#### Potential global radiation

Potential global radiation for daily mean values is given
as a function of the solar constant, _{}_{}daylength, latitude and
declination, *D*_{ec}:

(4.90)

where 1360 is the solar constant (Wm^{-2}), 60 is
the number of seconds per minute and *a*_{2} is given by:

(4.91)

where *lat* is latitude. The declination,
*D*_{ec}_{}_{}, is given by Eq. (4.88) and the
daylength, ∆*t*_{max},_{ }_{}_{} 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):

(4.92)

where 86400 is the number of seconds per day and
*a*_{3} is a geometric scaling function given by:

(4.93)

where *p*_{x}_{}_{} and
*p*_{y}_{}_{} 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):

(4.94)

*S*_{X}, *S*_{Y} and
*S*_{Z} are geometric functions related to the suns position at the
sky given by:

(4.95)

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

(4.96)

and

(4.97)

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

(4.98)

and

(4.99)

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

(4.100)

and

(4.101)