||Light pollution is a widely distributed form of anthropogenic pollution that threatens both biodiversity and human health. One of the most popular indicators is known as night sky brightness (NSB), measured with photometric techniques. In the study, we report results of the very first, long-term photometric survey of NSB over Poland's capital, Warsaw, for 636 nights between 2014 and 2016 using a sky quality meter (SQM). Data were collected for all-weather conditions and, for the first time, we simultaneously use two independent sources of cloud amount data: surface-based (SYNOP) and satellite-based (Meteosat/SEVIRI). Results show that Warsaw is significantly polluted by light, with average NSB of 18.65 ± 0.06 magSQM/arcsec2 (15 times higher than unpolluted sky). Zenithal NSB is almost unaffected by moonlight. During astronomical nights, cloud cover was the dominant determinant of NSB, increasing by 7 times for overcast sky. In general, the sky brightened by ∼0.2 magSQM/arcsec2 for each 10% increase in cloud fraction. Satellite-based cloud amount data was found to be a very reliable alternative to traditional SYNOP observations. No statistically significant difference was found for average NSB calculated using satellite and SYNOP datasets. This finding is of particular importance, since the coverage of surface-based data is limited, while satellite observations can be obtained for any location on Earth, and collocate with any NSB photometric station. Our investigation also highlighted that SYNOP data are unreliable when cloud amount is low. This is due to the different fields of view for SQM (20°) and SYNOP (180°) observations of broken cloud.