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TC Watch > TC Policy [Refresh] [中文]
 HKWW's TC Policy (Version 2.7) |
The following tropical cyclone policy came into effect on May 28, 2008 (Wednesday). Click here to view the first version of the TC policy.
Version 2 edit history:
Version 2.1 edited on June 22, 2008 (Sunday) : Released the 800 km constraint on two issuances per day.
Version 2.2 edited on April 1, 2009 (Wednesday) : Corrected a minor error.
Version 2.3 edited on June 18, 2009 (Thursday) : Added "Forecast Positions".
Version 2.4 edited on August 15, 2009 (Saturday) : Added parameters in "Current Storm Information" and "TC Naming".
Version 2.5 edited on July 12, 2010 (Monday) : Due to time constraint, the number of bulletins guaranteed on a day with TC signals is reduced.
Version 2.6 edited on August 19, 2013 (Monday) : Time of bulletin is updated.
Version 2.7
edited on September 15, 2017 (Friday) : Version includes minor amendment after introduction of the new TC report format.
A. Area of Responsibility (AOR)
A tropical cyclone (with at least tropical depression strength, as defined in Section B) is said to have entered Hong Kong Weather Watch (HKWW)'s AOR whenever when it is in the region bounded by the following lines:
- Latitudes: 0 degrees and 50 degrees north; and
- Longitudes: 100 degrees east and 180 degrees.
The following shows the AOR of HKWW graphically (click on the picture to view the full-size map):
Tropical cyclone reports are provided only when there exists tropical cyclones within the aforementioned region.
B. Tropical Cyclone Categorization
HKWW uses the following to categorize tropical cyclones based on their maximum 1-minute sustained wind speeds (background colour of each category indicates the colour code used in our TC track map):
TC Category |
Winds (knots) |
Winds (km/h) |
Winds (m/s) |
Tropical Depression |
22 - 33 |
41 - 62 |
11.4 - 17.2 |
Tropical Storm |
34 - 48 |
63 - 87 |
17.3 - 24.1 |
Severe Tropical Storm |
49 - 63 |
88 - 117 |
24.2 - 32.5 |
Typhoon (Category 1) |
64 - 82 |
118 - 153 |
32.6 - 42.5 |
Typhoon (Category 2) |
83 - 95 |
154 - 177 |
42.6 - 49.1 |
Typhoon (Category 3) |
96 - 113 |
178 - 209 |
49.2 - 58.0 |
Typhoon (Category 4) |
114 - 129 |
210 - 239 |
58.1 - 66.3 |
Super Typhoon (Category 4) |
130 - 135 |
240 - 249 |
66.4 - 69.1 |
Super Typhoon (Category 5) |
136 or above |
250 or above |
69.2 or above |
C. Tropical Cyclone Report - Frequency and Format
1. Frequency
As far as time is allowed, tropical cyclone reports are to be issued with the following frequencies:
Condition(s) |
Report Frequency |
| When the Tropical Cyclone Signal Number 8 or above is issued | At least twice per day [1] |
| When the Tropical Cyclone Signal Number 1 or 3 is issued, or when a tropical cyclone is expected to post a threat (or other effects) to the territory | At least once per day [1] |
| Other circumstances | Once per day [2] |
[1] Reports may be issued any time within a day. Please stay tuned for the latest information.
[2] Reports are normally issued before midnight Hong Kong Time.
The proposed update time of the bulletins are normally specified near the end of each tropical cyclone report.
Please also note that the above table serves as a guideline only; when time is not allowed a slight deviation from the schedule is possible.
2. Format
A tropical cyclone report normally consists of the following parts:
- Basic system profile, including
1. Name and classification of the tropical cyclone;
2. JTWC number / International number;
3. Bulletin number;
4. Time of report, usually the time of issuance of the bulletin;
5. Time of observation, on which the current and forecast storm information are based;
6. Position, in terms of latitude and longitude;
7. Maximum sustained wind speed (1-minute and 10-minute averages) and gusts;
8. Pressure;
9. Past 6-hour movement (direction and speed);
10. Areas currently affected.
- Forecast Track (more on this will be mentioned in Section E)
- Forecast Potisions, which provides detailed information about the forecast track shown, including but not limited to latitudes/longitudes and velocities
- Overview, which normally describes the overall status of the system and its position relative to other places
- TC naming, which includes background information about the tropical cyclone's name
- Movement analysis, which usually states how the storm moved in the past 24 hours and the reasoning of the forecast track
- Intensity analysis, which usually includes the T-number of the system and the changes in the intensity of the tropical cyclone together with reasoning
- Effects on Hong Kong
- Next update, the proposed update time for the next bulletin
- Various images provided by other agencies / organizations
A part on the estimated likelihood of the issuance of tropical cyclone signals by the Hong Kong Observatory will also be included if the storm is expected to enter the 800 km boundary.
D. Units Used
Our bulletin will normally use the following units in various measurements. Some other units may also be used depending on the circumstances.
- Time: In Hong Kong Standard Time (HKT) which is 8 hours ahead of the Greenwhich Mean Time (= GMT+8).
- Distance: In kilometres (km) and nautical miles (NM).
- Speed: In knots (nautical miles per hour), supplemented with kilometres per hour (km/h).
- Pressure: In hectopascals (hPa).
E. The Forecast Track
Forecast tracks of HKWW are either drawn after integrating the outputs of different models and agencies, or drawn directly using forecast track given by the Joint Typhoon Warning Center (JTWC). The latter is only used when time is not allowed for thorough analyses of the models.
For tracks drawn after analyses (i.e. not JTWC tracks), we provide forecasts of at most 72 hours ahead if the tropical cyclone is of tropical depression intensity, or at most 120 hours ahead if it is of tropical storm intensity or above. The shaded area represents the 70% probability potential track area of the storm during the forecast period, i.e., there is a 70% chance that the storm is within the shaded region at the corresponding time point.
F. Abbreviations Used
Commonly-used abbreviations are listed as follows:
- HKO = Hong Kong Observatory
- JTWC = Joint Typhoon Warning Center
- JMA = Japan Meteorological Agency
- CWB = Central Weather Bureau
- CMA / NMC = China Meteorological Administration / National Meteorological Center
Last Accessed: Sat Mar 25 2023 21:03:32 HKT
Last Modified: Fri Jun 03 2022
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