4.2.3.2 Diffused solar radiation
The calculation is based on the fundamental equation expressed by the WMO guide nr. 8 that relates the
global, direct and diffuse solar radiation. The formula given is 8.2:
where I corresponds to direct solar radiation, ζ is the angle of solar elevation, so I cos(ζ) is the horizontal
component of I, G is global solar radiation and D is diffuse solar radiation.
The calculation of diffuse radiation is then obtained by the formula:
𝐷 = 𝐺 − 𝐼 cos 𝜁
where G is the global radiation corresponding to the measurement obtained from a flat installed pyrometer,
I is the direct radiation obtained from the heliometer (or another similar sensor), ζ obtained from the
measurement calculated by Alpha-Log.
Fig. 24 – Example of calculated measurement configuration of type "Diffuse solar radiation"
4.2.3.3 Duration of insolation (heliophania)
You can choose between three different types of calculated measurement:
➢ Sun visibility (direct radiation)
➢ Sun Visibility (Step Algorithm)
➢ Sun visibility (Meteo France)
The first type refers to the standard method indicated by the WMO, which is based on the excess of the
threshold of 120 W/m
2
of direct radiation.
Fig. 25 – Example of calculated measurement configuration of type "Sun visibility (direct radiation)”
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