The local tropical cyclone warning signal system, commonly known as the "typhoon signals," has a long history^[1] and takes root in people’s minds. The number of the "typhoon signal" issued by the Hong Kong Observatory is often the focus of attention by the public in every tropical cyclone season. Meanwhile, do you know what factors the Observatory considers when deciding whether to issue tropical cyclone warning signals?

Let us first revisit the definitions of each tropical cyclone warning signal:

It can thus be seen that the impact of tropical cyclones on the winds over Hong Kong forms the basis of the warning signals. Therefore, when deciding whether to issue or change a signal, apart from the intensity of the tropical cyclone as it approaches Hong Kong and the distance between the storm's centre and the territory, the Observatory also takes the following factors into account:

1. Landfall position of a tropical cyclone (east of / west of Hong Kong) and its speed of movement

• Landfall to the west of Hong Kong: In the Northern Hemisphere, the circulation of a tropical cyclone rotates anticlockwise. On the right semicircle, the wind direction coincides with the direction of movement (usually northwestwards), so the winds are generally stronger and this side is known as the "dangerous semicircle" ^[2]. Therefore, when a tropical cyclone passes to the south of Hong Kong and makes landfall to the west, Hong Kong is affected by the east to southeasterly winds of the "dangerous semicircle." Moreover, since the winds blow over Hong Kong from the sea, they are less sheltered by terrain, and so the winds are stronger. The faster the tropical cyclone moves, the more pronounced the effect of the "dangerous semicircle" becomes.

• Landfall to the east of Hong Kong: Conversely, when a tropical cyclone makes landfall to the east of Hong Kong, its left semicircle — also known as the "navigable semicircle" — generally has weaker winds, and since the winds blow over Hong Kong from inland, they are more affected by terrain sheltering effect. Winds over Hong Kong will therefore be weaker than they would be if the same storm had made landfall to the west of Hong Kong.

• Take the Severe Tropical Storms Tapah and Mitag in 2025 as an example. Although the two storms were of similar intensity and both passed more than 100 kilometres from Hong Kong, their different tracks led to somewhat different impacts on the territory. Tapah, which made landfall to the west of Hong Kong, brought gale force winds to many places over the territory, and the Observatory had to issue the Gale or Storm Signal, No. 8. By contrast, under the influence of Mitag, which made landfall to the east of Hong Kong, the territory was situated on the storm's left semicircle and recorded only generally strong winds, so the Observatory only needed to issue the Strong Wind Signal, No. 3 (Figure 1).

• It should be noted that every tropical cyclone is unique. Apart from the location of landfall, the impact of a tropical cyclone on Hong Kong also depends on many other factors, so it cannot be generalised that a storm making landfall to the east of Hong Kong necessarily poses a lesser threat to the territory. For example, the Observatory needed to issue the Increasing Gale or Storm Signal, No.9 for Severe Typhoon Noul, which made landfall to the east of Hong Kong in 2026.

2. Size of a tropical cyclone's circulation and its wind structure

• The impact of a tropical cyclone on Hong Kong's winds is closely related to the size of its circulation and its wind structure. For example, the area of hurricane force winds of Super Typhoon Ragasa in 2025 was extensive. Although Ragasa passed about 120 km south of Hong Kong, its extensive area of hurricane force winds persistently battered the southern part of the territory (Figure 2, left), and the Observatory had to issue the Hurricane Signal, No. 10. It was the furthest tropical cyclone from Hong Kong since 1946 that necessitated the issuance of the Hurricane Signal, No. 10.

• On the other hand, Typhoon Higos in 2020 had a rather compact circulation (Figure 2, right). Even though the closest distance between Higos and the Hong Kong Observatory was only about 80 km, closer to Hong Kong than Ragasa, its eyewall with winds reaching hurricane force did not affect the territory, and the Observatory did not need to issue the Hurricane Signal, No. 10.

3. Combined effect of a tropical cyclone and the northeast monsoon

• In autumn, the coast of southern China is sometimes under the combined effect of a tropical cyclone and the northeast monsoon. Since a tropical cyclone is a low-pressure system while the northeast monsoon is a high-pressure system, the pressure gradient between the two is relatively large, bringing stronger winds to the affected areas^[3]. In such cases, even though the centre of a tropical cyclone is further away from Hong Kong, the winds experienced by the territory can still be quite strong. For example, when Typhoon Kompasu passed about 360 km to the south of Hong Kong in mid-October 2021, under the combined effect of Kompasu and the northeast monsoon, Hong Kong experienced strong to gale force east to northeasterly winds, with occasional storm force winds offshore, necessitating the issuance of the Gale or Storm Signal, No. 8 by the Observatory (Figure 3). By comparison, Typhoon Chanthu in late July 2010, of similar intensity to Kompasu, passed about 330 km to the southwest of Hong Kong. Although Chanthu came closer to Hong Kong at its closest approach than Kompasu did, the winds over Hong Kong generally only reached strong force in the absence of contribution of the northeast monsoon as it was summer at that time, so Signal No. 8 was not required.

