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

TL;DR

This study systematically measures night sky brightness at 26 locations in Eastern Austria, analyzing seasonal, lunar, and cloud effects, and identifies sites suitable for dark sky reserves with potential long-term light pollution trends.

Contribution

It provides a comprehensive analysis of night sky brightness variations across multiple locations, introducing new visualization methods and correlating light pollution with air pollution data.

Findings

Remote locations can experience up to 1 magnitude darkening due to clouds.

Most locations see up to 15 times brighter skies under cloudy conditions.

Five sites have low enough light pollution to establish dark sky reserves.

Abstract

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.