Looking at a typhoon track predicted by an immensely powerful computer – which might have a million or more processors, compared with the two or four in your desktop – it would be easy to think this information is definitive. But a track from another computer model may disagree, and the actual outcome could differ from either of them.
Typhoon forecasts are only really reliable perhaps three or fewer days from landfall, and for rainstorms, which can also wreak havoc, there may be discrepancies with forecasts made a day or even hours in advance. In our age of supercomputers and artificial intelligence, there is no shortage of experts investigating the weather, especially in Hong Kong, which is prone to severe storms that often take the city by surprise.“A fearful rain storm occurred at Hong Kong on May 29,” reported The Argus in Australia in June 1889. “The centre of the town is described as being simply a wreck […] Tramcars and tramlines were washed away […] Every street was flooded.” The Hong Kong Observatory reported hundreds of landslides on Hong Kong Island and at least 27 people killed.
Even today, that storm holds several Hong Kong rainfall records, including for 24-hour rainfall, though 1889 was not the deadliest storm to have hit the city. That distinction is held by a succession of rainstorms that arrived from June 16 to 18, 1972, killing around 150 people, 67 of whom died when a 13-storey residential block on Kotewall Road toppled and smashed its way down a slope.
The Observatory originated as a result of a typhoon that struck on September 22, 1874, killing around 2,000 people in Hong Kong – which then comprised only Hong Kong Island and the Kowloon peninsula. According to a history of the Observatory, this showed “an urgent need for an organisation that could predict the arrival of storms and issue warnings”, leading to its establishment in 1883.
As recounted in a 2017 paper titled “Uncertain skies: Forecasting typhoons in Hong Kong ca. 1874-1906”, by Fiona Williamson of Singapore Management University, the Observatory soon became involved in developing forecasting techniques, particularly for typhoons. The first director, William Doberck, published a pamphlet on typhoons, and Williamson reports that towards the end of that century, there were heightened public expectations regarding weather prediction.
One typhoon that arrived with little warning on September 18, 1906, dashed public hopes about typhoon forecasts. Though they had anticipated only a few thunderstorms, Observatory meteorologists noticed an abrupt air pressure change in the morning, and hoisted the black drum – a tropical cyclone warning – at 8am. Just 40 minutes later, the typhoon gun sounded over the harbour, sending panicked sampan and junk operators fleeing for safety. For many, it was too late: the typhoon’s onslaught, lasting under two hours, killed at least 10,000 people, many of them on boats that were sunk and smashed in Victoria Harbour. The next year, a government report on the typhoon quoted Doberck stating, “Meteorology is not an exact science. Nothing can be predicted with certainty.”
Doberck’s statement has since held true, including for one of Hong Kong’s most destructive typhoons, which swept across the territory on September 2, 1937. There was some warning, with the No 1 signal hoisted on August 31, yet it was thought the storm would pass to the south of the city, until a course change was noticed the following afternoon and the No 8 signal was raised. The situation was ripe for disaster, with Time magazine later reporting the harbour was “unusually jampacked with shipping”, including many vessels that had sought refuge following Japan’s invasion of Shanghai. Plus, the hurricane-force winds arrived after midnight, coinciding with high tide.
As a pithy report in the Waikato Times noted on September 3, 1937, “The waterfront is strewn with large and small craft which have been blown ashore. Among them are 21 seagoing vessels […] The roads are littered with the debris of houses and motor-cars.” A South China Morning Post reporter headed to Tai Po, and described “scenes of desolation”, including bodies strewn around the streets and the wreckage of buildings in Tai Po Market, as the result of a storm surge that swept in like a tidal wave. Around 11,000 people were killed.
While the Observatory stored documents on these and other natural disasters, they received little attention until weatherman doubling as history major Shun Chi-ming became director.
Shun tells of becoming fascinated by physics as a boy, and finding science, rather than philosophy, offered the most concrete approach to understanding the world. While studying physics at the University of Hong Kong, he took a course on atmospheric physics.
