\[fig:IFAdarkness\] clearly show that the NSB values we measure at our urban site IfA vary within a large range. The darkest skies reached at IfA are characterized by an NSB slightly above (= darker than) 19.25 , corresponding to a zenithal luminance of 2.15 . The brightest skies reached at IfA under overcast conditions have an NSB below (= brighter than) 15 , corresponding to a zenithal luminance of more than 100 , which is more than 0.1  and can easily be measured even with a simple luminance meter. As Fig. \[fig:IFAdarkness\] shows, not all NSB values occur equally often. The contour plot – which is based on more than 2 $\times$ 10$^{6}$ individual data points – shows two values which occur much more often than others: one is 19.1  – with a trend to still slightly larger (darker) values as the early morning hours (with less traffic and less artificial lights) are reached. These values are characteristic for clear skies (no clouds, little aerosol content of the air, relatively little amount of backscattering). The other range of frequently occurring NSBs is about 16.3, corresponding to 33. This value is about 13 times larger than the former one and corresponds 