Empirical relations relate the values of radiation (global or diffused) with meteorological parameters like sunshine hours, cloud cover and precipitations.
The monthly average daily global radiation can be estimated with the help of the Angstrom Correlation.
The modified Angstrom Correlation is defined as ?/??=?+?(s/S), where ?=monthly average daily global radiation (Wh/m2/day), ??=monthly average clear sky daily global radiation for the location in a given day, s=actual sunshine duration in a day, S= monthly mean length of the day in hours, a and b = empirical coefficients.
Gopinathan correlation introduces one more parameter - elevation.
Several models have been proposed for estimation of hourly, daily and monthly global and diffuse solar radiation on horizontal and inclined surfaces.
For calculation of solar radiation on tilted surfaces, the parameters to be considered are beam radiation, diffuse radiation as well as reflected radiation.
Tilt factor for beam radiation is given by the ratio of beam radiation flux falling on a tilted surface to that falling on a horizontal surface.
Tilt factor for diffuse radiation is given by ratio of diffuse radiation flux falling on a tilted surface to that falling on a horizontal surface.