The empirical relation between solar radiation and hours of bright sunshine in the high‐altitude tropics

The relationship between total solar radiation on a horizontal surface (Q), as measured by a pyranometer, and hours of bright sunshine (n), as measured by a Campbell-Stokes recorder, has been examined and it is shown that there is little practical gain in weighting the hourly sunshine records by the sine of the apparent solar altitude. In the non-dimensional equation values of a and b derived from daily records are shown to vary widely from month to month but their distribution appears to be substantially normal. Actual mean values of a and b, for a five-year period (1938–1943) at Kabete, Kenya (altitude 6,000 ft) differed slightly but non-significantly according to the length of period in which the data were grouped, the slope of the equation increasing with increased smoothing. The sum of a and b has been shown to be approximately constant (≃ 0·82) at Kabete, where the optical air mass varies little throughout the year. For checking the consistency of the data from various parts of the world, it is essential to reduce the differing apparent transmission coefficients to standard measure. In practice, a simple correction to optical air-mass unity has proved adequate for the records available at this station.