Abstract

Artificial light at night has affected most of the natural nocturnal landscapes worldwide and the subsequent light pollution has diverse effects on flora, fauna and human well-being. To evaluate the environmental impacts of light pollution, it is crucial to understand both the natural and artificial components of light at night under all weather conditions. The night sky brightness for clear skies is relatively well understood and a reference point for a lower limit is defined. However, no such reference point exists for cloudy skies. While some studies have examined the brightening of the night sky by clouds in urban areas, the published data on the (natural) darkening by clouds is very sparse. Knowledge of reference points for the illumination of natural nocturnal environments however, is essential for experimental design and ecological modeling to assess the impacts of light pollution. Here we use differential all-sky photometry with a commercial digital camera to investigate how clouds darken sky brightness at two rural sites. The spatially resolved data enables us to identify and study the nearly unpolluted parts of the sky and to set an upper limit on ground illumination for overcast nights at sites without light pollution.

Highlights

  • Understanding of the Earth’s illuminance on cloudy nights under natural conditions is a major knowledge gap. This reference point is essential for developing new experiments and performing ecological modeling in ALAN research

  • We have investigated the spectral color shift and the attenuation of the sky brightness by nocturnal clouds

  • Our results suggest an upper limit on natural horizontal illuminance under overcast conditions of 0.34 mlx

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Summary

Methods

The first set of observations were made at the LakeLab 36.5′′N, 13° 1′42.7′′E), a floating ecological research platform installed in the southwestern bay of Lake Stechlin, a clearwater lake with a surface area of 4.25 km[2] and a maximum depth of 69.5 m. The lake is situated about 75 km north of the city of Berlin in a forested area. The entire structure is anchored to the lake bottom to prevent drift. For more details about the LakeLab, see our previous report of the zenith brightness over several months[35]. The new World Atlas of artificial NSB4 predicts a clear sky luminance at zenith of 0.22 mcd/m2 at this site[56]

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