Hong Kong tropical cyclone warning signals
The Hong Kong tropical cyclone warning signals (Chinese: 熱帶氣旋警告信號), or informally typhoon signals (Chinese: 颱風信號), are a set of signals used to indicate the threat or effects of a tropical cyclone. The Hong Kong Observatory issues the warning signal if a tropical cyclone is centred within 800 kilometres (500 mi) of Hong Kong and may affect Hong Kong later.
The signals are represented as a set of numbers and symbols. Previously lights are also used at night.
The Hong Kong Observatory began issuing warnings for tropical cyclones in 1884, when it began to monitor and report news of tropical cyclones within the South China Sea, and the areas around Guam and Manila. The monitoring efforts later expanded to gathering information from various seafaring vessels. These works were done in conjunction with the Marine Department until 1886. At first, the monitoring and warning services were geared mainly towards seafarers, and not towards the residents of Hong Kong.
The forefathers of the warning signals that Hong Kong residents are familiar with today began in 1884, when a numerical warning system, consisting of symbols that are a combination of drums, balls, and cones was hoisted at various places in Victoria Harbour. At first, the signals only gave information as to which cardinal direction winds from the tropical cyclone are coming from. Revisions of the system in 1904 added alerts as to which ordinal directions the winds are coming from. Until 1917, there were two sets of warning signals, one set with black symbols, and one with red. The black symbols indicate that the tropical cyclone in question is less than 300 miles (480 km) from Hong Kong, while the red symbols indicate that the tropical cyclone is over 300 miles (480 km) away from Hong Kong.
The alert system was also supplemented by a custom of firing guns (later cannons and bombs, whose louder sounds were considered an improvement) when a tropical cyclone hits Hong Kong, which was the system most Hong Kong residents at the time rely on for severe weather updates, as a scant few pay attention to the warning system that was intended for seafarers. The frequency of the firing was also an indication of how intense the tropical cyclone was, with three being the indication of a very intense cyclone. This custom ended in 1937, as the local residents began to rely on radio, newspapers, and notices at ferry piers for tropical cyclone information.
A system that partially resembles the modern warning system was devised in 1917, after a meeting in Japan on the unification of cyclone warning standards failed to come to an agreement. Influenced by that meeting, the Observatory reformed its existing system, with the warning consisting of 7 levels (4 of which ultimately became the 4 no. 8 signals that Hong Kong residents are familiar with today). The system was marketed to other countries along the South China Sea, to no avail. The system was later repurposed for domestic use, but by request of Hong Kong's Chamber of Commerce, another set of warning signals intended for tropical cyclones outside of Hong Kong was also used by the Hong Kong Observatory. The practice of maintaining two distinct sets of warning systems, with different purposes, persisted until 1962.
Eventually, over the years, the storm signals deviated from their original purpose of serving mariners, and became a system for the domestic population. The 10-level warning system was eventually slimmed down to the 5 level system that is in use today, with signals 5, 6, 7, and 8 consolidated into the current four signal 8s.
In the past, the signals were physically hoisted at many locations in Hong Kong; there were 42 signal stations around the territory in the 1960s. However, as radio and television weather reports became increasingly effective, the need to hoist physical signals diminished. The last signal station, Cheung Chau aeronautical meteorological station on Cheung Chau, was decommissioned on 1 January 2002. Accordingly, the observatory has replaced the word hoist with issue in its official terminology, although the phrase "Signal No. __ has been hoisted" is still widely used by the public and the mass media. Weather authorities in Macao, however, still use the term "hoisted" when issuing their tropical cyclone warnings, the system of which is based heavily upon Hong Kong's.
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 
In accordance with legal codes and common practices in Hong Kong, once any signals higher than No. 3 are issued, all government agencies will shut down their operations. Schools will immediately cease their sessions, as well as the financial markets and a majority of the private sector. Non-essential workers will be released from work in a staggered manner as to avoid overwhelming public transportation. Public transit agencies that operate on the sea or on surface streets and highways will continue to operate but may cease operation on short notice. At the same time, employers who require those to work during the No. 8 signal must provide a safe work environment and shelter after work should transportation be unavailable. Such arrangements must be worked out beforehand between the employer and employee. In an effort to minimise the potential disruption, the Hong Kong Observatory usually gives the territory an advance warning of about two hours before actually issuing Signal No. 8.
