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