Hong Kong's Tropical Cyclone Warning Signals
Tropical Cyclones : Tropical cyclones normally occur during the months of May to November, and are particularly prevalent during September.
Tropical cyclone advisory bulletins and/or warnings are issued by the Hong Kong observatory whenever a tropical cyclone centred within 800 km of Hong Kong poses a threat to the territory.
These advisory bulletins include the tropical cyclone warning signal issued and its significance, the latest position and expected movement of the centre of the tropical cyclone, information on the wind strength, rainfall and sea level in the territory and advice on precautionary measures.
The weather in different parts of Hong Kong cannot be simply inferred from the signal issued. Simply knowing what signal is issued is not enough. You should take note of the latest tropical cyclone information and related announcements broadcast on radio and TV, and given in the Hong Kong Observatory's Internet website(http://www.hko.gov.hk and http://www.weather.gov.hk) and Dial-a-Weather system (Tel. No.: 1878 200) to decide on the actions to take in response to the signal issued.
Tropical cyclone signals are to warn you of the threat of winds associated with the tropical cyclone. Owing to local topographical conditions or the presence of buildings nearby, winds at your locality may be substantially different from the general wind strength over Hong Kong. Winds are often stronger over offshore waters and on high ground. Winds are less strong in areas sheltered from the prevailing wind direction. The Hong Kong Observatory provides to the public detailed information on regional wind and rain through a diversity of channels, especially the Internet. Members of the public should consider their own circumstances and level of acceptable risk when taking precautions in response to warnings.
For your own safety, you should stay alert at all times and be prepared to deal with changing conditions at short notice. Remember that areas that may have been previously sheltered may quickly become exposed.
Neither should you relax precautions when the tropical cyclone is moving away from Hong Kong, as high winds are likely to persist for some time. You should stay indoors where it is safe until winds moderate.
Information on other hazards associated with the tropical cyclone will be included in warning bulletins. When necessary, separate warnings of heavy rain, flooding and landslips will be issued.
History of the Hong Kong Tropical Cyclone Warning Signals : Starting from 1884, a system of drum, ball and cone was employed to give information to the mariners in the harbour on the existence and approximate location of a tropical cyclone. For the local public a typhoon gun was used to warn imminent gale force winds brought about by tropical cyclones. In 1907, explosive bombs replaced the typhoon gun as they made louder sounds and were considered an improvement over the firing of a gun. The last typhoon boom was exploded in 1937.
In 1917, the first numbered signal system geared to the warning of wind conditions in Hong Kong was introduced. The numbers were from 1 to 7 with numbers 2 to 5 signifying gale force winds expected from the four quadrants, namely N, S, E and W.
In 1931, the signals were amended to 1 to 10 with signals 2 and 3 signifying strong winds from SW and SE respectively, signal 4 being a non-local signal, signals 5 to 8 signifying gales from the four quadrants, namely NW, SW, NE ad SE, signal 9 signifying increasing gales and signal 10 indicating the threat of hurricane force winds. Signals 2, 3 and 4 were used intermittently afterwards and were discontinued in the late 1930s.
In 1956, the No. 3 Strong Wind Signal was introduced between the No. 1 Stand-by Signal and the gale signals.
Starting from 1 January 1973, signals 5 to 8 were replaced by 8 NW, 8 SW, 8 NE and 8 SE respectively so as to avoid misunderstanding by the public. This system has been in use ever since.
Originally, the signals were intended mainly for the benefits of mariners but have over the years been also adopted for use by the public. Starting from 1987, the Observatory issued the Pre-No.8 Special Announcement to give an advance notice to the public when the No.8 signal is expected within two hours.
Meaning of Tropical Cyclone Warning Signals and What you should do :
This is a stand-by signal, indicating that a tropical cyclone is centred within about 800 km of Hong Kong and may affect the territory.
Action - If you are planning an outing, remember that there is a tropical cyclone near Hong Kong which may affect your plans. Beware that strong winds may occur over offshore waters. Listen to radio and TV broadcasts or browse the Observatory website on the progress of the tropical cyclone.
Strong wind is expected or blowing generally in Hong Kong near sea level, with a sustained speed of 41-62 km/h (kilometres per hour), and gusts which may exceed 110 km/h, and the wind condition is expected to persist. Winds are normally expected to become generally strong in Hong Kong within 12 hours after the issue of this signal. Winds over offshore waters and on high ground may reach gale force.
Action - Secure all loose objects, particularly those on balconies and rooftops. Flower pots and other objects likely to be blown away should be taken indoors. Secure hoardings, scaffoldings and temporary structures. Drains should be cleared to avoid blockage and overflows. Stay away from the shoreline and not to engage in water sports. Fishing vessels should seek shelter without delay. Listen to radio and TV announcements and browse the Observatory website for further information about the tropical cyclone.
