How dark can it get at night? Examining how clouds darken the sky via all-sky differential photometry
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 set an upper limit on the illumination of overcast sites without light pollution. We investigate how clouds alter the sky brightness and color temperature at two rural sites. The spatially resolved data enables us to identify and study the nearly unpolluted parts of the sky, even in a non-ideal scenario. We observe cloud attenuation and red shift not only at zenith, but for most parts of the sky, reducing luminance and illuminance levels for overcast conditions. Our results represent a first step towards finding a reference point for cloudy skies in unlit areas within the context of ecological light pollution.
Introduction Daily, lunar and seasonal cycles of natural light have been key forms of environmental information for organisms since shortly after the first emergence of life. Artificial light at night (ALAN) has rapidly increased since the 19th century and has been introduced outdoors in places, at times, spectra and intensities at which it does not naturally occur at night. Global simulations of skyglow (one form of light pollution) on clear nights demonstrate the dramatic extent to which artificial night sky brightness (NSB) has altered nightscapes worldwide, with for example 88% of the EU experiencing light pollution on clear nights. We have recently shown that ALAN is growing in area and intensity by more than 2% per year on a global scale, with individual countries having increases of more than 10% per year.
ALAN thus has become an anthropogenic stressor - light pollution, that can affect humans, animals, plants and microorganisms. Recent findings include disruption of pollination or the disturbance of reproduction of mammals. Despite the many studies regarding the impact of ALAN on the environment, a comprehensive quantitative understanding of the amount of artificial and natural light in the nocturnal environment for all weather conditions and how it is perceived by organisms is still lacking. In particular, while reference values for sky luminance and illuminance exists for clear nocturnal skies, these reference points have not been defined for cloudy conditions. However, knowledge of such reference points is needed for the design of experiments and for ecological modeling to fully evaluate the environmental impacts of ALAN.
Atmospheric conditions can change light levels rapidly during the day and the night. In a natural setting with no ALAN, clouds should darken the night sky in most cases. For large distances around today’s urban areas however, clouds usually amplify the effects of ALAN, creating a complete reversal of natural conditions. NSB data on rural sites with low ALAN levels, however, is sparse and in general few studies with imaging devices and clouds exist.
One reason for these knowledge gaps in light pollution research is due to the fragmentation of expertise across very different disciplines and the lack of widely applicable commercial measurement tools until recently. For decades, NSB monitoring was mainly of interest for astronomers as they were first to study light pollution. For logical reasons, astronomers mainly concentrated their research on clear skies, but also performed comprehensive observations at several important (often very remote) astronomical sites, for example at high altitudes or latitudes. The advent of small photometers customized for NSB measurements (mainly the Sky Quality Meter: SQM) has sparked NSB research beyond professional astronomy. Nowadays scientists from different fields, amateur astronomers, and citizen scientists obtain NSB data from local to global scale. However, these single channel sensors usually provide zenith NSB data in a single spectral band, which provides only limited information of the nocturnal night light environment. Commercial digital cameras with fisheye lenses are a promising tool for all-sky nighttime photometry to fill this gap, yielding spatially resolved NSB data of nearly the full hemisphere in three spectral channels. A more comprehensive discussion of state-of-the-art NSB measurement techniques can be found in a recent review. Recently, we reported on lowered zenith luminance values for cloudy nights using an SQM at Lake Stechlin, Germany while others observed both, brightening and darkening by clouds in SQM datasets at Montsec Astronomical Park, Spain and in Austria. All-sky imaging data of cloud amplification were investigated in Madrid, Spain and near Montsec Astronomical Park, Spain. However, imaging data on the darkening by clouds in the context of ALAN has not been published, yet. To set a lower limit for nighttime luminance and illuminance such data is essential as it provides information of the NSB across the full hemisphere.
In this work, we use differential all-sky photometry with a commercial DSLR camera to investigate the changes that clouds have upon the NSB of both the entire sky, and segments of the sky. We examine two rural locations: one 70 km north of Berlin, Germany (LakeLab, Lake Stechlin, Brandenburg) and one 180 km east of Cape Town, south Africa (Night Sky Caravan Farm, Bonnievale). The calculated all-sky luminance maps and the derived color corrected temperature (CCT) from the three spectral channels of the DSLR camera enable us to study the NSB distribution in detail. The spatially resolved data allows to identify and study the minimally polluted parts of the sky for locations that are not completely free of artificial skyglow. Our results fill a research gap in the context of ecological light pollution and we hope this work triggers more such spatially resolved NSB measurements on a global scale to increase understanding of the impacts of ALAN.