per se a cloudy night – hence, there is a second reason for such nights to be brighter, which is the backscattering of the upward light by the clouds. The combination of those two effects leads to extremely large values of the NSB during some winter nights. The fact that the mean NSB values shown in the upper half of Fig. \[fig:lightcurve\] reach values of 15 is due to the influence of clouded, snowy nights during the first weeks of 2013. This value corresponds to a zenithal luminance of 0.1, which is 461 times the natural NSB. Under the assumption of a uniform sky brightness of 0.1  (from the zenith to the horizon), an illuminance meter would display 0.31lx – which is more than in the case of full moon. In reality, the sky is always brighter near the horizon than at the zenith, such that our measurement of 0.1, according to our experiences, corresponds to about 0.5lx.

A night sky brightness of 15 or 0.1 indicates very bright skies from the astronomical point of view, and yet we detected still larger NSB values at certain times – not as mean values for whole nights, but as records lasting at 