"Brian Wakem" wrote in message ...

Really? The highest I recorded in the whole of December 2014 was 160w/sqm
and I'm pretty sure the sun came out at some point.

I asked in this newsgroup a few months ago if it was possible to
mathematically determine if the sun was 'out' using the w/sqm along with
time of day and time of year but I didn't get far.
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From memory, I think you'll find that the threshold is actually 120W/sqm
_direct_ irradiance. I'm guessing you were probably measuring with a sensor
that records _total_ (ie direct plus indirect) irradiance, which is
obviously something rather different.

There are well-established algorithms for testing for bright sunshine (ie if
the sun is 'out'), which also need lat/long and TZ as inputs of course if
you are using a total irradiance sensor. It can only be an approximate or
rough estimate because of the limitations of using total irradiance, but it
works tolerably well. It's not possible to summarise these methods in a
short post here, but if still interested you might want to Google a bit
harder. To implement the algorithms in a script or program does require some
understanding of the trig functions involved.

John Dann
www.weatherstations.co.uk