and September. Even with our sampling rate of one measurement every 7 seconds, bright lightning can have a strong impact in our night sky photometry. During two nights in June and August 2012, our NSB curves measured at Vienna (IfA) show “spikes” due to lightning with a baseline value of 16 and peak values up to 11, which corresponds to an increase by a factor of 100. At the LFOA site, in contrast, we find even an increase by a factor of 10000 (!) due to lightning during the night from June 20 to June 21, 2012 (see Fig. \[fig:FOAlightning\]). Since lightning has extremely high luminance, but cover only a tiny fraction of the sky, it might be more interesting to measure the illuminances (in lux) caused by lightning. To the best of our knowledge, this has not been done in a systematic way so far.

A similar effect as the one caused by thunderstorms is observed – at least in Vienna – during New Year’s Eve. The effect of local fireworks is again clearly detected in the NSB measurements. In this case, we measured a short-term increase of the sky brightness from 18  to 16  around midnight. Again, this 