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  • How much do you know about Tropical Cyclone Signal No. 1?

  • Thursday, 12th November 2015

When a tropical cyclone enters within about 800 kilometres of Hong Kong and may affect our territory, the Hong Kong Observatory will issue the Standby Signal No. 1 to remind the public to take timely precautions against strong winds. Some organisations concerned will arrange additional manpower to take up duty or to stand by, so as to prepare for the adverse weather that the tropical cyclone may bring.

Another situation that warrants a No. 1 Signal is slightly different from the above. This refers to the late stage of the passage of a tropical cyclone, when the No. 1 Signal is issued to replace the Strong Wind Signal No. 3. Such scenario occurs mostly when a tropical cyclone is departing from Hong Kong and local winds have generally subsided below strong force while strong winds are still affecting some offshore waters. On one hand, the No.1 Signal notifies the public that winds over the territory are generally moderating and the threat of tropical cyclone to most people is reducing gradually. Those organisations that have arranged extra manpower may consider standing down. On the other hand, since offshore winds remain strong with high seas and swells over some waters, members of the public should remain on the alert, stay away from the shoreline and avoid engaging in water sports.

Let's take Typhoon Kalmaegi in 2014 as an example (Figure 1). As Kalmaegi came within around 800 kilometres of Hong Kong on 14 September night, the Observatory issued the Standby Signal No. 1 to remind the public to take timely precautions against high winds. Later when Kalmaegi continued to edge closer and local winds gradually strengthened, the Observatory issued Signal No. 3 and Signal No. 8 successively. On the night of 16 September, Kalmaegi departed from Hong Kong and moved into Beibu Wan. Locally, wind strength at many places such as Kai Tak and Chek Lap Kok fell below strong force. However, strong winds still persisted over offshore regions such as Cheung Chau (Figure 2). Signal No. 1 was therefore issued by the Observatory to replace Signal No. 3, indicating that threats posed by Kalmaegi were abating but the public should remain vigilant since offshore winds were still strong.


Figure 1

Figure 1      Track of Typhoon Kalmaegi from 12 to 17 September 2014, across Luzon
and the South China Sea, and into Beibu Wan.



Figure 2

Figure 2      Local wind strength and warning signals on 16 and 17 September 2014. After Signal No. 3
was replaced by Signal No. 1, winds remained strong at Cheung Chau but have
moderated at Kai Tak and Chek Lap Kok.



On occasions, particularly in autumn, monsoon may dominate over Hong Kong after a tropical cyclone departs and weakens. If strong winds continue to prevail, the Observatory will issue the Strong Monsoon Signal instead to keep the public alert of the windy condition.

Public safety is always the Observatory's top priority in considering warning signals. Despite the small size of Hong Kong, wind strength across the territory can still vary greatly. In the future, when a tropical cyclone is departing and people do not experience high winds at where they are, they may hopefully understand why the Observatory still keeps the No. 1 Signal in force for some time.



T.S. Tsoi



Last revision date: <13 Jan 2016>