Using all-sky differential photometry to investigate how nocturnal clouds darken the night sky in rural areas

TL;DR

This study uses all-sky differential photometry with a digital camera to measure how clouds darken the night sky in rural areas, providing essential reference points for natural nocturnal illumination under overcast conditions.

Contribution

It introduces a novel application of all-sky photometry to quantify cloud-induced darkening in rural environments, filling a gap in understanding natural night sky brightness.

Findings

Clouds can significantly darken the night sky in rural areas.

The study establishes upper limits on ground illumination during overcast nights.

Data helps differentiate natural from artificial light pollution effects.

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.