Defining standard skies for Hong Kong

A CIE International Daylight Monitoring Programme (IDMP) general class station was established in 2000 at the Chinese University of Hong Kong. The station was upgraded to Research Class IDMP Station in April 2003. The study, which includes the sky luminance scan data collected from April 2003 to May 2005, firstly fits the data to the CIE Standard General Sky definitions, which consist of 15 luminance distributions for modeling the sky from the heavily overcast sky to cloudless clear sky. Then the paper proposes a reduced set of CIE general skies (and their probability of occurrence) to represent the sky conditions of Hong Kong. This reduced set will be known in this paper as “Hong Kong Representative Sky” (HKRS). Further, the paper evaluates the sensitivity of vertical sky component (VSC) to differences between the HKRS and the standard CIE Overcast Sky model. Comparing with observed data, it is demonstrated that the HKRS could give better results. A reduction in error of approximately 20–40% could be expected, depending on the orientation of a surface. Using the HKRS, building designers could better predict daylight availability of their design. Energy saving and more sustainable buildings might result.