How much brighter are Hong Kong’s urban night skies than its darkest rural skies?

Details

The first comprehensive light pollution survey in Hong Kong measured and monitored night-sky brightness around the city using a portable light-sensing device called the Sky Quality Meter. The survey ran for 15 months beginning in March 2008 and collected 1,957 data sets at 199 distinct urban and rural locations across all 18 Administrative Districts of Hong Kong.

The survey found severe environmental light pollution. Urban night skies averaged 15.0 mag arcsec⁻², about 100 times brighter than the darkest rural sites at 20.1 mag arcsec⁻². The researchers said this indicates that high lighting densities in densely populated residential and commercial areas lead to light pollution. At the worst-polluted urban location studied, the sky measured 13.2 mag arcsec⁻² and could be over 500 times brighter than the darkest sites in Hong Kong.

Observed sky brightness was affected by human factors such as land use and population density at observation sites, as well as meteorological and/or environmental factors. Skies at 9:30 p.m. local time were generally brighter than at 11:30 p.m., which the researchers attributed to some public and commercial lights being switched off later at night. With the limited survey sampling, however, they could establish no concrete relationship between observed sky brightness and air pollutant concentrations.

The researchers said the survey results would provide an important database for the public to assess whether new rules and regulations are needed to control outdoor lighting in Hong Kong.