Chris Hogg wrote:
I have a Davis automatic weather station with solar radiation sensor.
The latter records in watts/m^2, and, quite reasonably, records levels
of solar radiation even on dull days. But it doesn't tell me the
traditional hours of sunshine, as such. Is there a threshold value of
w/m^2 that is generally accepted as being equivalent to actual
sunshine, either rule of thumb or scientific? Put another way, what's
the minimum strength of solar radiation in w/m^2 required to burn the
strip of paper in a glass sphere type of sunshine recorder?

A sunshine recorder and a solar radiation sensor are different
instruments. The solar radiation sensor on your Davis is measuring the
total solar radiation from the whole sky whereas a traditional sunshine
recorder only measures the light from the sun (as it's focused or tracked).

There are some interesting letters in the last couple of issues of the
COL bulletin - see http://www.met.rdg.ac.uk/~brugge/col.html

Jonathan
Canterbury

You have to distinguish clearly between sunshine intensity measured as
direct irradiance and as global irradiance. Direct irradiance
involves, most simply, a sensor pointing directly at the sun and
tracking it across the sky through the day. This requires an expensive
instrument, although there are more cost-effective methods of
simulating the measurement of direct irradiance eg by using two fixed
sensors, one fitted with shade bands, but this is still not simple or
cheap.

But if you can measure direct irradiance then any value 120W/sqm
(IIRC) qualifies as bright sunshine.

The solar sensors of more affordable stations such as the Davis VP/VP2
measure global or whole-sky irradiance. There is then no simple
threshold value that can be used to enumerate bright sunshine hours.

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

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

On Fri, 5 May 2006 22:36:32 +0100, Steven Briggs
wrote:

I been thinking about doing just that.

As you might have guessed, I have some standalone software that does
just that and computes a range of other monthly summary values from
the Weatherlink archive files. The calculation of bright sunshine
values was calibrated against several months' worth of CS data and
there's also proving to be respectable agreement with the Instromet
sunshine recorder. The software is almost ready for general use - I
just need to find time to tidy up some loose ends.

John Dann
www.weatherstations.co.uk