is only a lower limit of the influence of fireworks on the NSB, since our SQMs are pointed toward the zenith, and hardly any fireworks rocket would reach the local zenith at the observatory, which is surrounded by a large park.

![During the night from June 20th to June 21st 2012, a short term increase in the NSB from about 20 up to 10  is found at our mountaintop observing station LFOA – corresponding to an increase in the NSB by a factor of 10000 due to lightning.[]{data-label="fig:FOAlightning"}](Figure7.eps){width="10.5cm"}

Implications of our results for the transition from mesopic to scoptopic vision
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As shown in Figure \[fig:histo\], the mean zenithal NSB values in Vienna (location IfA) are in the range 15 to 19.5. The black vertical line represents a sky brightness of . This corresponds to a luminance of or 3 . We take this value as the lower limit of the cones in human vision. At smaller luminances, the color-insensitive rods entirely dominate our vision. If the NSB is brighter than , the adaptation of the human eye to darkness is very incomplete. The partial visibility of our Milky Way at least near the zenith requires a value which is 