The first and the most widely used correlation for estimating monthly average daily global solar radiation was proposed by Angstrom , who derived a linear relationship between the ratio of average daily global radiation to the corresponding value on acompletely clear day (H/H0) at a given location and the ratio of average daily sunshine duration to the maximum possible sunshine duration.

\( H/H_C=a+b(S/S_0) \)

A basic diffificulty with Equaction lies in the defifinition of the term Hc. Prescott and the others have modifified the method to base it on extraterrestrial radiation on a horizontal surface rather than on clear day radiation and therefore proposed the following relation:

\( H/H_0=a+b(S/S_0) \)

The values of the monthly average daily extraterrestrial irradiation (H0)can be calculated from the following equation:

\( H_0(Wℎ/m^2 day )= \)\( 24/π[1+0.033cos(360n/365)][cos⁡φcos⁡δsin⁡ω_s+(ω_ssin⁡φsin⁡δ)π/180] \)\( I \)_{\( sc \)}

The solar declination \( (δ) \)and the mean sunrise hour angle \( (ω_s) \) can be calculated by following equations:

\( δ=23.45sin⁡[360/365(284+n)] \)

\( 𝜔_s=(−tan⁡𝜑tan⁡𝛿))cos \)^{\( -1 \)}

For a given month, the maximum possible sunshine duration (monthly average day length, S0) can be computed by using the following equation:

\( S_0=2/15 *𝜔_s \)