To most people, these cryptic words may mean no more than a riddle. But for pilots, they represent
the weather forecast for the Hong Kong International Airport in the coming 24 hours.
Charted waters and continents
There are now more choices of wind and temperature charts on the web-based Aviation Meteorological Information Dissemination System (AMIDS) for aviation users. The figure 5 presents the available choices.
Weather observations at the airport
Fig. 6 & Fig. 7 A band of cumulus clouds associated with a cold front,
Late winter and early spring are the favourable seasons for fog in Hong Kong. Fog is formed when warm and moist air from the southeast moves over the cool waters near the South China coast. On 17 January 2002 dense fog affected the Hong Kong International Airport, with the local visibility going down to 200 m at one time that morning.
Fig. 8 Passenger Terminal Building (PTB) immersed in a layer of dense fog, with
its top barely visible (picture taken at 8:30 a.m., 17 January 2002 by D.S. Lau).
Inset shows the PTB in a clear fine day.
The Hong Kong Observatory received from an airport personnel a report of very windy conditions resembling a
tornado on Chek Lap Kok Island in the evening of 20 May 2002. The event occurred at around 8:30 p.m. and dustbins were blown around
violently at Catering Road East, Chek Lap Kok.
Thunderstorms were affecting Hong Kong around that time. Warnings of strong monsoon and thunderstorms were in force. The Observatory had also issued alerts of significant windshear in advance.
The event was well observed by the Observatory's Terminal Doppler Weather Radar. This is the first time the radar detected a tornado since its operation in 1998. Located at Tai Lam Chung, this radar is specifically used for detecting weather phenomena giving rise to windshear in severe weather at the airport. The radar picture taken at the time shows opposing winds exceeding 90 kilometres per hour and affecting an area one kilometre across over the eastern part of Chek Lap Kok. This is typical of tornadoes.
A tornado is a fast rotating column of air extending from thunder clouds to the ground. Locally, occurrences of tornado are mostly associated with unstable weather during the rain season from May to September.
On record, there are a total of 6 cases of tornadoes in Hong Kong, including the above report. The last reported tornado occurred in Fanling on 17 May 1994.
A tornado over waters is called a waterspout. Since 1959, there were a total of 36 cases of waterspout and waterspout-like funnel cloud within 460 kilometres (250 nautical miles) of Hong Kong.
Fig. 9 Picture taken by the Terminal Doppler Weather Radar of the Hong Kong Observatory
at 8:31 p.m. on 20 May 2002. Purple/blue indicates winds blowing towards the northeast,
while orange/yellow indicates winds blowing towards the southwest.
Two patches of opposing winds closely packed together is the typical pattern of tornadoes.
In the afternoon of 18 January 2002, four aircraft reported encountering windshear while landing at the Hong Kong International Airport on 07L runway (western end of the north runway). They reported windshear magnitudes of 15 to 20 knots during the period 3:00 to 4:15 p.m. Hong Kong Time (HKT).
The weather pattern that day (see the Weather Chart) was characterized by a ridge of high pressure ahead of a cold front over south China. The ridge brought easterlies to the surface, with winds veering to the south on hilltops of Lantau.
Fig.10 Weather Chart.
The time sequence of the wind observations shown below depicts the local wind disturbances near the airport during the period. While winds over the airport remained generally easterly, the newly-installed weather buoy saw winds changing from northeasterly to northwesterly, then southerly and finally northeasterly. This successive change in wind direction at the buoy, against rather steady winds in the background, suggests the existence of small vortices one by one, over the western part of the airport, and their subsequent movement to the northwest. These small vortices brought significant changes to the headwind locally, as amply demonstrated in the accompanying figure which shows the headwind changes between the weather buoy and the anemometer near the 07L touchdown zone. It is believed that the four aircraft encountered windshear associated with such headwind changes.
The windshear alert had been issued in advance. We were happy to have received the following compliment from one of the pilots:
"Thanks to the controller's advice we increased our bug speed accordingly and were ready to deal with the w/s (windshear) encounter".
Fig. 11 Time sequence of the wind observations
Fig.12 Time series of head wind change (relative to aircraft landing on runway 07L)
between the buoy and the anemometer at the western end of runway 07L.
Time and magnitude of the four windshear events are also indicated on the graph.
Paul K L Ho
At 1:14 p.m. HKT on 8 April 2002, the Observatory captured a satellite picture (right) of widespread dust over northeast China. The dust was far from Hong Kong (more than 1,500 kilometres) and an extensive rainband over the East China Sea had the effect of washing out any dust moving south. Hong Kong was not affected by the duststorms.
Fig.13 Satellite image captured by NOAA-16 at 1:14 p.m. (Hong Kong time) on 8 April 2002.
The Observatory delivered two briefings on windshear and turbulence alerting service on 14 March 2002, one to airline representatives and pilots and one to air traffic control (ATC) personnel. The objective of the briefings was to present the findings of a review on the windshear and turbulence alerting service for the airport, improvements achieved and the future plan. The total audience was over 60, and both briefings were followed by lively questions and discussions. A technical note on the subject "Windshear and turbulence alerting service at the Hong Kong International Airport a Review" is now available on the Observatory's website, under "Publication and Records".
Fig.14 Mr. Benjamin Fong, Chief Air Traffic Control Officer of the Civil Aviation Department (left)
introducing the speakers Mr. C.M. Shun (middle) and Dr. C.M. Cheng (right) to ATC colleagues.
