Correlating beam radiation with sunshine duration

For seven locations in north and central India, monthly averaged data of daily beam irradiation on a horizontal surface, I, have been correlated with bright sunshine duration, s, using the relation (I/I₀) = b₀(s/S′) where I₀ is the monthly mean value of extraterrestrial irradiation over a day, S′ is the corresponding duration over which Campbell-Stokes type sunshine recorders remain sensitive. It is found that b₀ is a constant that is period and region dependent. Monthly estimates of beam radiation using the relation above show that for the seven stations rms errors are within 3–6%. Tests made for other stations in India also gave good estimates. For the computation of I, global radiation is not required unlike most estimation techniques and therefore the method should find a wide application.It may be observed that b₀ values turn out to be roughly equal to exp(-md T ) where m is the relative airmass, d is the Raleigh optical thickness and T is the Linke turbidity factor.