Hong Kong tropical cyclone warning signals are issued by the Hong Kong Observatory to warn of a potential threat or effects of a tropical cyclone in the greater Hong Kong area. The signals are represented with a set of numbers and symbols. Previously, lights were also used at night. The warning system currently in use in Hong Kong is based on a signal level from the lowest level, Hurricane Signal No.1, to the highest level, Hurricane Signal No.10. The signaled numbers may change in accordance with the conditions. Once any signal that is higher than No.3 is issued, government agencies, schools, financial markets, and most of the private sector close their operations. During major storms like Typhoon Hato in 2017 and Typhoon Mangkhut in 2018, public transit was suspended and schools and businesses were closed.

History

Beginning

In May 1884, the Hong Kong Observatory began warning of surrounding tropical cyclones by notifying newspapers and telegraph companies. Later that year, the observatory implemented the first visual warning system in August. It used an arrangement of red drums, cones, and balls to indicate the presence of a tropical cyclone around Hong Kong and its relative position. The signal was hoisted on a mast in front of the Tsim Sha Tsui Police Barracks. Sailors primarily used it to help plot their course. The signal did not indicate any threat of a typhoon directly impacting Hong Kong. In August 1884, the Hong Kong Observatory began using a "typhoon gun" to warn residents of hazardous wind conditions. One shot was fired to indicate when strong gales were expected. Two were fired when hurricane-force winds were expected and was fired again each time the wind changed direction. The typhoon gun was fired on 21 August 1884, although no gale-force wind was recorded at the observatory. Initially, the same gun was also used to announce mail deliveries from London. This caused confusion for residents, so in 1886, the use of guns for mail deliveries was discontinued.

Revised visual signal

In 1890, a black color began to be used with the drum, cone, and ball visual signal to indicate the distance of a storm from Hong Kong. Red continued to be used for storms at least 300 km away, while black was used for storms closer than 300 km. Night signals using lanterns were introduced in late 1890. Warnings were provided by the firing of the typhoon gun and the night signals, while other signals provided information on the positions of surrounding tropical cyclones. Starting around 1898, the visual signals (both day and night) began to be raised at the Godown Company in Kowloon and (by day only) at the Harbour Office in Sheung Wan and on H.M.'s Receiving Ship. In February 1897, the storm signals created by Admiral Robert FitzRoy in 1861 were introduced in Hong Kong with a minor modification. The modified storm warning system consisted of storm signals in the form of a cone or drum and night signals. A cone pointing upward was hoisted to warn of gales from the north or east, while a cone pointing downward warned of gales from the south or west. A drum was added to the cone when a strong gale that might reach hurricane force was expected. The night signal consisted of three differently-colored lanterns hung on a triangular frame, pointing upwards or downwards. No lanterns were hoisted to represent the drum.

Committee suggestions

In January 1898, at the suggestions of the Committee of the Chamber of Commerce, the signals were reverted to the system that had been in use in Hong Kong from 1884 to 1896, as the original system was considered to be better interpreted. After the 1906 Hong Kong typhoon, a committee decided to abolish the typhoon gun in 1907. Instead, a system of explosive bombs began to be used. A black cross was also hoisted at the same time to indicate winds of hurricane force. The night signals were changed to a series of three vertical lights in green and red. Additional signals using a cone were hoisted at nine outlying stations, including Waglan Island, Gap Rock, and Aberdeen, to inform passing ships that storm signals were hoisted in Victoria Harbour.

First Numbered System

In 1917, Hong Kong first began using numbered signals. The new system consisted of seven signals (No.1–7). Signal No.1 was used as a standby signal, Signals No.2–5 were respectively used for gales coming from each cardinal direction (north, south, east, and west), Signal No.6 for gales of increasing force, and Signal No.7 for hurricane-force winds. The hurricane signal was accompanied by three explosives fired at the Tsim Sha Tsui police barracks and Harbour Office. The local signal code was revised on 1 March 1931 following the recommendations at the Conference of Directors of Far Eastern Weather Services in 1930. The signal system was extended to ten signals (No.1–10), although Signal No.4 was only used in the Philippines but not in Hong Kong, as the non-local signals covered the information it conveyed. In 1935, the signal system was revised as agreed between the Observatory and the Central Weather Bureau of Manila. In the revised system, Signals No.2–4 were not used in Hong Kong, while Signal No.9 was not used in Manila. The last typhoon bomb was set off in September 1937 during the passage of an intense typhoon. After the Second World War, the previous tropical cyclone warning system was reinstated.

