Typical night sky brightness at two locations: the top plot is Kitt Peak in Arizona, the bottom is near the center of Berlin. NSU is how bright it is compared to a starry sky.

Our group participates in a paper about skyglow in which researchers from 12 countries continuously measured the night sky brightness during the summers of 2011 and 2012. The published paper is open access, so you can read it for free here!. The main results are (copied from the Loss of the Night Blog):

• At 7 of the sites the sky was always at least 10 times brighter than a starlit sky.
• At 30 of the sites, the sky was at least twice as bright as a starlit sky at least 95% of the time.
• The relationship between typical brightness of overcast and clear skies changes at city limits.
• In areas without artificial light, overcast nights are WAY brighter when the moon is up, in contrast, in areas with a lot of light pollution, the moon's effect on overcast nights is almost absent.