Cheung Chi-fai – Updated on Mar 06, 2009 – SCMP
A collision between warm air and a mass of cold air from the north resulted in the earliest yet rainstorm warning yesterday.
The Observatory issued amber rainstorm signals twice after a thunderstorm brewed across the border, moved down to Hong Kong and abruptly transformed the weather from soggy and cloudy into rainy and windy.
The rainstorm signals lasted from 2pm to 3pm and from 4.10pm to 5.10pm and brought about 30mm of rain in northwest New Territories, where gusts reached 90km/h.
The wind blew a cargo container from a stack of cargo boxes at a storage site in Yuen Long. It crashed to the ground next to a truck but no one was injured. Firemen rescued six construction workers who were trapped in a crane that had broken down under the wind. One worker suffered a minor hand injury.
The Observatory forecast that the weather would remain unstable today, with occasional heavy rain and thunderstorms. The temperature is expected to fall to as low as 15 degrees Celsius.
Chan Chik-cheung, senior scientific officer at the Observatory, said the rainstorm warning was the earliest they had issued since the warning system was introduced in 1998. The previous record was set on March 23, 2002. Mr Chan said such collisions were common during the transition from winter to spring.
“We had warmer than usual weather in February, and when the cold air meets the warm air it creates a strong convection of air currents so that warm air is forced upwards quickly and rainstorms are formed.”
He said the weather developed so quickly that staff had no idea the second storm was forming when they cancelled the first warning.
Johnny Chan Chung-leung, director of the Guy Carpenter Asia-Pacific Climate Impact Centre at City University, suspected that winters in Asia had changed in recent years.
He said it seemed that over the past few years, a cold month in winter was often followed by a warm month and then a cold month again.
But more studies were required to find out if this happened to be a long-term trend.
“The frequency of changes has increased, though it is hard to tell now if it is related to global warming or it is just a pattern of regional climate shift,” Mr Chan said.