‘C’). Main roads, however, are still kept at constant illuminance throughout the whole night. At 12 p.m., most of the decorative facade lighting is switched off (this effect is called ‘F’ below). The sky brightness, therefore, shows two discontinuous steps towards lower values at 11 and 12 p.m., of which the first one is larger. This is shown in Fig. \[fig:curfew\].

On average, step C results in a night sky brightness decrease by 0.18 , while step F leads to a decrease (= improvement of the sky quality) by 0.09 .

It would be worthwhile to monitor similar effects of curfews in other cities and communities, also to demonstrate this benefit for the nocturnal environment.

![Detection of the curfew at 11 p.m. (C) and reduction of facade lighting at 12 p.m. (F) in Vienna on March 21st, 2012.[]{data-label="fig:curfew"}](Figure5.eps){width="12cm"}

Influence of snow and other seasonal variations
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Snow has a strong influence on the NSB, especially in urban regions. The reflectance of snow is clearly much larger than that of bitumen. Hence, streets, pavements and squares covered by (fresh) snow reflect a huge portion of the downward directed radiation of streetlamps back to the sky. Moreover, a night with snowfall is 