Visibility observations at four stations in Hong Kong were analyzed. The locations of the four stations, namely the Hong Kong Observatory (HKO), Hong Kong International Airport (HKIA), Cheung Chau (CC) and Waglan Island (WI), are shown in Figure 1. HKO and HKIA are located in the urban area by the side of the Victoria Harbour. CC and WI are outlying islands situated over the southwestern and so...