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Wednesday, 10th September 2008

Typhoon eye kills

Before talking about typhoon eyes, let me express my sincere thanks to all those who care about the Observatory. In the past two weeks or so, many friends and people unknown to us have called by phone, sent letters and e-mail messages, and written letters to the editor and in internet forum, to say positive things about our work. They encourage us to continue to serve Hong Kong with science and with love. This visible support has helped remove to some extent the negative atmosphere in the department arising from groundless scolding which my colleagues suffered after Nuri.

Now, back to business. Nuri's centre crossed Hong Kong on 22 August. During that period, winds and rain in many places were light and some even had glimpses of the blue sky. This is typical weather associated with eye passage. Hidden behind the apparent calmness is a great potential to kill. In hourly tropical cyclone warning bulletins issued on that day, we kept on prompting people to prepare for the sudden jump in wind strength. Our greatest regret is that still people got killed during this critical period.

Typhoon eyes don't cross Hong Kong often. It is important to learn from the experience. Let us first look at the variation in wind speed (figure 1). There were two obvious peaks, one before eye arrival, another one after. Winds were relatively light in the middle. That was when the eye passed overhead.

Figure 1
Figure 1 : The wind traces at the eight reference anemometer stations for tropical cyclone signals during the passage of Nuri. "#3" and "#8" indicate the threshold wind speed values for the number 3 and number 8 signals.

The danger of the typhoon eye lies in its apparent calmness. People are misled into thinking that the storm is over and are lured to go outdoors or even into the sea. Then maddening squalls strike and kill instantly. When Nuri struck on 22 August, we realized that this could happen. To cope with this, the number 9 signal was issued. We repeated and repeated in our warning bulletins the message that the eye would bring sudden changes in wind. On this occasion the eye was rather big (see discussion below and figure 3) and its movement was rather slow. Thus the number 9 signal was in force for an exceptionally long duration.

In a mature typhoon, the eye is an area of fine weather apart from being calm. It is surrounded by a circular "eye wall" consisting of towering cumulonimbus clouds more than 10 kilometres high. In weaker typhoons (as well as severe tropical storms), the eye wall is broken and incomplete. Furthermore, some low clouds also exist inside the eye. Sunshine would get through cracks in the cloud cover and the blue sky is seen here or there. This was the situation in Nuri as it approached Hong Kong (figure 2).

Figure 2
Figure 2 : The infrared satellite picture of Typhoon Nuri at 7 a.m. 22 August. The circular eye was incomplete though clearly distinguishable. Note its size which was comparable to that of the whole Hong Kong.

The radar also helps us "see" the typhoon eye. To make things easier for people who are not familiar with meteorology, I have picked the 3-dimensional radar picture at 5 p.m. on that day (figure 3). According to surface wind observations, Nuri's centre was located in the Saikung area. The diameter of the circular eye wall exceeded 100 kilometres. Even the closest point was more than 20 kilometres away.

Figure 3
Figure 3 : 3-dimensional radar picture at 5 p.m. 22 August

Several people have taken photographs of blue sky being seen in an overcast sky during the passage of Nuri. There were discussions about whether it was the "eye". The photograph seen by most was taken by Mr Alman Li (figure 4). As explained above, the eye wall then was tens of kilometres away. What one sees in the photograph is an opening in the low cloud layer in the eye and the eye wall was too far in the distance to be captured. Thus it is not appropriate to label the small piece of blue sky as the typhoon eye. Nevertheless, considering the picture was taken at a place close to the centre of Nuri as it landed around Saikung at the time, there is no doubt that the photograph captured what it looked like inside the eye. It is therefore still a most valuable record of the episode.

Figure 4
Figure 4 : Photograph looking towards the north-east from the Silverstrand in Saikung area, around 5 p.m. 22 August. There was a glimpse of the blue sky in spite of the overcast condition.
Courtesy of Mr Alman Li.

When Nuri's centre passed overhead the Observatory headquarters, rain stopped. Hundreds of dragonflies filled the somewhat eerie calmness. The roaring south-westerly squalls in the "tail" of the typhoon which followed were however truly frightening. In relaying his experience to my colleagues, the photographer Mr Li reported the same: being in the eye gave him a wonderful soothing feeling, but being struck by rain squalls later in western Kowloon was horrible. Beyond calmness, there is often another storm in the waiting.

I take this opportunity to remind everybody to take care with typhoon eyes. Hidden behind the calmness of the typhoon eye is a great potency to kill. Next time one passes over Hong Kong, please do be prepared for sudden changes in weather. Please help me spread the message. Don't let people die avoidable deaths in the furious squalls following the calm.

C.Y. Lam

P.S. Observatory colleagues will provide technical discussions in greater detail in the "education resource" section of the website later. They will cover also the complex changes in Nuri's eye as it crossed Hong Kong.

Last revision date: <17 Jan 2013>