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We present an analysis of the zenithal night sky brightness (henceforth: NSB)measurements at 26 locations in Eastern Austria focussing on the years2015-2016, both during clear and cloudy to overcast nights. All measurementshave been performed with 'Sky Quality Meters' (SQMs). For some of thelocations, simultaneous aerosol content measurements are available, such thatwe were able to find a correlation between light pollution and air pollution atthose stations. For all locations, we examined the circalunar periodicity ofthe NSB, seasonal variations as well as long-term trends in the recorded lightpollution. For several remote locations, a darkening of the night sky due toclouds by up to 1 magnitude is recorded - indicating a very low level of lightpollution -, while for the majority of the examined locations, a brightening ofthe night sky by up to a factor of 15 occurs due to clouds. We present suitableways to plot and analyze huge long-term NSB datasets, such as mean-NSBhistograms, circalunar, annual ('hourglass') and cumulative ('jellyfish')plots. We show that five of the examined locations reach sufficiently lowlevels of light pollution - with NSB values down to 21.8 mag$_{SQM}$/arcsec$^2$- as to allow the establishment of dark sky reserves. Based on the 'hourglass'plots, we find a strong circalunar periodicity of the NSB in small towns andvillages (< 5.000 inhabitants), with amplitudes of of up to 5 magnitudes. Usingthe 'jellyfish' plots, on the other hand, we demonstrate that the examined cityskies brighten by up to 3 magnitudes under cloudy conditions, which stronglydominate in those cumulative data representations. The long-term development ofthe night sky brightness was evaluated based on the 2012-17 data for one of oursites, possibly indicating a slight (~2%) decrease of the mean zenithal NSB atthe Vienna University Observatory.

Systematic measurements of the night sky brightness at 26 locations in Eastern Austria