Decade-Long Photometric Observations of Light Pollution and Cloud Effects on Night Sky Brightness in Hong Kong

This study examines how night sky brightness (NSB) in Hong Kong has evolved over the past decade. It combines recent datasets covering 2019–2023 with the earlier dataset analyzed in a previous study (2010–2013) . This study emphasizes the importance of long-term monitoring in the context of light pollution variations resulting from urban development and increasing public awareness. Photometric data were collected nightly and continuously from multiple locations equipped with a Sky Quality Meter, covering both urban and suburban settings. The in situ observation frequency was at sub-minute intervals, characterizing nighttime profiles with a temporal resolution that other monitoring systems (e.g., satellites) cannot provide. Analysis reveals that Hong Kong’s night skies are substantially brighter than the International Astronomical Union’s (IAU) dark sky standard, with urban areas exceeding 100× the standard brightness on average. By comparing early- and late-night observations, we establish a robust indicator for assessing the direct impact of light pollution, concluding that early evening skies are brighter than late-night skies due to the variation in artificial lighting. Urban regions demonstrated more pronounced post-midnight darkening, a trend consistent with increased light pollution awareness and enhanced compliance with late-night lighting protocols. Additionally, this study introduces remotely sensed infrared (IR) sky temperature as a novel cloud amount indicator, demonstrating a strong positive correlation between cloud amount and NSB, particularly in urban areas. Our findings highlight the urgent need for effective light pollution mitigation strategies in rapidly developing cities like Hong Kong, offering valuable insights for similar initiatives worldwide.