to completely overcast conditions. The values between 16 and 19 are measured when the sky is partly cloudy, or cloudless but hazy, or clear but not moonless. Furthermore Fig. \[fig:IFAdarkness\] shows that the trend of NSB development in Vienna during one night typically follows a gradient of 0.1  per hour.

It is also interesting to compare our “best” measured values for the urban conditions at IfA with the predited sky brightness that we extracted from Cinzano, Falchi & Elvidge [@Cinzano2001] in their “World Atlas of the artificial night sky brightness”. We find that our urban observatory IfA has a predicted clear-sky NSB of approximately 10 times the natural night sky brightness. Our above reported best value of 19.25 or 2.15  amounts to 12.5 times the natural zenithal night sky brightness, for which we assume 0.172 (Duriscoe, priv. comm.; see also Tab. \[t:conversion\]).

![Density plot of all measurements obtained at Vienna (IfA) from April 2012 to April 2013. A gradient of 0.1  per hour and two dominant NSB ranges can be identified.[]{data-label="fig:IFAdarkness"}](Figure4.eps){width="11.5cm"}

Detecting the curfew(s) in Vienna (11 p.m., 12 p.m.)
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The city of Vienna reduces a substantial fraction of its street lighting at 11 p.m. (curfew, in the following: 