Dear Umbra, Our radio station provides daily information regarding ultraviolet ratings. I am curious about what these ratings actually represent, and why they change so dramatically. For example, for the past few years, most ratings have been between three and seven. Now we are getting ratings of 10. I doubt the ratings are directly related to Dobson units, which measure the amount of atmospheric ozone present, because they seem to change from day to day (and do not seem particularly related to cloud cover -- or do Dobson units change that frequently?). Any clarification would be welcomed. JackHamilton, Ontario, Canada …
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