Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 130)

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Figure C 2.1. Penam bra functions of diffusom eters for 45 degrees solar elevation.
Com paring the calculated and m easured sky functions it is seen, that the m easured ones are m uch
larger then the calculated ones. This is illustrated by Figure C 2.2 where the m easuredlargest, sm allest
and m ean function is seen altogether with ones calculated for different aerosol m odels, for 45 degrees
solar elevation. All calculated functions are below the m easured m ean one. The variability of the calculated
values is a factor of 5, while for the m easured ones is about 10. The reason of difference between
the calculated and m easured values is that, that the real cloudless atm osphere usually contains haze
particles too, while the m odel atm ospheres are restricted to aerosol particles only. Figure C 2.3 shows
the sm allest, m ean and largest m easured functions together with ones calculated for atm osphere including
haze particles too. Since the optical depth of haze contam ination is very sm all, therefore the direct
and diffuse radiation changes slightly, but the circum solar radiation m ight vary by several tim es.