Pilots not familiar with this alerting principle may be caught by surprise when on approaching the airport they first experience a gain event (windshear of +15 kt) instead of a loss event (microburst of -30 kt) at the first encounter (3 mile final)" location only to encounter a loss of -30 kt a couple of seconds later at 1 mile final. This issue was raised by the Civil Aviation Department and subsequently discussed by the International Federation of Air Line Pilots' Associations (IFALPA) in late 2002. The pilots generally felt that an indication of the maximum intensity and an indication of location in terms of approach or departure corridor (without further details on which section of the corridor) would suffice for their preparation for landing or take-off. A new phraseology has thus been drawn up, and under the new arrangement the alert in the above example will become:
Caution. Microburst minus 30 knots on final approach".
A trial has started on 1 April 2003 and will last for 6 months. Users are encouraged to provide feedback to the Hong Kong Observatory (telephone: 2926 8434, fax: 2375 2645, email: email@example.com).
Kind words from the aviation community
The number of products on the Observatory's on-line aviation weather information service, the AMIDS (Aviation Meteorological Information Dissemination System), now tops 200. The products range from runway winds, satellite cloud images to en-route and destination weather information. A newcomer is the computerized medium-level significant weather charts in colour.
Here is a snapshot of feedback from aviation users:
Your efforts on improving the AMIDS and relevant services for HKG's aviation community are much appreciated."- from an airline.
...weather forecast on HKO web site is very useful."- from a pilot.
An 18-year Observatory-EMSD collaboration
The Hong Kong Observatory (HKO) and the Electrical and Mechanical Services Department (EMSD) jointly put a new electronics device into operation at the automatic weather station at Ngong Ping in May 2003. The work represents the effort of Ms. Cindy Man, an engineering graduate from EMSD and working at HKO. Since 1985, the Observatory has been collaborating with EMSD in providing attachment opportunities for engineering graduates.
Located at an exposed and remote hilltop of Lantau Island to the south of the Hong Kong International Airport, the station at Ngong Ping relies solely on wind power to operate its equipment. With the electricity generated, the station automatically measures the wind and temperature there and sends the data back to HKO by wireless transmission. Cindy has made a device which lets us know instantly whether the wind is producing enough electrical energy for the station", said Dr. C.M. Tam, Senior Scientific Officer of the HKO.
Cindy was pleased that her work could serve a scientific purpose. She said: It is one thing learning from books and quite another putting what you have learnt into practice. Colleagues at the Observatory helped me a lot in overcoming difficulties in the circuit design. Journeys to inspect the station at Ngong Ping and to figure out how to install the device there were exhausting because there was no road transport and we had to carry the equipment on foot. However, I find the experience very rewarding, especially now that my work can be used to explore the potential of such renewable energy as wind power in Hong Kong."
Weather observations at the airport
(Photo: Courtesy of Mr. C.S.Yuen)
The picture shows a special optical phenomenon called halo, seen over Chek Lap Kok on 17 April 2003. Halos result from the refraction or reflection of light by ice crystals suspended in the atmosphere.
Quiz on aviation weather
Which of the following is always true for the atmosphere?
a. The humidity decreases with height
b. The temperature decreases with height
c. The wind speed decreases with height
d. The pressure decreases with height
The process by which water vapour turns into water droplets to form clouds is called …|
Which of the following is highest in the sky?
What is usually associated with low pressure?
b. cumulus clouds
c. windy weather
An area of low pressure with a circulation centre is called …
a. an anticyclone
b. a cyclone
c. a trough of low pressure
d. a ridge of high pressure
Which of the following on the Beaufort scale of wind force is equivalent to strong winds?
Sea breeze develops under fine and light wind conditions. At the Hong Kong International Airport, sea breeze usually sets in from the west. With the background wind from the east, the sea breeze converges with the background wind to produce windshear, i.e. a gain in the headwind to an aircraft (Figure 11). Yet sea breeze can also develop under stronger wind conditions. A case in point occurred on 20 April 2003, when against a background easterly wind of 15 knots (Figure 12) a 10-knot sea breeze developed west of the airport near midday. This produced a windshear of 25 knots on the airfield. Indeed, an aircraft reported a headwind gain of 25 to 30 knots while making a landing from the west around the time.
On the Observatory's LIDAR (LIght Detection And Ranging) system (Figure 13), a sea-breeze 'front' was clearly discernible over an area on the western part of the airport where outgoing winds (the background wind, warm colours) and incoming winds (sea breeze, cold colours) met. This new piece of equipment had enabled the Observatory to issue in advance a windshear alert for that occasion.
The Observatory is happy to announce the deployment of a new weather buoy since early 2003. This is the third buoy that the Observatory launched in the past two years to enhance weather monitoring at the airport.
The first two weather buoys are located over the waters west of the airport (WB1 and WB2 in Figure 14 ). Their deployment over the past two years has demonstrated their usefulness in providing alerts of windshear. The new weather buoy is located east of the airport to cover the waters over that area.
It did not take long for the new weather buoy to demonstrate its effectiveness. In the afternoon of 6 February 2003, an eastward-advancing sea breeze affected the airfield. The front of the sea breeze (red dotted line in Figure 14) passed the airport in the early afternoon, bringing westerly winds to the airfield against the prevailing easterlies. The convergence between the two airstreams signaled the existence of significant low-level windshear over the eastern flight corridors, with a magnitude of over 20 knots as indicated by the difference in the headwind between station R1E and the new weather buoy WB3. As revealed in Figure 15, the new weather buoy enabled a windshear alert half an hour earlier.
While sea breeze usually causes a headwind gain to an aircraft, some aircraft did experience a headwind loss. Why was that?