• However, the northeast monsoon can be either a boost or a hindrance to the winds in Hong Kong. Since tropical cyclones rely mainly on warm and moist air as their source of energy, when cooler and drier northeast monsoon intrudes into their structure, tropical cyclones may weaken or even dissipate. Take Krosa in 2013 as an example, although Krosa had intensified into a severe typhoon over the northern part of the South China Sea, it weakened rapidly with the arrival of a dry northeast monsoon. Meanwhile, the northeast monsoon can also affect the tracks of tropical cyclones. For example, Krosa initially moved on a west-northwesterly track over the South China Sea, but was subsequently steered by the northeast monsoon to turn towards the southwest and move away from Hong Kong (Figure 4).

• On a related note, when Hong Kong is under the combined effect of a tropical cyclone and the northeast monsoon, if the local weather is initially dominated by the monsoon, the Observatory will first issue the Strong Monsoon Signal. However, as the tropical cyclone gradually approaches and the local winds come to be dominated by the tropical cyclone, the Observatory will directly issue the Strong Wind Signal, No. 3 to replace the Strong Monsoon Signal, without issuing the Standby Signal, No. 1. In the case of Kompasu, the weather over Hong Kong was initially dominated by the northeast monsoon, and the Observatory first issued the Strong Monsoon Signal. As Kompasu entered the northeastern part of the South China Sea, the Observatory then directly issued the Strong Wind Signal, No. 3, replacing the Strong Monsoon Signal.

4. Uncertainties in tropical cyclone forecasting

• As the atmosphere is a chaotic system, computer forecast models can produce very different results from only slight differences in initial conditions when simulating weather changes. Forecasts from different models often diverge, and the divergence grows as the forecast period lengthens. If there is significant divergence among different models, it indicates a complex atmospheric situation and greater forecast uncertainty; if the models largely agree, the forecast uncertainty is relatively small. As a tropical cyclone approaches Hong Kong, slight changes in its track or intensity can have very different impacts on local winds.

• For example, after Super Typhoon Mangkhut in 2018 crossed the northern part of Luzon, it moved rapidly on a west-northwestwards track across the northern part of the South China Sea towards the coast of Guangdong, and made landfall near Taishan in Guangdong. The track probability forecast showed relatively little divergence among different models of the computer ensemble prediction system (Figure 5, left). Therefore, the Observatory alerted the public several days in advance that Mangkhut would pose considerable threat.

• In the case of Severe Tropical Storm Nalgae in 2022, by contrast, the models of the ensemble prediction system diverged more widely in their track forecasts over the northern part of the South China Sea (Figure 5, right). Given the large forecast uncertainty, the lead time for warnings was relatively shorter.

The tropical cyclone warning signal system is a science-based service, with public safety as its top priority. Owing to the limitations of weather forecasting technology, the longer the lead time for warnings, the higher the forecast uncertainty. Nonetheless, to enable members of the public to better prepare for the upcoming deterioration of weather, thereby minimising casualties and losses, the Observatory will, where technology permits, endeavour to provide sufficient lead time. As a result, when a tropical cyclone warning signal is issued, the weather may not yet have deteriorated immediately. At the same time, in light of forecast uncertainty and the limitations of forecasting technology, meeting the prescribed wind force criteria should not be the only and ultimate goal in issuing tropical cyclone warning signals; public safety is the paramount consideration.

• Do not let your guard down: After a tropical cyclone warning signal is issued, even if the winds in your area have not yet strengthened, you should complete all precautionary measures, as the weather can deteriorate rapidly.

• Be aware of differences in wind speed: Due to local topography and the built environment, wind conditions in different parts of Hong Kong can vary appreciably. Members of the public should refer to the latest tropical cyclone information, as well as regional wind data, provided on the Observatory's website and the "MyObservatory" mobile app, and take appropriate precautionary measures in response to the warnings.

• Check flight status: Aircraft take-off and landing are mainly affected by crosswinds (winds blowing across the side of the aircraft) and associated wind shear or turbulence, and whether flights can operate does not necessarily relate directly to the tropical cyclone warning signals. When it is anticipated that the weather may cause serious disruption to air traffic, the Observatory will remind travellers in the tropical cyclone bulletin to check with airlines on flight status before heading to the airport.

• Beware of heavy rain risks: Tropical cyclones may also bring heavy showers. The public should also pay attention to rainstorm-related warnings or special advisories issued by the Observatory, and guard against flooding and landslides.

• Be alert to storm surge and overtopping waves: Affected by storm surge^[4], low-lying areas may experience inundation or backflow of seawater. The public should avoid entering potentially affected areas and stay away from dangerous locations. Overtopping waves^[5] may also occur along the coast; the public should stay away from the shoreline and not to engage in water sports.