  We present an analysis of the zenithal night sky brightness (henceforth: NSB)
measurements at 26 locations in Eastern Austria focussing on the years
2015-2016, both during clear and cloudy to overcast nights. All measurements
have been performed with 'Sky Quality Meters' (SQMs). For some of the
locations, simultaneous aerosol content measurements are available, such that
we were able to find a correlation between light pollution and air pollution at
those stations. For all locations, we examined the circalunar periodicity of
the NSB, seasonal variations as well as long-term trends in the recorded light
pollution. For several remote locations, a darkening of the night sky due to
clouds by up to 1 magnitude is recorded - indicating a very low level of light
pollution -, while for the majority of the examined locations, a brightening of
the night sky by up to a factor of 15 occurs due to clouds. We present suitable
ways to plot and analyze huge long-term NSB datasets, such as mean-NSB
histograms, circalunar, annual ('hourglass') and cumulative ('jellyfish')
plots. We show that five of the examined locations reach sufficiently low
levels 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 and
villages (< 5.000 inhabitants), with amplitudes of of up to 5 magnitudes. Using
the 'jellyfish' plots, on the other hand, we demonstrate that the examined city
skies brighten by up to 3 magnitudes under cloudy conditions, which strongly
dominate in those cumulative data representations. The long-term development of
the night sky brightness was evaluated based on the 2012-17 data for one of our
sites, possibly indicating a slight (~2%) decrease of the mean zenithal NSB at
the Vienna University Observatory.
