Multi-band zenith amplification factor of Madrid night sky brightness under overcast conditions

Urban areas such as Madrid have undergone shifts in artificial light emissions due to the transition from high-pressure sodium to LED streetlights. We analyze a decade-long (2010–2020) high-resolution photometric dataset, which covers a major streetlight retrofit in 2015, obtained with an All-Sky Transmission Monitor Camera (ASTMON) at the Universidad Complutense de Madrid (UCM) observatory, using the Johnson B, V, and R filters. From this dataset, we quantified the Amplification Factor (AF), defined as the ratio of zenith night sky brightness under overcast conditions to that under moonless clear skies, based on aggregated observations from Winter nights. Our findings show a yearly decreasing trend in AF for the Johnson V and R bands after midnight, with values dropping from 19 . 2 − 0 . 1 + 0 . 1 to 16 . 0 − 0 . 1 + 0 . 1 and from 24 . 2 − 0 . 1 + 0 . 1 to 20 . 8 − 0 . 1 + 0 . 1 , respectively, from 2011 to 2019. These changes are consistent with the city’s transition to LED lighting and suggests that the AF is a sensitive indicator of changes in urban lighting technologies. • A decade-long multi-band analysis of the zenith amplification factor of night sky brightness under overcast conditions in Madrid is presented. • Amplification factors were wavelength dependent, with Johnson V and R showing decreasing trends after midnight following the 2015 streetlight retrofit. • A baseline photometric nights classification for overcast and clear conditions, allowed to compute progressive darkening effects on consecutive photometric nights.