"Local Strong Wind Signal"

A conference on Storm Warning Procedures held in Manila in May 1949 decided to bring the black ball into use in Hong Kong as the international warning signal for strong winds on 1 January 1950. This "Local Strong Wind Signal" aimed to warn small aircraft of the onset of strong winds that were not expected to reach gale force. The Local Strong Wind Signal was occasionally hoisted to warn of the winds associated with tropical cyclones between 1950 and 1956. In some cases, it was also followed by gale warnings (i.e., Signals No.5–8). Moreover, the Director of the Observatory reported in the Annual Report that owing to the lack of weather information from the mainland, it was very difficult to use the signal effectively for giving warning of strong monsoon winds in winter. With effect from 15 April 1956, the Strong Monsoon Signal (Black Ball) and the new Tropical Cyclone Strong Wind Signal No.3 (inverted T) were introduced to delineate the use of signals for monsoon systems and tropical cyclones. The Strong Monsoon Signal was used only as a warning against strong winter and summer monsoon winds and the black ball was displayed whenever monsoon winds were forecast or known to exceed 21 knots (40 kilometres per hour) in Victoria Harbour or coastal waters. Occasionally, when Hong Kong was under the combined effect of a monsoon and a tropical cyclone, the Strong Monsoon Signal might be replaced by tropical cyclone signals and vice versa, depending on the synoptic conditions at the time. This system remains in use today.

Renumbering

Originally, Gale or Storm Signals 5, 6, 7, and 8 were different only in terms of local wind direction. In 1971–1972, a review of the local storm warning system was conducted by the Observatory. Letters and questionnaires were sent to shipping companies, government departments, and other organisations to find out whether the majority of people in Hong Kong wanted to change the existing storm warning signals to make them simpler in that increasing signal number would indicate increasing winds. Starting from 1 January 1973, signals numbered 5, 6, 7, and 8 were re-numbered as 8NW, 8SW, 8NE, and 8SE, respectively, to avoid giving the impression that the interchange of Signals 5, 6, 7, and 8 carried a meaning of increasing or diminishing wind strength. This system remains in use today. Since the last major revision in the local signal system in 1973, some adjustments have also been made in the interim years, including the introduction of the Pre-8 advance alert in 1987 and the setting up of a network of eight reference stations for considering the issuance of Signals No.3 and No.8 in 2007.

Signal stations

History

In 1917, a numbered signal system was implemented for warning wind conditions in the territory. Initially, the local day signals were displayed at the mast head of the storm signal mast on Blackhead Hill, the Harbour Office, HMS Tamar, Green Island signal mast, the flagstaff of the premises of Kowloon Wharf and Godown Company in Kowloon, the flagstaff on the premises of the Standard Oil Company in Lai Chi Kok, and the flagstaff near the Field Officer's Quarters at Lei Yue Mun. Night signals were displayed at the tower of the Railway Station, on HMS Tamar and on the Harbour Office flagstaff. Supplementary signals in the form of a cone were also displayed at Gap Rock, Waglan Island, Stanley, Aberdeen, Shau Kei Wan, Sai Kung, Sha Tau Kok, and Tai Po whenever local signals were displayed in the harbour. Since the China Seas Storm Signal Code started to include a time signal code at the mast head which formerly was reserved for local signals, at the suggestion of the Director of the Observatory, hoisting of local signals was moved to the Observatory wireless mast. The night and day signals signals started to be displayed on the Observatory wireless mast on 3 October 1919 and 1 June 1920 respectively. In 1933, the wireless mast was removed from its original position to make way for the Director's Quarters. The new wireless mast was set up a little to the northeast of the Main (1883) Building. The local signal stations were initially set up in the harbour and the outlying islands. Signal stations gradually increased in number across the territory in addition to signal stations at the Hong Kong Observatory Headquarters and outstations at Cheung Chau and Waglan Island. A number of old signal masts in the New Territories and the outlying islands had to be repaired. Most of the signal stations were located at government buildings of the Hong Kong Police Force and the Marine Department. In 1963, a visual signal was completed at Lau Fau Shan Police Station in Deep Bay. Night stations were installed at the San Miguel Brewery in Sham Tseng and the Port Signal Station at North Point. In 1964, a storm signal station established at the Marine Licensing Office, Shau Kei Wan, was brought into operation. In 1965, storm signal stations, displaying both day and night signals were established at the Marine Police Operations Bases at Tai Po Kau and Tai Lam Chung, at the Plover Cove Police Station, and at Divisional Police Headquarters, Yuen Long. Signal stations at the Mobil Oil Depot Lai Chi Kok and at Hei Ling Chau Police Station ceased operation. In 1967, a signal station was established at Peng Chau Police Station. The number of signal stations in Hong Kong peaked at 42 according to the Observatory's 1968–1969 annual report. Aberdeen Police Station was vacated in October 1969. The station at Marine Department Headquarters was moved to Port Communication Centre in April 1970. The station at Cheung Chau Police Station was moved to the meteorological station in 1971.