The system was originally intended to serve as warnings for mariners, the issuance of signals was based on wind speeds measured around the Victoria Harbour. However, the system has also been adopted for use by the public over the years, and with ongoing urbanization, the system of using Victoria Harbour as a reference point for the issuance of signals had it limitations. Hong Kong's topography, urban environment, and the relocation of Hong Kong International Airport required the need for more regionally based wind information. This was evident in the criticism of the Hong Kong Observatory during Typhoon Prapiroon in 2006 when considerably more severe conditions occurred in urban areas outside of Victoria Harbour while the No. 3 signal was in effect. In response, the Hong Kong Observatory, from the tropical cyclone season in 2007, the issuance of the No 3. and No 8. signals would be based on a network of 8 near sea-level reference anemometers over Hong Kong.
Meaning of signals
The Saffir–Simpson hurricane wind scale is a classification used for some Western Hemisphere tropical cyclones. However Hong Kong has a similar official five-level definition of the signals as shown in the table below.
The highest level, No. 10, is rarely issued. As of 24 July 2012, there have been only 13 No. 10 warnings in the last 65 years, and only two in the last 28 years (Typhoon York in 1999, and Typhoon Vicente in 2012). Prior to 1983, No. 10 warnings occurred at more frequent intervals of four years or less.
|Sustained Wind Speed
(km/h, (mph, Beaufort scale))
(No longer in use)
|1||Stand-by (戒備)||N/A||N/A||A tropical cyclone is centred within 800 kilometres of Hong Kong and may later
affect the territory, or there are strong winds in Hong Kong waters
|3||Strong winds (強風)||41 – 62 (26–37, Beaufort Force 6–7)||may exceed 110 (69)||Strong winds are expected or blowing generally in Hong Kong near the sea level, and the wind condition is expected to persist|
|8 NE||Gale or storm force winds
|63 – 117 (38–73, Beaufort Force 8–11)||may exceed 180 (113)||Gale or storm force winds are expected or blowing generally in Hong Kong near the sea level from the NE quadrant, and the wind condition is expected to persist|
|8 NW||Gale or storm force winds are expected or blowing generally in Hong Kong near the sea level from the NW quadrant, and the wind condition is expected to persist|
|8 SE||Gale or storm force winds are expected or blowing generally in Hong Kong near the sea level from the SE quadrant, and the wind condition is expected to persist|
|8 SW||Gale or storm force winds are expected or blowing generally in Hong Kong near the sea level from the SW quadrant, and the wind condition is expected to persist|
gale or storm force winds
|88 – 117, increasing (55–73, Beaufort Force 10–11)||N/A||Gale or storm force winds are increasing or expected to increase significantly in strength.|
|10||Hurricane (颶風)||>118 (74+, Beaufort Force 12)||may exceed 220 (138)||Hurricane force winds. Eye of typhoon may be passing directly over Hong Kong.|
In Macau, the territory's Meteorological and Geophysical Bureau maintains a very similar system. The bureau has maintained the practice of hoisting the warning signals (as well as its nighttime light signals), even as Hong Kong abandoned the practice in 2002. The signals are hoisted at Guia Fortress and the Fortaleza do Monte.
- "Modernisation of The City" (in Chinese and English). Hong Kong Observatory. 2003. p. 68. Retrieved 2008-08-24.
- "History of the Hong Kong Tropical Cyclone Warning Signals". Hong Kong Observatory.
- "Code of Practice in times of Typhoons and Rainstorms". Labour Department.
- "Related Advisory Information and Arrangements from Bureaux and Departments in Tropical Cyclone Situations". Hong Kong Observatory.
- "Regional Wind Information". Hong Kong Observatory.
- "Review of the Tropical Cyclone Warning System in 2006 and New Measures in 2007". Hong Kong Observatory.
- "Meaning of Tropical Cyclone Signals and the relevant recommended safety precautions". Macao Meteorological and Geophysical Bureau. Retrieved 2013-07-15.