To answer the question, let's look at the formation and structure of sea breeze. As the land surface warms up in a sunny day with light winds, the air rises and allows the cooler air over the sea to come in, thus setting up a sea breeze.
Figure 16 shows a vertical scan of the atmosphere taken by the LIDAR (LIght Detection And Ranging) on a fine, sunny day in April 2003. It shows sea breezes (green) against a background of easterly winds (yellow and brown). Interestingly, it reveals an undulating top for the sea breeze.
Figure 17 is a schematic diagram showing the sea breeze structure. Under normal circumstances, an aircraft that comes in with the sea breeze will experience a headwind gain when crossing the leading edge of the sea breeze. However, if within the sea breeze the tailwind is increasing (from point A to point B) instead of decreasing then it translates into a headwind loss for the aircraft.
Another situation that will lead to a headwind loss arises when the aircraft flies into the sea breeze, as shown in Figure 18. In this instance, the aircraft flying from point a to point b will experience a change from headwind to tailwind, resulting in a headwind loss. Indeed, an aircraft landing at the airport near midday that day reported encounter of a headwind loss followed by a headwind gain. This would be consistent with a flight path in which the aircraft flew from point a to b (loss) and then from b to c (gain).
The Observatory delivered two briefings on windshear and turbulence alerting service on 12 March 2003, one to airline representatives and pilots and one to air traffic control (ATC) personnel. This year's briefings highlighted the usefulness of new weather sensors - LIDAR and weather buoys - in windshear detection, as well as the improved performance of the windshear and turbulence alerting service over the past years. With over 50 participants, both briefings were concluded with lively questions and answers. The presentation in electronic form is available on request.
Observatory open day
March 2003 is celebration time for the Observatory. Following the 120-year Anniversary and Time Capsule Laying Ceremony on 2 March, pleasant weather continued to greet visitors to the Observatory Open Day during the weekend of 22-23 March. Despite concerns over SARS, over 4000 members of the public showed up.
Meteorological briefing for ATC personnel
At the request of the Civil Aviation Department, the Observatory's L.O. Li gave a briefing to a group of ATC personnel on 25 March 2003. As part of a training course for ATC personnel, the briefing covered such topics as the climatology and significant weather within the Hong Kong airspace.
The Japan GMS-5 geostationary meteorological satellite has been serving this part of the world since the early 1990. Starting from 22 May 2003, it is backed up by GOES-9. Located above the equator at longitude 155°E, GOES-9 is operated by the joint effort of JMA and US NOAA NESDIS. The replacement of GMS-5, the MTSAT, is scheduled to launch in early 2004.
Note: JMA = Japan Meteorological Agency ; NOAA = National Oceanic and Atmospheric Administration ; NESDIS = National Environmental Satellite Data and Information Service
Meetings and visits
Air transport symposium
The Observatory participated in the 4th International Symposium on Electronics in the Air Transport Industry (SEATI) in Hong Kong on 8 January 2003, with a presentation of a paper 'Recent Developments to Enhance Windshear and Turbulence Alerting at the Hong Kong International Airport' by C.M. Shun, C.M. Cheng and H.G. Wai. The symposium offered a good opportunity for the Observatory to interact with the air transport industry.
Three scientists from the Environmental Technology Laboratory (ETL) of the U.S. National Oceanic and Atmospheric Administration (NOAA) visited the Observatory from 17 February to 1 March 2003. The purpose was to exchange knowledge and experience in the application of LIght Detection And Ranging (LIDAR) system to weather monitoring. The scientists shared with the Observatory their experience in using LIDAR for studying weather phenomena near mountainous terrain in the U.S. and Europe, while the Observatory presented LIDAR images of weather patterns never observed before in Hong Kong. There were useful discussions on optimizing and evaluating LIDAR performance, and potential areas for joint studies were identified.
In mid-February 2003, the Observatory's Ms. Sharon Lau and Mr. Y.T. Wong visited the Korea Meteorological Administration (KMA) and its Airport Meteorological Office (AMO) at Inchon. The visit enabled a better understanding of the operation of the AMO and maintenance of meteorological facilities in support of the aviation weather services there.
Mr. C.Y. Lam succeeded Dr. H.K. Lam as Director of the Observatory on 14 March 2003. Dr. Lam retired after 32 years of distinguished service at the Observatory. Mr. Lam, the new Director, has been in charge of the public weather services for over seven years and prior to that was responsible for the planning and development of meteorological facilities for the Chek Lap Kok airport.
'Service based on Science' is the guiding philosophy of the Observatory", said Mr. Lam in a meeting with the press after taking up the Director post, the Observatory will proactively respond to the society's ever-changing needs, and will adopt the appropriate technologies and measures to meet these needs."
At the Observatory, C.M. Cheng works on high-impact weather, while S.T. Chan works on windshear and turbulence alerting. But do not expect to see them always immersed in scientific studies. As a matter of fact, they are winners of the best TV weather presenter for the past two quarters respectively. The panel of judges includes an expert from Radio Television Hong Kong. Our congratulations to both of them!
Colleagues of the Observatory and their family pictured here on a tree planting day in April 2003, an annual event to make the Observatory headquarters greener and prettier.
|Telephone and Fax Numbers|
|Enquiry on flight documents||(852) 2910 6922|
|Officer-in-charge AMO||(852) 2910 6300|
(852) 2922 5805
|Duty Forecaster AMO||(852) 2910 6920|
(852) 2922 5806
|Fax||(852) 2910 0080|
(852) 187 8200 (Cantonese)|
(852) 187 8202 (Putonghua)
(852) 187 8066 (English)
Telephone Information Enquiry System
(852) 2926 1133
Hong Kong Observatory Home Page
Web Page for Aviation Weather Services
Editor this issue C. W. Wu
Hong Kong Observatory:134A Nathan Road, Kowloon, Hong Kong