
281Fundamentals of Solar Radiation
© 2008 Taylor & Francis Group, LLC
Collares-Pereira and Rabl (C-P&R) also introduced the sunset hour angle h
ss
in their cor-
relation to account for the seasonal variation in the diffuse component. The C-P&R correla-
tion is
D
H
h
h
ss ss
=+ −
−+ −
0.775 0.347 0.505 0.02619
−cos(2 1.8)
T
K
(5.54)
where h
ss
is the sunset hour angle in radians. The C-P&R correlation agrees well with the cor-
relations for India [19], Israel [20], and Canada [21] and is, therefore, preferred to Equation 5.53.
The monthly average beam component B
–
h
on a horizontal surface can be readily calcu-
lated by simple subtracti