Solar and Infrared Radiation Measurements

Appendix H: Sunshine Duration

The measurement of sunshine hours or percent sunshine was the typical “measurement” of irradiance until the last few decades. This measurement is still made routinely in many parts of the world. A number of ways to estimate sunshine duration are explained in the CIMO Guide (WMO, 2008).

The measurement has been used to estimate global horizontal irradiance using this equation from Ångström (1924):

$$\raisebox{1ex}{$GHI$}\!\left/ \!\raisebox{-1ex}{$GH{I}_0$}\right.=a+b.\raisebox{1ex}{$S$}\!\left/ \!\raisebox{-1ex}{$S_0$}\right.\left(\mathrm{H.1}\right)$$

In this equation GHI_o is the global irradiance at the top of the atmosphere, S_o is the total possible minutes of sunshine during the day, S is the sum of measured minutes of sunshine, and a and b are constants that are empirically derived for each site. These estimates were used when the use of pyranometers ...