“I didn’t know that with physics and maths you could earn a job in the Hong Kong Observatory,” he says.
“You can apply theoretical studies to real-life situations, and save some lives.” Upon graduating, he applied to the Observatory, was accepted, and became a scientific officer.
After assignments including radiation monitoring and seismology, and a forecasting course for senior scientific officers, Shun spent six weeks in Britain with the European Centre for Medium-Range Weather Forecasts – which today operates the world’s premier computer-forecast model.
“It was around 1990, no one trusted the computer models, the results were very coarse,” recalls Shun. “I helped with international cooperation, to provide intensive observations over East Asia – and we found that with more data, the model performed better.”
Before the 1990s, the best method for forecasting a typhoon depended on measuring the speed it was moving at, along with assessing the climatology – the overall weather pattern, with consideration of the time of year. This, plus comparison with previous storms with similar weather patterns, helped predict its future movement.
“Even with AI today, there is basically this analogue process, supported by sophisticated deep learning,” says Shun. “The basic principle hasn’t changed, but there’s a big data set.”
Shun was made director of the Observatory in 2011, and soon became interested in historical storms. During a visit to Macau, he says, he viewed an exhibition on the 1874 typhoon, “in which 8 per cent of the population died”. Reading documents on storms such as those in 1906 and 1937, he became concerned about the potential for further storm surges. And though there had been no major surge since Typhoon Wanda, which had killed at least 130 people in 1962, Shun was always alert to possible dangers.
When Typhoon Hato approached in 2017, Shun had to decide if it could be another Wanda, or more akin to Hagupit – which in September 2008 passed south of Hong Kong, causing gales and a storm surge that mostly affected southern Lantau and Cheung Chau.
“I looked at 1937 and 1936 typhoons, which were of similar strength. The 1936 typhoon track overlapped the track of Hato, and was further south than Wanda,” says Shun. “So I decided it would be more like Hagupit.” Though Shun issued the No 10 signal, and hurricane-force winds were accompanied by a substantial storm surge, Hong Kong was spared a Wanda-like assault.
A year later, Shun and colleagues became far more concerned as Super Typhoon Mangkhut formed over the western North Pacific, and took aim at Hong Kong. Their computer model indicated the potential for an immense storm surge, raising water levels by more than five metres in Victoria Harbour, and an additional two metres in Tolo Harbour: no one at the Observatory had experienced anything like this. Looking for historical comparisons, Shun figured it would be similar to the 1937 typhoon or 1962’s Wanda. “It was stressful making this decision,” he says, but “once I said it could be like Wanda, there was no turning around.”
The Observatory started alerting government departments a week before Mangkhut’s predicted arrival, and held press conferences three and two days in advance. “During Mangkhut, I worked over 40 hours, with just one or two hours’ sleep,” says Shun. “It was really exciting; I didn’t want to miss it.”
The enhanced Observatory staff included two forecasters rather than one, and a social-media team that disseminated information and showed Shun images coming in as the surge flooded homes in Sai Kung and giant waves pounded a Heng Fa Chuen housing estate on eastern Hong Kong Island.
Though the government reported more than 458 people injured, tens of thousands downed trees and a 3.88-metre surge in Victoria Harbour, akin to that during Wanda, Shun considers Hong Kong lucky, having been helped by Mangkhut’s brush with north Luzon, in the Philippines, reducing the typhoon’s intensity, and low tide coinciding with the storm’s closest approach.
Shun, who retired from the Observatory in 2020, recalls the stress of issuing typhoon signals. “You need to be careful in striking the right balance,” he says. “It’s a multimillion-dollar or even multibillion-dollar decision.” He recalls issuing the No 8 signal for Typhoon Linfa in 2015, when “nothing happened”, but has no regrets. “It’s better to be safe than sorry.”
Currently, Shun is helping the Hong Kong University of Science and Technology establish a climate centre focused on adaptation and resilience, in preparation for more extreme typhoons as global temperatures continue to rise. “We’re gathering historical climate information to develop a detailed model,” he says, “such as showing the places in Hong Kong with the heaviest rain and highest temperatures.”
