at SZA 60 and ±6% at SZA 80). It should be noted that this error
applies mainly to the direct irradiance component; therefore for
large SZAs the angular response error in GHI would be even smaller.
The large deviations of the ratios at large SZAs may have been
caused by obstacles close to the horizon, other local effects, and by
uncertainties in the measured GHI which at these SZAs has very
small values. Finally, the noise in the ratio can be attributed to effects
from varying aerosols, thin clouds that were not captured by
the cloud screening algorithm, variations in water vapor which are
not taken into account, and by uncertainties in the model
estimates.
The patterns shown in Fig. 2 are typical for all stations as far as it
concerns the general shape, although there are differences with
respect to absolute deviations. This is an additional indication for
Solar zenith angle dependence of the ratio model/measurements under clear skies for Thessaloniki. Each panel corresponds to a different SW radiation scheme. Data points
for different months are plotted in different colors. Negative SZAs correspond to morning hours (i.e. before local noon) (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.).