Abstract
The potential of Campbell‐Stokes records of sunshine hours has not been exploited. Since 1924 various forms of the Angstrom equation have been used to estimate mean daily global radiation from the mean daily fraction of possible ‘bright’ sunshine. the Angstrom equation gives an equal radiation weight to all hours of sunshine and consequently the relationship is weak. As measured by the Campbell‐Stokes instrument ‘bright’ sunshine may be measured and recorded within individual hours from the hour markings on the card. Most observers routinely add up the sunshine measured for individual hours to give the daily total of sunshine hours.
Adding monthly mean fractions of ‘bright’ sunshine within each hour, weighted by the cosine of the mid‐hour zenith angle and a mid‐hour atmospheric transmission, gives standardized radiation hours which are quite precisely related to mean daily direct radiation. Making assumptions about the effects of atmospheric scattering and absorption allows calculation of diffuse radiation income as well. Thus a proper use of the information content of sunshine records can give good estimates of direct, diffuse and global radiation. These may be used in the absence of other radiation records or for the direct/diffuse partitioning of radiation as recorded by solarimeters or pyrheliometers.