Long term trend of the annual number of hours of reduced visibility in Hong Kong from 1968 to 2010

     The number of hours of reduced visibility (visibility below 8 km excluding cases of rain, fog and mist) observed at the Hong Kong Observatory Headquarters since 1968 has been updated with data collected in 2010 (http://www.hko.gov.hk/cis/statistic/hko_redvis_statistic_e.htm).  The figure below shows that there has been a generally improving trend since the peak in the mid-2000s.  Nonetheless, the present level is still well above the 1000-hour per year mark exceeded after 2002 and is many times the levels enjoyed prior to the 1990s. 

     Apart from fog, mist, rain and other meteorological phenomena involving water droplets, reduced visibility occurs mainly as a result of the absorption and scattering of light by particulates suspended in the atmosphere.  Suspended particulates form mainly as a result of human activities such as vehicular traffic, construction, fossil-fuel power generation, cooking and burning of vegetation.  As suspended particulates are carried and dispersed by the wind, reduced visibility is significantly influenced by meteorological factors such as wind direction, wind speed and atmospheric stability.

     Reduced visibility is observed at the Hong Kong Observatory Headquarters more often during the cooler months, and less in summer time. 

Annual total number of hours of reduced visibility observed at the Hong Kong Observatory
visibility below 8 km excluding cases of rain, fog and mist).