In message , John Dann
writes
However, it is possible to estimate the predicted global irradiance
for a given location and a given date/time given clear-sky conditions.
Then by comparing the actual and predicted values and defining a %
value above which you consider the sun to be shining brightly then you
can certainly estimate bright sunshine hours. It takes some analysis
to get a good value for the % cutoff. But the results over a month can
agree well with CS data, though totals on any given day have a larger
error.

John Dann
www.weatherstations.co.uk

I been thinking about doing just that.
I already have spreadsheet that gives the maximum solar radiation curve
throughout the day, next step was to load in the data from Weatherlink
and sum up the times for which the measured level exceeds some
percentage of the maximum. About 70% would be my starting threshold I
think, but could be tweaked from there.
Then expand to a month and generate a nice monthly summary page. I
already post the NOAA summaries on my site, but would rather have a
slightly different data set, including an approximate measure of
sun-hours.

--
steve

Weather at Scotton, Knaresborough, North Yorkshire
http://www.knaresboroughweather.co.uk