Gale or storm force wind is expected or blowing generally in Hong Kong near sea level, with a sustained wind speed of 63-117 km/h from the quarter indicated and gusts which may exceed 180 km/h, and the wind condition is expected to persist. Winds are normally expected to reach gale force generally in Hong Kong within 12 hours after the No.8 signal replaces the No.3 signal.
Action - Complete all precautions now before gales commence. Lock all windows and doors. Fit bars into positions and insert reinforced shutters and gates if available. Adhesive tape fixed to large window panes in exposed positions will reduce damage by broken glass.
Do not stand near windows on the exposed side of your home. Move all furniture and valuables away from these areas. Make sure you have a safe place to shelter, should windows be broken. Now is the time to decide which rooms you will use to shelter if the windows on the exposed side of your home become broken.
Owners of neon signs should arrange for the electricity supply to their signs to be switched off.
Park your car where it is least likely to be damaged.
Avoid staying in the street. Return home as soon as possible if conditions so permit.
Gale or storm force wind is increasing or expected to increase significantly in strength.
Action - Stay indoors. Stay away from exposed windows and doors to avoid flying debris. Close all interior doors and make sure children are confined to the least exposed part of your home. Do not touch electrical cables that have been blown loose. You should fix broken windows and doors only when there is no danger in doing so.
If you are away from home, find a safe place and remain there until the danger is over.
Hurricane force wind is expected or blowing with sustained wind speed reaching upwards from 118 km/h and gusts that may exceed 220 km/h.
Action - The same precautions as above apply.
Remember that if the eye of the typhoon passes directly over Hong Kong, there may be a temporary lull lasting a few minutes to several hours. Do not relax your guard, as there will be a sudden resumption of violent winds from a different direction. Remain where you are if protected and be prepared for destructive winds.
Reference for the Issue of No.3 and No.8 Signals
Starting from 2007, the reference for the issue of No.3 and No.8 signals expanded from the Victoria Harbour to a network of eight near-sea level reference anemometers covering the whole of Hong Kong as depicted in the following figure.
Network of reference anemometers in the tropical cyclone warning system
The reference anemometers were selected on account of their good exposure and geographical distribution, taking into account the natural separation by Hong Kong mountain ranges. Together, they provide a broad picture of the wind condition in Hong Kong.
The No.3 or No.8 signal, as the case may be, will be issued when half or more anemometers in the reference network register or are expected to register sustained strong winds or gale/storm force winds and the wind condition is expected to persist. The wind speed range of the No.3 signal is 41-62 km/h and that of the No.8 signal is 63-117 km/h.
When a tropical cyclone is located relatively far from Hong Kong, occasionally an outer rainband associated with the tropical cyclone might sweep across the territory, bringing squalls which cause transient winds at four or more of the reference anemometers exceeding the prescribed lower thresholds for the issue of No.3 or No.8 signals. In that case, as the wind condition is not expected to persist after the passage of the rainband, the Observatory would not issue the respective signals.
Regional Wind Information
Due to local topography and the built environment, wind condition in different parts of Hong Kong can vary appreciably. A numbered tropical cyclone signal can provide a general warning for the public, but it has an inherent limitation in communicating varying wind speeds at different locations. The public should take note of the regional wind condition and take appropriate response actions.
Currently, the Observatory provides information on regional wind condition on its website (http://www.weather.gov.hk or http://www.hko.gov.hk) and through the Dial-a-Weather service (telephone: 187 8200). It also has special arrangements with individual sectors, such as the marine and aviation sectors, to meet their specific needs. Starting from 2007, the Observatory further enhances the dissemination of regional wind information by highlighting in media broadcast of its tropical cyclone bulletins those areas with wind speeds significantly higher than the general wind condition of Hong Kong. A webpage showing the regional distribution of wind strength was also added to the Observatory website with strong and gale force winds presented in an easily comprehensible graphical form. A description of how winds of different forces manifest themselves on land and over the sea is also available on the webpage (http://www.weather.gov.hk/wxinfo/ts/wind_gale_e.htm).
Considering that aircraft operations at Chek Lap Kok are mainly affected by crosswinds (i.e. winds blowing towards the aircraft from the side) and the associated windshear and turbulence rather than wind speed per se, in situations when the wind conditions likely to cause significant disruptions to air traffic are expected, the Observatory will add an advisory to its tropical cyclone bulletin that the traveling public check with airlines before departing for the airport commencing 2007. The public should listen to the radio or TV broadcasts or browse the Observatory website for information on the tropical cyclone bulletins.
Work and School Arrangements in Tropical Cyclone and Rainstorm Situations
Code of Practice in times of Typhoons and Rainstorms (Labour Department)
Tropical Cyclones and Heavy Persistent Rain Arrangements for Kindergartens and Day Schools (Education Bureau)
Tropical Cyclones and Heavy Persistent Rain Arrangements for Evening Schools (Education Bureau)