Fig. 6
(a) Relative sky brightness in term of NSB at TST in Hong Kong during clear Earth Hour evenings. (b) Same for cloudy evenings. Each NSB measurement was converted to a linear quantity with Equation 1 in Methods then presented as a ratio with reference to the average brightness measured 30 minutes before and after the event each year. The 2011 light curve is plotted as circles to indicate its sparse temporal sampling resolution (150 seconds per data point) compared to the others (60 or 30 seconds per data point). The 2013, 2015 and 2021 light curves have been corrected for moonlight (see Methods for details). The 2011, 2013 and 2015 light curves have also been corrected for shielding on the sensor’s housing (Supplementary Note 9 and Supplementary Fig. S6). The 2021 data collected before 20:50 and corrected for cloud (Supplementary Note 8, Supplementary Figs. S3–S5) and moon contributions are shown here in a gray dashed curve for completeness only. Vertical dashed lines mark the start and end of the lights-out period.
