The incoming short wave radiation can be estimated based on several formulae. If the solar radiation at the exterior of the atmosphere is known, the incoming short wave radiation can be estimated based on an empirical formula of Black et al 1954:

where is the mean monthly cloudiness as decimal fraction and is the extraterrestrial-radiation for the whole month in cal/cm²/day.

Another method of estimating is by means of the equation:

where are empirical constants, are observed duration of sunshine hours per day and are maximum possible duration of sunshine hours.

Location	a	b	Source
World	0.23	0.48	Black et al. 1954
World	0.23*cos	0.52	Glover and McCulloch 1958
S.E. England	0.18	0.55	Penman 1948
Virgina U.S.A	0.22	0.54	in Penman 1948
Canberra Australia	0.25	0.54	in Penman 1948
Brisbane Australia	0.23 to 0.35	0.38 to 0.54	Cartledge 1973
West Africa	-0.12 to 0.26	0.99 to 0.50	Davies 1966