The patterns of measured and historical UV Indices at Palmer are similar to those at McMurdo, but the magnitudes are different. The highest UV Index occurs in spring, reaching a maximum of 14.8. The 90th percentile is considerably enhanced during spring. Particularly large UV-B levels were observed in November and early December during years when the polar vortex became unstable and air masses with low ozone concentration moved toward the Antarctic Peninsula. In combination with the relatively high solar elevations in those months, it led to UV intensities exceeding San Diego's summer levels. UV Indices at Palmer during summer were always lower than 8. Historical measurements were estimated based on the average ozone column calculated from TOMS/Nimbus-7 measurements of the years 1978 -1980. The grey range in Fig. 3.6(b) indicates variability by clouds only. This variability is much larger than the effect of ozone variations on historical UV Indices at South Pole (Fig. 3.3). The range indicated in Fig. 3.6(b) should be a good estimate for the actual variability at Palmer if past year-to-year changes in total ozone were similar at South Pole and Palmer. We believe that this assumption is justified. Recent measurements for mid-September to mid-November are on average 30% - 60% larger than the historical average (Fig. 3.7(b)). The differences for the 90th percentile are between 60% -100%.
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