San Diego is located at 32°N where the depletion of stratospheric ozone has been small: total ozone averaged over the latitude band 35°N - 60°N was about 3% lower during the period 2002 - 2005 than during 1964 -1980 (WMO, 2007). The maximum summer-time UV Index is 12.0 (Fig. 3.8(a)). The average and 90th percentile for July are about 9.5 and 11, respectively. UV levels are generally lower during the months preceding the summer solstice due to a combination of larger ozone columns in spring compared to summer (the average ozone column for May is 320 DU; that for July is 295 DU), and frequent coastal fog during the months of May and June, also known as "June gloom". This is particularly evident in the dip of the 10th percentile around 1 June. Model calculations and historical data are not available for San Diego.
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