Beaufort wind scale is an integral part of Hong Kong Observatory’s storm warning system

PUBLISHED : Monday, 14 August, 2017, 5:45pm
UPDATED : Monday, 14 August, 2017, 9:41pm

I refer to the letter by Peter Mallen (“Storm warning system should be updated”, July 27) on Hong Kong’s tropical cyclone warning system and I thank him for his praise of our work at the Observatory.

I would also like to respond to the question on the use of the Beaufort wind scale in the tropical cyclone warning system and the issue of closing down the city under the No 8 signal.

While the Beaufort wind scale was developed for mariners back in 1805, it has been revised several times and is currently widely used for weather reporting on land and at sea. This scale, which relates wind force description with wind speed (for example, “strong” force in scale six and seven corresponds to a wind speed of 41 to 62 km/h), is adopted by the UN World Meteorological Organisation (WMO), including its “Global Guide to Tropical Cyclone Forecasting”.

Precautions and emergency responses to high winds vary from one city to another, depending on the city’s exposure and vulnerability to hazards, which in turn is related to the city’s demographic distribution and infrastructure. Hong Kong is densely populated and highly built-up with high rises everywhere.

Loose objects, like temporary scaffolding, shop signs and flower pots, can be blown away in gales and land on busy roads. Trees can be blown down by strong winds, not to mention gales. Debris can damage glass panels, causing more debris to fall to the ground. In a crowded city like Hong Kong, the threat of gales under the No 8 signal cannot be underestimated.

Another threat is storm surge. Being on the coast, Hong Kong is vulnerable to sudden rising water in a storm surge brought by high onshore winds associated with the rotating storm.

When this happens, low-lying areas near the shore will be submerged – this could be life-threatening and requires evacuation. Severe Typhoon Hagupit in 2008 is an example.

The century-old tropical cyclone warning system is a proven system, which is understood by the public.

It has been demonstrated that it is highly effective in reducing casualties and fatalities. It has been subject to periodic revisions, the last one being an extension of the warning criteria to cover the whole territory, instead of just Victoria Harbour, in 2007.

The Observatory will continue to seek improvements to ensure that the system will evolve with the changing society while ensuring public safety.

C. M. Cheng, assistant director, Hong Kong Observatory