Visibility in Hong Kong is Worsening, Reduced Visibility Hits Record High in 2004

The Hong Kong Observatory released its findings on long-term visibility change in Hong Kong today (6 January 2005). The results showed that visibility in Hong Kong has a long-term deteriorating trend, and instances of reduced visibility reached a record high in 2004.

Assistant Director of the Hong Kong Observatory Mr. Yeung Kai-hing noted that in Hong Kong, instances of reduced visibility, meaning visibility below 8 km excluding cases of rain, fog and mist, were occurring more often. He said, "Between 1968 and 1986, the percentage of time that reduced visibility occurred at the Hong Kong Observatory Headquarters rose at a rate of 0.8% per decade. Between 1986 and 2004 the rate became significantly greater, at 5.7% per decade or 7 times faster than that of the previous period. In 2004, low visibility occurred some 18% of the time, the highest on record. For the seven months February, April, June, August, September, October and December 2004, new records were set for the corresponding months."

At the Chek Lap Kok International Airport, the situation was similar. In 2004, reduced visibility occurred 24% of the time, the highest on record at that location. For the eight months February, May, June, August, September, October, November and December 2004, new records were set for the corresponding months.

In the eight years between 1997 and 2004, reduced visibility occurred at the Chek Lap Kok International Airport 13% of the time on average, far higher than the average of 4% observed between 1980 and 1982 by the Observatory staff at Chek Lap Kok before the new airport was built.

Apart from fog, mist, rain and other meteorological phenomena involving water droplets, reduced visibility occurred mainly as a result of the absorption and scattering of light by particulates suspended in the atmosphere. In Hong Kong and other parts of southern China, suspended particulates form mainly as a result of human activities such as construction, vehicular traffic, 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.

Seasonally, reduced visibility occurred at the Observatory Headquarters more often in winter and spring, and less in summer and autumn. This is because Hong Kong is generally under the influence of the continental northeast monsoon in winter and spring, while under the influence of maritime southwesterly winds in summer and early autumn. For all the four seasons, the percentage of time with reduced visibility was on the increase. Mr. Yeung pointed out that the most obvious change was in autumn. Before the mid-1980s, reduced visibility seldom occurred in that season. After the mid-1980s, reduced visibility became increasingly frequent in autumn, reaching 10% of the time in the last few years. This change could be due to Hong Kong experiencing northeast monsoon winds from inland in late autumn and an increase in sources of suspended particulates inland.

In winter and spring, reduced visibility usually occurred in association with weak northerly surges of the monsoon or when the northeast monsoon affecting Hong Kong was subsiding. In summer and early autumn, reduced visibility was frequently a consequence of northerly or northwesterly winds and higher atmospheric stability induced by tropical cyclones located a few hundred kilometers east of Hong Kong.

At the Observatory Headquarters, there was no significant change in wind speeds between 1968 and 1986. High density urban development caused wind speeds to decrease at a rate of 1.69 kilometres per hour (0.47 metres per second) per decade between 1986 and 2004. While there was little change between 1968 and 1986, the frequency of westerly winds increased by 3.4% per decade between 1986 and 2004. Mr. Yeung reckoned that decreasing wind speeds and increasing frequency of westerly winds might have contributed to the marked rise in reduced visibility at the Observatory Headquarters between the mid-1980's and the present day.

Last year, the Observatory installed a satellite reception system to receive on a daily basis imageries from an imaging spectroradiometer (MODIS) aboard NASA's Earth Observing Satellite. Visible imageries received which showed the spatial coverage of smoke and haze were made available to the public through the Observatory's website in November last year (2004). Recently, the Observatory further processed the satellite data received to produce images of Aerosol Optical Depth (AOD). These AOD imageries show more distinctly the distribution of visibility in the Pearl River Delta area, and will be made available to the public on the Observatory's website http://www.weather.gov.hk/wxinfo/intersat/modis/sat.html beginning today.

Mr. Yeung noted that there was considerable public concern in reduced visibility in Hong Kong, and hoped that the various types of visibility information collected by the Observatory would be useful for research into visibility by tertiary institutes and other organizations in Hong Kong.

Assistant Director of the Hong Kong Observatory, Mr. YEUNG Kai-hing describing the long-term visibility change in Hong Kong

Aerosol Optical Depth distribution over the Pearl River Estuary at 11 a.m. on 6 January 2005