Closing progressively

With the development of alternative communication channels in disseminating weather information and warnings, the signal stations were progressively closed in the late 1970s. The wireless mast for hoisting signals at the Observatory was dismantled in 1978 for the construction of the new Centenary Building to the east of the Main (1883) Building. On 31 March 1980, there were 37 signal stations in Hong Kong. In 1983, only 15 signal stations were retained to provide visual display of signals, mainly to vessels in or near the harbour. The signal stations at Waglan Island and Marine Department Port Communication Centre ceased to operate with effect from 30 June 1988. The signal station at Green Island ceased to operate with effect from 15 October 1989.

Decommissioning

Starting on 1 April 2000, signals would no longer be hoisted at the signal stations at Aberdeen Marine Office, Sha Tau Kok Sewage Treatment Works and Tai Lam Small Boat Unit Headquarters; from 21 April 2001, signals would no longer be hoisted at the signal station at Lau Fau Shan Police Station; and from 1 January 2002, signals would no longer be hoisted at the signal station at Cheung Chau Aeronautical Meteorological Station. The signals are now preserved as historical exhibits, such as those displayed at the Observatory Headquarters and Cheung Chau Meteorological Station.

Current system

In accordance with legal codes and customs in Hong Kong, once any signal higher than No. 3 is issued, all government agencies shut down their operations. Schools, financial markets, and most of the private sector closes. Non-essential workers are released from work in a staggered manner so as to avoid overwhelming public transportation. Public transit agencies generally continue to operate initially but may cease operation at short notice. The Hong Kong Observatory has since 1987 issued warnings two hours in advance of the issuance of the No. 8 warning signal.

Criticism

In its early days, the system relied upon wind speeds measured around Victoria Harbour. As the emphasis shifted to serving the urbanised populace, such observations were seen as lacking relevance. The Hong Kong Observatory was criticised during Typhoon Prapiroon in 2006, when conditions in urban areas were much more severe than those on the harbour, which had justified only a No. 3 signal being issued. In response, in 2007, the Hong Kong Observatory broadened its network to eight near-sea-level reference anemometers around Hong Kong. According to the new system, the No.3 and No.8 signals will be issued when half or more anemometers in the reference network register sustained winds of 41 to 62 km/h and 63 to 117 km/h respectively. In 2013, Lau Fau Shan replaced Wetland Park as a reference anemometer station. The current eight reference anemometer stations include Cheung Chau, Hong Kong International Airport, Sai Kung, Kai Tak, Lau Fau Shan, Tsing Yi Shell Oil Depot, Sha Tin, and Ta Kwu Ling.

Signal No. 10 occurrences

When Typhoon Hato hit Hong Kong in August 2017, Signal No.10 was issued, leading to flights being suspended or cancelled. Flights later resumed as soon as it was safe, including throughout the night to make up for lost time. MTR cancelled train services in open sections of the line and maintained a limited service underground on an adjusted schedule. All schools were closed for the day, and the government opened temporary shelters. Trees fell and flooding occurred. The stock market (and other businesses) were suspended for part or a whole day. When Typhoon Mangkhut hit Hong Kong in September 2018 with Signal No.10 being issued, all of the aforementioned closure happened. The Hong Kong Observatory and the Hong Kong Government advised people to put masking tape on their windows to try and stop their windows from being blown away.

Meaning of signals

The Saffir–Simpson hurricane wind scale is a classification used for some Western Hemisphere tropical cyclones. Hong Kong has similar official five-level definition warning signals, which use descriptions of winds taken from the Beaufort Scale. The Hong Kong levels, however, do not correspond to the Beaufort Scale, which has 12 levels. The lowest level of the Hong Kong system No.1 does not correspond to any wind strength. Instead, it is an alert based on the distance of a storm. The highest level, Hurricane Signal No.10, is issued infrequently. There have been 17 No.10 warnings since 1946. From the 1980s to 2000s, two No.10 signals were issued, for Typhoon Ellen in 1983 and Typhoon York in 1999. Four of such signals have occurred since 2010, for Typhoon Vicente in 2012, Typhoon Hato in 2017, Typhoon Mangkhut in 2018, and Typhoon Saola in 2023. Signal numbers will change in accordance with the conditions. The Hong Kong Observatory website gives live updates of issue and cancel times for signals.

Hong Kong Observatory public guidance

The Hong Kong Observatory has given public guidance when signals are given. This include:

Macau counterpart

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. In 2017, Typhoon Hato also reached Signal number 10, but Macau raised the signal late, hours after Hong Kong.