Quoted from: Rietveld M R . A new method for estimating the regression coefficients in the formula relating solar radiation to sunshine[J]. Agricultural Meteorology, 1978, 19(2-3):243-252.

https://www.sci-hub.ren/10.1016/0002-1571(78)90014-6

Following the manner of Prescott, most regression equations relating solar radiation to sunshine hours involve a factor a + bn/ N, where a and b are constants dependent on the climate and the locality, and n/ N represents the ratio of actual to potential duration of sunshine. Examination of the several published values of a and b shows that a is related linearly, and b is related hyperbolically, to the appropriate mean value of n/ N. Use of these relationships to establish values of a and b results in a formula which provides more accurate estimates of radiation from sunshine data than by the extrapolated use of existing formulae.

Rietveld by using measured data collected from 42 stations located in different countries, has proposed a unifified correlation to compute the horizontal global solar radiation. Rietveld's model, which is claimed to be applicable anywhere in the world, is given in following equation:

\( 𝐻/𝐻_0 =0.18+0.62(𝑆/𝑆_0 ) \)

Rietveld also examined several published values of a and b coeffificients of the Angstrom–Prescott model and noted that constants a and b are related linearly to the appropriate mean value of S=S0 as follows:

\( 𝑎=0.10+0.24(𝑆/𝑆_0 ) \)

\( 𝑏=0.38+0.08(𝑆/𝑆_0 ) \)