Isn't this the inherent problem though with bright sunshine hours as a
metric - the value you get depends on how you measure it and there's no
absolute value in quite the same way as there is with eg temperature or
pressure. I suspect that the IM sensor is probably going to correlate better
with older records ie as measured with a Campbell-Stokes instrument, which
IIRC always struggled to burn a good trace for 30 minutes or so after
sunrise and before sunset even on a clear day.

OTOH the modern WMO definition of 120W/sqm of _direct_ irradiance is I
suppose more absolute in a sense and maybe will correlate more with the BL
sensor, but also seems a little more arbitrary. Presumably there are several
papers in the professional literature that analyse the correlation between
old and new approaches and under what circumstances they diverge.

On Thursday, 28 December 2017 16:53:52 UTC, JohnD wrote:
Isn't this the inherent problem though with bright sunshine hours as a
metric - the value you get depends on how you measure it and there's no
absolute value in quite the same way as there is with eg temperature or
pressure. I suspect that the IM sensor is probably going to correlate better
with older records ie as measured with a Campbell-Stokes instrument, which
IIRC always struggled to burn a good trace for 30 minutes or so after
sunrise and before sunset even on a clear day.

OTOH the modern WMO definition of 120W/sqm of _direct_ irradiance is I
suppose more absolute in a sense and maybe will correlate more with the BL
sensor, but also seems a little more arbitrary. Presumably there are several
papers in the professional literature that analyse the correlation between
old and new approaches and under what circumstances they diverge.

I totally agree John, hence I will run them both and at some point be able to calculate the deviation percentage between the two. But for accuracy from what I have seen the BL wins it for me.

Keith (Southend)