I noticed a few months ago that there is now a standard method in Meteobridge which estimates the sunshine duration based on pyranometer data. On first sight I was happy to see it uses exactly the same method as I am using for a lot of years already: the sun shines when radiation is more than 40% of the maximum possible value (and radiation is above some minimum level). I already implemented my old method on meteobridge by using the virtual sensors and that one matches nicely with my older calculations of the pre-meteobridge era.
I live close to the border with the Netherlands and can compare my sunshine hours with their values from official stations. My own estimates fit in very well there.
Now that these two are running in parallel, I see the Meteobridge method's limit is significantly lower than the one in my own calculation (although setting is also 40%), so it overestimates the amount of sunshine when clouds are not too dense. Now I wonder what it takes as the maximum possible radiation value. In my own calculation it is simply:
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solar constant * sin(solar elevation angle)
(Solar constant should be corrected for the variation in distance between sun and Earth during the year, but that effect is fairly small, like 2-3%). How is the maximum radiation value determined in the Meteobridge calculation?
