Well, you may have better luck in the sci.geo.meteorology news group.

Look in Physical Climatology by Sellers (Univ. of Chicago, 1965) or other books with any info on solar radiation.

basic equation is
cos Z = sin a sin b + cos a cos b cos c

where Z is the solar zenith angle, a is latitude, b is solar declination varying from about -23.5 deg on Dec 22 to 22.5 on June 21, and h is the local hour angle which is zero at solar noon.

Please look in other references for more info.