Given that typhoons are powered by warm ocean waters, it might seem likely that as the Pacific waters heat up, the number of typhoons will increase. Yet the true picture is not so simple. Even forecasting the number of typhoons in a given year remains far from an exact science. This May, StormGeo – which focuses on weather services for businesses – anticipated below normal typhoon activity in 2024, with perhaps 12 in all. The company’s senior meteorologist, Nancy Lin, cautions against taking this number as more than a curiosity, noting, “The primary objective of predicting the number of typhoons or hurricanes is to facilitate proactive planning, especially for weather-sensitive industries like shipping and offshore wind.”
Johnny Chan Chung-leung, science director of the Asia-Pacific Typhoon Collaborative Research Center in Shanghai and an emeritus professor at City University of Hong Kong, has spent four decades studying extreme weather, and is one of the world’s leading experts on typhoon science.
“El Nino is the most important influence on annual numbers of typhoons, but doesn’t explain everything,” he says. “Other factors include sea-surface temperatures in the Indian Ocean, and decadal changes.”
Chan has helped develop a system that he rates “quite good” for predicting the number of typhoons making landfall on the South China coast. “If water temperature was the only factor, the frequency would increase; but there are other things happening.”
Most studies of the area – including those by Chan – suggest a reduced tendency for clusters of clouds to begin rotating en masse, as required for the earliest stages of typhoons, hence there is a decrease in numbers of tropical cyclones. However, once typhoons develop, they are likely to be more intense than in previous decades.
Even as nascent storms are detected, computer models set to work on predicting their movements.
“If you look at track forecasting, the errors have decreased substantially in the last 20 years,” says Chan. “But there are some tracks that differ significantly from what is observed, and we need studies on why such predictions are so bad.
“Intensity forecasts haven’t really improved for about 20 years – especially for typhoons about to make landfall. That’s a worrying thing for the South China region. We don’t know all the physics behind rapid intensification, and models can’t capture the situation very well.”
This year, Chan was part of a team that published research linking stronger typhoons to warmer temperatures over the Tibetan Plateau, as a result of decreased vertical wind shear. “A typhoon is a three-dimensional thing, rising very high in the atmosphere, and vertical wind shear is the change of regional wind speed and direction with height,” says Chan. “With little shear, the typhoon is like a vertical cylinder, and can be very strong. But with stronger vertical shear, the cylinder becomes tilted, and there’s less likelihood of high intensity.”
While others had reported the decrease in wind shear, this was the first study to explain the underlying cause, as relatively high altitude winds from the warmer Tibetan Plateau can modify the atmospheric circulation over the Pacific.
Chan adds that the amount of rain from typhoons has also increased, particularly in coastal cities. This is partly because a warmer atmosphere can hold more moisture, but also results from warm air rising over built-up areas that are heated by the sun, and contain other sources of heat, such as air conditioners and traffic. Plus, pollution results in more airborne particles, around which water condenses, forming small drops that rise then agglomerate, freeze and fall as hail and intense rain.
These factors also explain why rainstorms are becoming more severe. The hourly rainfall record for Hong Kong was mostly below or just over 100mm until 2008, when it reached 145.5mm. Then there was a further jump to 158.1mm on September 7 last year.
And as wetter and wilder weather events lie ahead, Shun believes it is still useful to look back in time. “Another historic typhoon I’m interested in was in 1862, hitting Guangzhou and the west part of the Pearl River Delta,” he says. “While there were different estimates, perhaps 80,000 people were killed. It’s something for the Greater Bay Area to take a look at.”
As we go to press, the monsoon rains have mostly shifted north of Hong Kong, and the west Pacific is peaceful. Yet any day now, thunderstorms may begin clustering over the Pacific or the South China Sea, and they may begin spinning, gently at first, then faster, as a central vortex develops. And you may find yourself again on a Hong Kong beach, under a cloying sky, as the waves surge. And you may ask yourself, is this, already, another extreme weather event?
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