Tropical Cyclone Warning Signal No.5 in the Past
In the wake of Severe Typhoon Vicente, there were some press reports which suggested re-using the No. 5 signal as in the past. Let us have a quick review of the tropical cyclone warning signal system at present and in the past.
The Observatory has operated a numbered tropical cyclone warning signal system since 1917 to alert the public of the threat of high winds brought by tropical cyclones. The current signal system consists of the No. 1, 3, 8, 9, and 10 signals with their meaning given in Table 1. In essence, the No. 3 signal warns about strong winds (i.e. wind strength of 41-62 km/h), the No. 8 signal warns about gale or storm force winds (i.e. wind strength of 63-117 km/h), and the No. 10 signal warns about hurricane force winds (i.e. wind strength of 118 km/h or above). The descriptive terms of wind speeds, e.g. strong, gale, storm and hurricane, are based on the Beaufort scale (Table 2) which is widely adopted internationally. In general, a higher signal implies higher impact to society.
Prior to 1973 , the signals consisted of the No. 1, 3, 5, 6, 7, 8, 9 and 10, of which the No. 5, 6, 7 and 8 all warned about gale or storm force winds, coming from the directions of NW, SW, NE, SE respectively (Table 3). In order to strengthen the public understanding that all these four signals warned about the same wind strength, these signals were replaced by 8NW, 8SW, 8NE and 8SE respectively in 1973. This could avoid some misconception in the past, for example that the No. 7 signal was more severe than the No. 5 signal.
As a simple illustration, the following signals were issued for Vicente in chronological sequence: 1, 3, 8NE, 9, 10, 8SE, 3, 1. Under the old system, the signals would have been: 1, 3, 7, 9, 10, 8, 3, 1. In this case, there could be questions like: would the winds be less strong when the No. 7 was issued compared with the No. 8? Is there less urgency for us to return home when the No. 7 was issued compared with the No. 8? The answers to these questions are of course "No" for those familiar with the warning system, but perhaps not self-evident to every person on the street.
People may also ask: how about the warning signals of our neighbouring regions? In Macao, the warning signal system is more or less the same as in Hong Kong. In Guangzhou and Shenzhen, colour-coded systems instead of numbered systems are used, for example: the Blue signal warns about strong winds, the Yellow signal warns about gale force winds, the Orange signal warns about storm force winds, and the Red signal warns about hurricane force winds. Again, these signal systems are based on the Beaufort scale of wind speeds and there is no intermediate signal between the strong wind warning and the gale warning. Similar tropical cyclone warning systems based on the Beaufort scale of wind are also utilized in other countries including the United States, Australia and Fiji.
In Hong Kong, to allow employers sufficient time to release their employees in stages according to the work requirements so as to ensure safe trips back home and to minimize the stress on the public transport system, the Observatory normally issues the "Pre-No. 8 Special Announcement" once the No.8 Signal is expected to be issued within two hours. The Labour Department also issued a code of practice to provide advice and practical guidelines on work arrangements in times of tropical cyclones.
The above is only a very brief review of the tropical cyclone warning system at present and in the past. Further information on the history of the system, including the use of drum, ball and cone as well as the typhoon gun in the early years, can be found at the Observatory's webpage: http://www.hko.gov.hk/informtc/tcsignal_history.htm. We hope that this would help people better understand the signals so that we can all be better prepared next time a tropical cyclone hits.
After all, despite having fell a record-breaking number of trees in the past couple of decades, Severe Typhoon Vicente could only be considered as one of the weakest No. 10 cases if we compare its local wind records with those in the past No. 10 occasions (see press release on Vicente at: http://www.hko.gov.hk/press/SP/pre20120727e.htm). The revisit of the No.10 signal in 13 years reminds us of the severe impacts that tropical cyclones can bring about. Let us remain vigilant in the future, as more intense tropical cyclones are likely as global warming progresses.
John YK Leung and WH Lui
 Earlier history of the tropical cyclone warning system prior to 1956 can be found at the Observatory's webpage:
 The code of practice by the Labour Department can be found at http://www.labour.gov.hk/eng/public/wcp/Rainstorm.pdf
Table 1. The current tropical cyclone warning signal system operated by the Hong Kong Observatory
Table 2. Descriptive terms of wind speeds
Table 3. The tropical cyclone warning signal system operated by the Hong Kong Observatory between 1956 and 1972