Potential global radiation

Potential global radiation for daily mean values is given as a function of the solar constant, daylength, latitude and declination, Dec:


where 1360 is the solar constant (Wm-2), 60 is the number of seconds per minute and a2 is given by:




where lat is latitude. The declination, Dec, is given by Eq. (4.88) and the daylength, ∆tmax,  is given by Eq. (4.86). See viewing function Global radiation, potential.

Within day variation of potential global radiation is estimated as a function of hour of day, day of year and latitude following equation (4.92) -(4.101):


where 86400 is the number of seconds per day and a3 is a geometric scaling function given by:




where px and py are parameters defining the slope (m·m-1) of the surface in the north-south and the west-east direction respectively (see “Meteorological Data”). This function can also optionally be used for correction of measured global radiation if the ground is sloping and the measured values are representing a horizontal plane (see switch SlopeCorrMeasuredGlobal):



SX, SY and SZ are geometric functions related to the suns position at the sky given by:


where Φ is the azimuth angle and Λ is the elevation angle of the sun, which are given by






respectively. The arctanΦ, sinΦ and cosΦ expressions in equation (4.96)  are given by:







where Θ is the zenith angle and Ω is the hour angle of the sun defined by