Fig. 15 Airline representatives and pilots having the floor in the discussion
L.O. Li(There can be more than one answer.)
The annual survey on the aviation weather services provided by the Observatory was conducted in late March. We thank airlines for their returns. The survey covered products and service items and overall 98% of them were rated satisfactory or better. The airlines also provided valuable comments on areas regarding further improvement. We are working on the comments and will strive to do better.
To ensure our weather services for the airport go uninterrupted when Control Tower becomes inoperative, the Observatory has its backup facilities ready and running 24 hours a day. Drills are conducted regularly to ensure the competency of staff. Some of the drills also involve relevant government departments and the Airport Authority.
Fig.16 Ancillary Airport Meteorological Office
Cheung Chau Signal Station
calling it a day!
With rapid advancement in modern communication technology, nowadays the latest weather forecast and warning is readily available on various medias ranging from radio, TV, telephone to internet. To keep pace, the Observatory decommissioned Hong Kong's last signal station at Cheung Chau early this year. This marked the end of the tradition of hoisting visual signals for tropical cyclone and strong winds.
Back in the old days, Cheung Chau also served as an aeronautical weather station from 1953 till 1992, when it went automatic. Despite the decommissioning of the signal station, the automatic weather station there will continue to operate and provide weather observation round the clock.
Fig.17 Observatory staff and their family members at Cheung Chau Station, pictured here with the decommissioned No.10 typhoon signal.
Fig. 18 Observatory staff explaining the exhibits to visitors
Fig. 19 Photograph session with cartoon figures of the Observatory Weather Family
A delegation from Air Traffic Management Bureau, Civil Aviation Administration of China (CAAC/ATMB) participated in a seminar on aviation meteorological services and information exchange at the Observatory during 15 - 18 January 2002. The visit was carried out under the Memorandum of Long Term Technical Co-operation between CAAC/ATMB and the Observatory. The delegation visited the Observatory's facilities, including those at the Airport Meteorological Office. Apart from technical exchanges, discussion was made on the enhancement of weather information exchanges including the provision of numerical model output, automatic weather station and radar data.
Fig.20 The delegation from CAAC/ATMB pictured here with the Director and staff of the Observatory
For enhanced exchange of meteorological data, the Observatory's Messrs. L.O. Li and N.K. Cheng visited the Meteorological Center of CAAC's Central and South China Air Traffic Management Bureau (ATMB) in March 2002. During the visit, there were useful interactions on dissemination of weather data via the internet.
Fig.21 Colleagues of Meteorology Division of Central and South China ATMB pictured here with Observatory staff
Fig.22 N.K. Cheng giving a demonstration to ATMB counterparts
On 17 March, Observatory staff and their family members planted a
variety of trees in the compound. Apart from beautifying the grounds,
the event also conveyed the message of environment conservation.
Fig. 23 Tree Planting Day at the Observatory
Throughout my career at the Observatory, I had been in various jobs and there was no lack of excitement in each of them. However, a recent task to select a site for a new weather station on Lantau Island was quite a challenge to me - I had to be winched down from a hovering helicopter!
To enhance monitoring of windshear affecting the airport and to further improve forecasting techniques, the Observatory is adding new anemometers on Lantau. When I was assigned the task, the first thing that came into my mind was the picture of myself descending smartly from a helicopter and landing gracefully on the ground, like an action movie star! To make this scene come true, I took the winching class seriously, where I learnt how to avoid sliding off the rope and to keep my body steady to prevent injuries.
It was fine the day when I flew to Sham Wat on Lantau. However, I did not have the mood to appreciate the beautiful scenery. I just kept on mentally rehearsing the safety procedures. Upon arriving at the landing point, I readied the rope and waited nervously with my legs dangling outside the helicopter and my heart beating twice as fast! Soon, I was winched up a little and then slowly down. The few seconds before landing seemed to last for hours. After landing safely on the ground, I waited till I calmed down and got my breath back before starting the site-selection work.
When I finished my task on Lantau, I had again to be winched up to the helicopter. Second time over, I was not so frightened and even managed to smile before the camera something to show off to my kids!
Thinking about it, although winching was a little scary, it saved both time and energy when compared to hill-climbing. I really have to thank colleagues from the Government Flying Service. Without them, I would not have completed the task so smoothly.
Fig.24 The writer had her share of excitement during site selection on Lantau!
Mr. K.S. Leung, officer-in-charge of the Airport Meteorological Office(AMO), retired from the civil service in January 2002 after working 30 years at the Observatory. We wish him a happy retirement. He is succeeded by Ms. Sharon Lau. Ms. Lau has earlier involved in the planning of the meteorological facilities at Chek Lap Kok as well as the development of its windshear and turbulence alerting service. She welcomes suggestions and comments on ways to improve services and can be reached by phone at 2922 5805 or email email@example.com.
Fig.25 Participants of the 17th meeting of the Liaison Group on Aviation Weather Services bidding farewell to Mr. K.S. Leung (centre)
Fig.26 Ms. S.Y. Lau in her office
Telephone and Fax Numbers
Enquiry on flight documents
(852) 2910 6922
Hong Kong Observatory Home Page
Web Page for Aviation Weather Services
Editor this issue L.O. Li, M.T. Tsang
Hong Kong Observatory : 134A Nathan Road, Kowloon, Hong Kong