February 1998

Hong Kong Observatory

Striving for that precise forecast

By Annie Yeung

On 2 September 1937, a storm surge flooded the coast of Tolo Harbour and caused the deaths of 10,000 people. There was no warning.

At that time the Hong Kong Observatory had no satellite images to detect the onset of storm systems. The 1937 storm was not even given a name.

Some 60 years hence, the weather forecaster’s understanding has improved, and modern equipment such as satellites, radar and wind profilers are used to assist meteorologists.

Photo by Annie Yeung

To further improve the accuracy of weather forecasts, the Hong Kong Observatory maintains close contacts with overseas observatories.

According to Mr. Lee Boon-ying, a senior scientific officer at the Hong Kong Observatory, high-resolution pictures of atmospheric phenomena are now received from a Japanese geostationary satellite by a ground receiving stations. He said this information is useful for evaluating weather systems.

However, the Japanese Observatory calculates meteorological changes only at 24-hour time intervals and at a spatial interval of 100 kilometres.

“This information is not delicate enough,” said Mr. Lee.

To improve both the spatial and time resolution, the Hong Kong Observatory has recently imported a new computer software from Japan — a forerunner in the art of computer weather forecasting.

The Observatory will use the new software as a framework to devise a model suitable for Hong Kong.

Its target is to improve the time resolution from 24 hours to 6, and the spatial resolution from 100 kilometres to 20 . So the weather forecast will be more oriented to the region.

As technology in weather forecasts has improved over the years, public opinion toward the Observatory has also changed.

Mrs. Ng Tang Kit Yee, a housewife, said the Observatory has done a great job in weather forecasting.

Said she: “Both the accuracy and service of the Observatory have improved tremendously in recent years.

“Besides, the present ‘Dial-a-Weather’ service provides in-depth information on weather changes.

“I always refer to this information to decide what my little daughter should wear.

“I would give 90 percentile marks,” Mrs. Ng said, “if 100 means perfect.”

However, not everyone would be as generous as Mrs. Ng in grading the Observatory’s performance.

Mr. Wong Shing Kwan is an employe for a transportation company.

Said he: “The weather forecast is accurate sometimes, while in other times it is not.

“Inaccurate weather forecasts cause me inconvenience because my job involves the loading and unloading products outdoors.

“If the weather forecast miscalls a rainstorm, the work planned may have to be postponed.

“Generally, I think accuracy is quite high in forecasting cold spells. But for rainstorms in summer, the forecast is not that accurate.”

In fact, this opinion is shared by the forecasters themselves.

Said Mr. Lee: “The forecast is more accurate for cold spells than for rainstorms.”

Cold weather systems are bigger than rainstorm systems. Thus, the former are easier to detect.

“Our technology can trace a cold weather system formed as far away as Siberia,” said he. “But a rainfall system is less steady. It can be formed within 1 or 2 hours. We have less time to make the forecast, and accuracy is therefore lower.”

Dr. Shouquan Cheung, an assistant professor in the Department of Geography at The Chinese University of Hong Kong, recognised the difficulties faced by the Observatory. “Weather changes are very complex, especially in tropical regions.”

Mr. Lee concluded, “Even with modern science and advanced computers, technological limits still exist. There’s still a long way to go.”

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