  The night sky brightness at the RGN site (near the centre of Zagreb, Croatia)
was monitored form January 2012. to December 2017. The gathered data show that
the average night sky brightness in this period did not change significantly,
apart from differences caused by yearly variations in meteorological
parameters. The nightly minima, maxima and mean values of the sky brightness do
change considerably due to changes in meteorological conditions, often being
between 2 and 3 magnitudes. The seasonal probability curves and histograms are
constructed and are used to obtain additional information on the light
pollution at the RGN site. They reveal that the night sky brightness clutters
around two peaks, at about 15.0 mag/arcsec2 and at about 18.2 mag/arcsec2. The
tendency to slightly lower brightness values in spring and summer can also be
seen in the data. Two peaks correspond to cloudy and clear nights respectively,
the difference in brightness between them being about 3 magnitudes. A crude
clear/cloudy criterion can be defined too: the minimum between two peaks is
around 16.7 mag/arcsec2. The brightness values smaller than this are attributed
to clear nights and vice-versa. Comparison with Vienna and Hong-Kong indicates
that the light pollution of Zagreb is a few times larger.
