Observational studies of contributions of artificial and natural light factors to the night sky brightness measured through a monitoring network in Hong Kong

Abstract

Light pollution is a form of rapidly-growing and global-scale environmental degradation in which excessive outdoor lighting affects the natural environment, the ecosystem, and possibly even human health. Poorly designed outdoor lighting wastes energy and money, and robs the beautiful night sky. Effects of light pollution on the night sky can be evaluated by the skyglow caused by artificial lighting sources, through measurements of the night sky brightness (NSB). The Hong Kong Night Sky Brightness Monitoring Network (NSN) was established to monitor in detail the light pollution conditions in Hong Kong. Monitoring stations were set up throughout the city covering a wide range of urban and rural settings to continuously measure the variations of the NSB. Over 4.6 million zenith NSB measurements were collected from 18 distinct locations between May 2010 and March 2013. This huge dataset forms the backbone for studies of the temporal and geographical variations of this environmental parameter and its correlation with various natural and artificial factors.

The average NSB in Hong Kong, excluding data affected by the Moon, was 16.8 mag 〖arcsec〗^(-2), or about 80 times brighter than the dark site standard established by the International Astronomical Union. The urban night sky was on average 15 times brighter than that in a rural location. NSB is not only highly depended on locality-specific distribution of outdoor artificial lighting sources, but is also associated with the amount of lighting switched on at particular times of night. The switch-off of lighting near midnight led to over twofold reduction in the observed skyglow flux in urban areas.

The Moon was the major natural factor and could brighten a rural night sky as much as 6 mag 〖arcsec〗^(-2) at the zenith. On the other hand, in urban locations impacts of moonlight on NSB were barely detectable because it was overwhelmed by the artificial skyglow. The moonlight model of Krisciunas & Schaefer (1991) was tested against the observed night sky luminosity and was found to be about 30% accurate under cloudless and low luminosity conditions. Through comparing the NSB data with infrared sky measurements and visual cloud observations, it was concluded that the variation of cloud amount played a key role in determining the short-term and long-term observed fluctuations of NSB due to cloud’s back-scattering of upwelling city light to the ground, and clouds amplified light pollution more in urban than rural settings. Finally, the comparison between a night-time image taken from the International Space Station and ground-based NSB measurements reveals a strong correlation between the results taken from the two methods, suggesting good potential for the application of remote-sensing techniques on studying light pollution. This thesis shows the importance of continuing monitoring of light pollution, both by extending the operation of NSN, and complemented by examining high spatial resolution nocturnal remote-sensing data in the future. The above findings established the effects of artificial lighting on the night sky, and motivated the ways for light pollution reduction and developing dark sky protection policies in Hong Kong and beyond.