  Artificial skyglow is constantly growing on a global scale, with potential
ecological consequences ranging up to affecting biodiversity. To understand
these consequences, worldwide mapping of skyglow for all weather conditions is
urgently required. In particular, the amplification of skyglow by clouds needs
to be studied, as clouds can extend the reach of skyglow into remote areas not
affected by light pollution on clear nights. Here we use commercial digital
single lens reflex cameras with fisheye lenses for all-sky photometry. We track
the reach of skyglow from a peri-urban into a remote area on a clear and a
partly cloudy night by performing transects from the Spanish town of Balaguer
towards Montsec Astronomical Park. From one single all-sky image, we extract
zenith luminance, horizontal and scalar illuminance. While zenith luminance
reaches near-natural levels at 5km distance from the town on the clear night,
similar levels are only reached at 27km on the partly cloudy night. Our results
show the dramatic increase of the reach of skyglow even for moderate cloud
coverage at this site. The powerful and easy-to-use method promises to be
widely applicable for studies of ecological light pollution on a global scale
also by non-specialists in photometry.
