Hong Kong tropical cyclone warning signals
Tropical Cyclone Warning Signals (熱帶氣旋警告信號) or typhoon signals are a set of signals warning about the existence and approximate location of a tropical cyclone in Hong Kong. Currently, there are a group of 10 signals measuring the gale affecting Hong Kong. Hong Kong Observatory will issue the warning signal should a tropical cyclone enter the 800 km circle threathening Hong Kong.
In the past, the signals are hoisted at many locations of Hong Kong. In 1960s there are 42 signal stations. In 1970s, weather is reported effectively on radio and television. With these common and instant transmission media, the need of hoisting signal decreases and the last signal station, on Cheung Chau, was decommissioned on 1st January 2002. With hoisting signals became the past, the observatory replace the word hoist with issue.
Early days
From 1884, a system of drum, ball and cone informed the mariners the harbour on the existence and approximate location of a tropical cyclone. The local public warned by a typhoon gun the 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 bomb was exploded in 1937.
Numbered system
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. In 1931, the signals were amended to read 1,5,6,7,8,9,10 with signals 5 to 8 signifying gales from the four quadrants. In 1956, the No. 3 Strong Wind Signal was introduced between the No. 1 Stand By Signal and the gale signals.
Current system
Starting from 1st 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.
Meaning of signals
No. 1 - This is a stand-by signal, indicating that a tropical cyclone is centred within 800 km of Hong Kong and may later affect the territory.
No. 3 - Strong winds are expected or blowing in Victoria Harbour, with a sustained speed of 41-62 km/h (kilometres per hour). Gusts may exceed 110 km/h. Winds are normally expected to become generally stronger in the harbour areas within 12 hours after the issuing of this signal.
No. 8 (NW, SW, NE, SE) - Gale or storm force winds are expected or blowing in Victoria Harbour, with a sustained wind speed of 63-117 km/h from the quarter indicated. Gusts may exceed 180 km/h.
No. 9 - Gale or storm force winds are increasing or expected to increase significantly in strength.
No. 10 - Hurricane force winds are expected or blowing. Sustained wind speeds are reaching upwards from 118 km/h. Gusts may exceed 220 km/h.