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Wednesday, 9th July 2008

Knowledge brings worry

Rain fell intermittently on 7 July. To most people, it was just another rainy day. But to front-line weather forecasters, it was a very special, worrisome day.

We had expected the broad trough of low pressure in the South China Sea to edge close to Hong Kong, bringing continuous heavy rain. Indeed rain fell hard on 6 July. But an area of low pressure quietly emerged over the sea to the east of Hainan Island. It started moving towards the north-east, adding an element of unpredictability in the weather forecast.

While all evidence indicated that the area of low pressure would not strengthen too much, it did show up clearly as a vortex in the weather map and was moving closer to Hong Kong. The weather forecaster appreciated that it was something requiring attention and started mentioning it in the "General Situation" section of the weather bulletin for the public.

It was business as usual on 7 July, but tension gradually built up. The forecaster kept the area of low pressure under close watch as it crossed the 200 km circle. The Director could not resist wondering whether the system might intensify into a tropical depression with maximum sustained winds near the centre exceeding 40 km/hour. Should it happen, a number 1 signal would be required. With a weather system in such close proximity, computer outputs from numerical models were of little use. We were left with watching it closely and responding as the situation evolved.

As the area of low pressure skirted to the south of Hong Kong, local winds turned northerly. Weather stabilized and rain subsided. It even stopped for a while. By the evening, the vortex, which was very nearly a tropical cyclone, landed to the east of Hong Kong in the nearby Huizhou-Shanwei area where much rain fell. In Hong Kong, winds turned south-westerly and rain resumed. But the rainfall amount on 7 July was noticeably less than that on the previous day.

The area of low pressure and the associated vortex developed in a data-scarce area in the sea. There was a high degree of unpredictability. Had it intensified just a little more, it would have necessitated the issuance of the number 1 signal. Had it landed 100 km to the west, Hong Kong would have experienced much more wind and rain. A tiny shift in the atmosphere very often translates into major differences in weather at a particular spot. This is what the weather forecaster has to live with nearly everyday in his work. This is what makes weather forecasting difficult.

To most people in Hong Kong, 7 July was just another day. But for the weathermen who looked at the large volume of weather information, it was a day of great worries because of the low-pressure vortex.

Many people say: knowledge is power. Let me tell you: knowledge sometimes is the source of worry!

C Y Lam

Figure 1

Figure 2

Last revision date: <17 Jan 2013>