Scientists fooled by rapid growth of storm's ferocity

Typhoon Vicente took most people by surprise with its hurricane-force winds - not least atmospheric scientists puzzled by how it intensified in such a short time and distance.

'The typhoon's action was beyond the expectation of many scientists watching it,' said Professor Johnny Chan Chung-leung, dean of the School of Energy and Environment at City University.

'Even the United States, Japan and China didn't get it right this time. And everyone is discussing it now.'

A chair professor of atmospheric science, Chan said Vicente went through a rare 'rapid intensification' as it was developing in the South China Sea, transforming from a tropical depression into a severe typhoon within two days.

Chan said Vicente might have encountered a warm pool of water in the sea or there might have been some atmospheric flow pattern changes that gave rise to such rapid transformation. But more analysis was needed to find out the causes.

'There have been many advances in predicting the movement of a typhoon, but little progress in how to predict changes in intensity,' he said. Chan believed the typhoon, with a near-centre speed of 155km/h, brought hurricane force winds as it edged close to western Hong Kong.

He agreed this shattered some public misconceptions about the link between hurricane force winds and a 'direct hit'.

Records of approaches since 1971 showed that a direct hit by a typhoon was not a necessary condition for raising a typhoon signal 10.

Only typhoon Rose in 1971, Hope in 1979 and York in 1999, landed right, or partially, over Hong Kong. York was the last time the No 10 signal was raised.

Typhoon Ellen in 1983 headed directly towards the Pearl River Estuary and made landfall near Macau and Zhuhai. In 1975 Elsie took a very similar path to Vicente.

Both originated in the western Pacific and travelled northwestwards through the gulf between Taiwan and the Philippines. Both also landed in western Guangdong.

While the typhoon's approach is no guarantee of a hurricane signal, the Hong Kong Observatory relies on measuring wind speed to determine the need for escalating the signal.

Under its rule, when sustained wind speed hits more than 118km/h in some of its weather stations in the city, the hurricane signal or the No 10 signal is hoisted.

During the hour to 11.30am on Monday night, Ngong Ping and Tate's Cairn were already recording mean wind velocities of 156km/h and 125km/h.

Several other places like Cheung Chau and Tai Mei Tuk had wind speeds of more than 100km/h. In Ngong Ping, one gust of wind was measured at 255km/h.

Vicente is the 14th typhoon requiring the hoisting of the hurricane signal since 1946.

While the typhoon, classed for a brief two hours as a hurricane, injured about 130 people and uprooted a thousand trees, it caused less damage than York.

That typhoon sank a cargo ship, caused a blackout affecting 60,000 residents and killed two people during its 11-hour life as a hurricane.