Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 154)

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The cloud radiances have been tuned to the m easured ones, while the cloudless parts are the sam e
as calculated for the atm ospheric colum n containing the nam ed m odel aerosol. This latter does not
agree with the m easurem ents.
D 2.5 The pyrheliometers
Geom etrical differences can be found even am ongst the newly developed pyrheliom eters. The calculations
were m ade for 4 pyrheliom eter geom etry (see Fig. D 2.9). ABS represents PMO2, PMO5, Pacrad and
HF instrum ents. CRO3 is the Crom m elynck 3L pyrheliom eter that has the sm allest slope angle and
the largest lim it angle. The KIPP and NIP therm oelectric pyrheliom eters are used for continuous recording
of the direct radiation.
D 2.6 The effect of clouds
Calculations have been m ade for the cloudless atm osphere and for both types of cloudy situation.
The distance of clouds from the solar centre was 1, 2 and 3 degrees respectively.
The deviations of calculated outputs of pyrheliom eters between the clear and cloudy situations are
presented in Figs D2.10 - D2.13. In all cases the cloud increases the radiation entering into the instrum ent,
but the increase never reaches 1 percent.
D 2.7 Conclusions
The presence of clouds in the circum solar part of the sky increases the output of pyrheliom eters but
this increase does not exceed 1 percent.
It seem s that the observed changes in the output of a pyrheliom eter in cloudy conditions are related
m ainly to changes in the transm ission and scattering of the atm ospheric colum n, the scattering and
reflection of clouds have sm aller effect.
D 2.8 Reference
Noring, J.E., Grether, D.F. and Hunt A.J. 1991: Circum solar Radiation Data: The Lawerence Berkeley
Laboratory Reduced Data Base NREL/TP-262-4429