Anthropogenic disruption of the night sky darkness in urban and rural areas

Abstract

The growing emissions of artificial light to the atmosphere are producing, among other effects, a significant increase of the night sky brightness (NSB) above its expected natural values. A permanent sensor network has been deployed in Galicia (northwest of Iberian peninsula) to monitor the anthropogenic disruption of the night sky darkness in a countrywide area. The network is composed of 14 detectors integrated in automated weather stations of MeteoGalicia, the Galician public meteorological agency. Zenithal NSB readings are taken every minute and the results are openly available in real time for researchers, interested stakeholders and the public at large through a dedicated website. The measurements allow one to assess the extent of the loss of the natural night in urban, periurban, transition and dark rural sites, as well as its daily and monthly time courses. Two metrics are introduced here to characterize the disruption of the night darkness across the year: the significant magnitude (m1/3) and the moonlight modulation factor (γ). The significant magnitude shows that in clear and moonless nights the zenithal night sky in the analysed urban settings is typically 14–23 times brighter than expected from a nominal natural dark sky. This factor lies in the range 7–8 in periurban sites, 1.6–2.5 in transition regions and 0.8–1.6 in rural and mountain dark sky places. The presence of clouds in urban areas strongly enhances the amount of scattered light, easily reaching amplification factors in excess of 25, in comparison with the light scattered in the same places under clear sky conditions. The periodic NSB modulation due to the Moon, still clearly visible in transition and rural places, is barely notable at periurban locations and is practically lost at urban sites.

STARS4ALL Night Sky Brightness Photometer

“…The IR sensor allow us to estimate the cloud cover at the same time as the sky brightness is being measured. This is very important to discard measures of sky brightness affected by the light reflected into the clouds and to get a better statistical estimate of the sky brightness evolution [1], [2].…”

“…However, it is a simple and reliable method to monitor the evolution of the sky brightness. Analyzing the data taken along the night, every night along extended periods it is possible to obtain statistical results of the variation of the sky brightness with atmospheric conditions [1], [2], [12].…”

“…The increase of the nocturnal sky glow prevents citizens from observing a dark starry sky. Although the light sources are located mainly in urban areas it is necessary to travel far from these places to find a sky with unpolluted, natural sky [1], [2].…”

How Clouds are Amplifying (or not) the Effects of ALAN

“…Intercalibration consists in a cross-calibration method based in pairing all the instruments, so with this strategy all the devices will show the same value for the same situation. The initial accuracy of a SQM is around 10% and with the intercalibration could be improved to 1% [10,16,17].…”

High correlation but high scale-dependent variance between satellite measured night lights and terrestrial exposure

“…Third, the VIIRS DNB sensor is not sensitive to light <500 nm (Miller et al 2013). With the transition from lighting sources such as sodium vapor bulbs to LEDs the spectrum of ALAN is currently shifting to more blue light (Bará 2013, Luginbuhl et al 2014, Barentine et al 2018, and with it an increase in the potential for models which incorporate DNB data, such as the WA, to underreport total visible exposure. Ultimately, we find the WA can be used to make usable predictions of exposure to ALAN on the ground for ecological and epidemiological studies, and which could be further improved with the incorporation of additional satellite-based data sets with finer spatial and spectral resolutions.…”