
The main figure in the paper updates one that’s been used in several Ozone Assessment documents in the past. The most recent example was in Question 16 of their “Twenty Questions and Answers About the Ozone Layer”. Well worth a read.
The new version of the figure is quite striking. It puts the present maximum UVI values in stark context with what it was before ozone depletion began. The changes in Antarctica are spectacular, much larger than at high latitudes in the northern hemisphere. And in San Diego, any changes have been trivially small.
The figure shows the highest UVI ever measured for each day of the year since 1990 at Palmer, San Diego, and Barrow, compared with the reconstructed values for the pre-ozone hole period 1970–1976. The difference in the UVI at Palmer between then and post-1990 (indicated by yellow shading) illustrates the effect of Antarctic ozone depletion, which is particularly strong during spring.
As you can see, the highest UVI values observed at Palmer during the more recent period comfortably exceed those measured at San Diego despite the city’s much lower latitude.
Of course, it has to be remembered that these are only peak occurrences, not averages. Those high UVIs at Palmer correspond to days when the ozone hole is overhead. The average UVI at noon for any season (including spring) is far greater in San Diego than at Palmer Station.
Fortunately, because of the success of the Montreal Protocol on Protection of the Ozone Layer, further large increases in UV are not expected.
Thanks for reading this. Previous posts on the intersection between Ozone, UV, Climate, and Health can be found at my UV & You area at Substack. Click below to share or subscribe for occasional free updates.