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Thursday, 8th January 2009

Thinking about sunset

In the evening of 1 January, a friend sent me an SMS, telling me that he saw a sun "big and bright" during sunset. He asked me whether there had been any special astronomical phenomenon.

That day I had a hike in Sai Kung Country Park. I noticed that the sky was azure blue and distant hills showed sharp silhouettes. According to the records of the Observatory's visibility meter at the waterfront in Central (figure 1), the visibility in the Victoria Harbour exceeded 10 kilometres throughout that day. (For real-time visibility data everyday, you may visit


Fig. 1 Instrument-measured visibility in the Victoria Harbour on 1 January 2009

Because of the good visibility, as the sun was about to set below the horizon, its light beams reached the observer's eyes without being blocked by too many suspended particulates in the air. Thus the sun's bright disk was seen as it should be. Furthermore, as the sun neared the horizon, the hills provided a spatial reference for size. This tends to make people feel that the sun is bigger, compared with a sun high up in the sky with no sense of scale. So while my friend thought that he had seen a sun particularly "big and bright", it was just an ordinary sunset.

But then why did he think that it might be a special astronomical phenomenon? I believe that it is because ordinary sunset is becoming a rare sight. In recent years, the sun often disappears gradually behind a thick curtain of suspended particulates in the lower atmosphere at evening time long before it reaches the horizon. Day and night are no longer separated by a sharp line. "Sunset" is just a fuzzy concept. Some people including my friend have forgotten how real sunset looks like. To me, "sunset" carries the image of the sun forcefully making its way into the horizon, seen through clear pristine air.

The air in Hong Kong has become increasingly turbid over a period of some twenty years. In 1997, the Observatory issued for the first time a press release about the subject but it did not catch the eye of too many people(1). It has become a hot topic only after the rapid deterioration in more recent years. Figure 2 shows the trend in the annual total number of hours of reduced visibility at the Observatory headquarters not related to rain or mist/fog or high relative humidity (>=95%). That is, it refers only to situations where the reduction in visibility is due to suspended particulates in the air. In this study, a threshold value of 8 km is used to define low visibility.


Fig. 2 Annual total number of hours with visibility at the Hong Kong Observatory Headquarters below 8 km from 1968 to 2008 (relative humidity below 95 % and not counting rain, mist or fog)

In the first twenty years (1968-1987), there were about 300 hours of low visibility each year. The numbers fluctuated somewhat but the overall upward trend was slight. An obvious change took place in the last twenty years, with a major upward swing. By 1998, the annual number of hours of low visibility doubled and exceeded 600. In less than ten years and before reaching 2008, it doubled again, going beyond 1,200 hours. Hazy days increased in number so rapidly that it could not possibly escape people's notice. It led to people expressing concern.

Nowadays, normal sunset is so rarely seen that it has ended up being mistaken by people as a "special astronomical phenomenon". I am afraid the turbidity of our air has reached a point where we must look it in the face and wonder what has gone wrong. Why is the sky increasingly turbid? Are human beings responsible? What has happened in the last twenty years? What could we do to stop further deterioration?

Finally, I invite you to think about this: is it important in life to be able to see real sunset?

CY Lam

Note (1): The press release was issued in April 1997. Only the New Evening Post (no longer in existence) reported it.

Note (1):The press release was issued in April 1997. Only the New Evening Post (no longer in existence) reported it.

Last revision date: <17 Jan 2013>