  Artificial light at night has affected most of the natural nocturnal
landscapes worldwide and the subsequent light pollution has diverse effects on
flora, fauna and human well-being. To evaluate the environmental impacts of
light pollution, it is crucial to understand both the natural and artificial
components of light at night under all weather conditions. The night sky
brightness for clear skies is relatively well understood and a reference point
for a lower limit is defined. However, no such reference point exists for
cloudy skies. While some studies have examined the brightening of the night sky
by clouds in urban areas, the published data on the (natural) darkening by
clouds is very sparse. Knowledge of reference points for the illumination of
natural nocturnal environments however, is essential for experimental design
and ecological modeling to assess the impacts of light pollution. Here we use
differential all-sky photometry with a commercial digital camera to investigate
how clouds darken sky brightness at two rural sites. The spatially resolved
data enables us to identify and study the nearly unpolluted parts of the sky
and to set an upper limit on ground illumination for overcast nights at sites
without light pollution.
