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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

  • No tropical cyclone warning signal in August and September

  • Wednesday, 4th November 2015

August and September are normally the peak months of tropical cyclone activity in Hong Kong with an average of about three tropical cyclones (TCs) affecting Hong Kong, necessitating the issuance of tropical cyclone warning signals[1]. However, in 2015, no tropical cyclone warning signal was issued in August and September, the first time since 1946. So what are the reasons behind this "no TC warning" phenomenon?

Less tropical cyclones entering the South China Sea from the western North Pacific

Firstly, except Soudelor and Dujuan, all TCs formed over the western North Pacific (WNP) in August and September this year (Figure 1) recurved to the north and did not enter the South China Sea (SCS). Even for Soudelor and Dujuan which entered the SCS, they weakened after moving across Taiwan and Fujian and did not directly affect Hong Kong. The reason for less TCs from WNP entering the SCS can be mainly attributed to the current El Nino event. Some studies[2,3] suggest that the above normal sea surface temperature over the central and eastern equatorial Pacific in El Nino years displaces the breeding ground of TC in the WNP further east (Figure 2). This increases the chance for TCs to recurve and turn northwards when moving across the WNP, resulting in less TCs entering the SCS.

Figure 1

Figure 1      Tropical cyclone tracks in August and September 2015

Figure 2

Figure 2      Tropical cyclone genesis positions for January - September 2015.
Distribution of the long-term average[1] of tropical cyclone genesis
for January - September is shown in shaded colour.

Less tropical cyclones forming in the South China Sea

Secondly, only one tropical cyclone (Vamco) formed in the SCS in August and September 2015, less than the long-term average of two to three[1]. Vamco moved towards Vietnam and did not directly affect Hong Kong as well. Less TCs forming in the SCS in August and September 2015 was mainly due to the weaker than normal southwesterly airstream over the SCS. This led to less moisture transport and weaker convergence zone in the SCS, which in turn hindered the formation of TCs in the region.

CHOY Chun-wing and WU Man-chi


[1] Long-term average of 1961 to 2010

[2] Goh, A. Z.-C. and Chan, J. C. L., 2010: Interannual and interdecadal variations of tropical cyclone activity in the South China Sea. Int. J. Climatol., 30: 827-843

[3] 厄爾尼諾與西北太平洋的熱帶氣旋,陳營華、張文瀾、胡文志、區展衡,發表於第十二屆粵港澳重要天氣研討會,香港,一九九八年一月十五至十七日 (Chinese version only)

  • The Scorching Summer of 2015

  • Friday, 25th September 2015

The U.S. National Oceanic and Atmospheric Administration recently announced that the globally averaged temperature over land and ocean surfaces in summer (June to August) 2015 was the highest ever recorded for the globe since 1880[1].

Hong Kong also experienced the hottest summer ever since records began in 1884 with the mean temperature for June to August 2015 reaching 29.4 degrees, breaking the previous record of 29.3 degrees set not so long ago last year.

June 2015 was actually the hottest June on record (Table 1). Under the subsidence effect associated with Typhoon Soudelor, the temperature at the Hong Kong Observatory soared to a record-breaking 36.3 degrees on 8 August 2015, beating the previous extreme of 36.1 degrees set in 1990[2]. More details on the record-breaking temperatures this summer are summarized in Table 2.

Table 1

Table 2

In fact, under the combined influence of global warming and urbanization development, there is a very significant long-term rising trend in the summer mean temperatures in Hong Kong (Figure 1). Record-breaking temperatures are evidently becoming more frequent, with four of the six hottest summers occurring within the past seven years (Table 3). Looking forward, if we fail to reduce greenhouse gas emission as soon as possible, the latest climate projections suggest that under a high emission scenario, the mean temperature in Hong Kong is expected to rise 1.5 - 3.0oC by the mid-21st century (2051-2060) as compared to the 1986-2005 average[3]. This implies that very hot days, hot nights and extreme temperature records in future summers would only keep increasing, bringing negative impacts to our living conditions, especially for the underprivileged who have to stay in congested and poorly ventilated environment. The very hot conditions will also mean enhanced air-conditioning demand and hence more energy consumption. This will create a vicious negative feedback cycle with a profound implication of adverse social and economic consequences in the long run.

Figure 1

Figure 1 - Long Term Trend of Summer Mean Temperature (June - August) recorded at the Hong Kong Observatory

Table 3

K W Li and K M Leung

References :

[1] NOAA Global Analysis - August 2015

[2] Record-breaking high temperature

[3] Hong Kong temperature projections