The hard thing with that, is determining what constitutes that the sun is actually Shining. You could use the UV and solar radiation together, but you’re still just making a best guess.
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indeed it isn’t easy. But with lots of data, an empirical formula could be used (that is what people did with the 69000 data points from 670 sites mentioned in the article above.
Even the ‘official’ Campbell–Stokes recorder is a bit unreliable as it might burn the paper less easy when there is a lot of moisture. Personally i define ‘sun is shining’ as ‘i can see hard shadows’ which is of course impossible to determine with the current weatherflow. The campell-stokes data on the other hand could probably be approximated quite good from radiance, location and time of the year.
Cumulus software has a rule when the solarradiation is 80 percent of the max possible radiation on the given lat/long the sun is shining i know its not perfect but it should calculate the sunshine hours/minutes close.
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I’ll look into that… sounds like it could work.
I’m currently using the UV numbers to make a best guess on clouds, sun, etc. Maybe I can combine the two…
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