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Wednesday, 26th March 2014

Clear Air Turbulence - CAT Encountering in the Air

Aircrafts in the air aloft may sometimes experience marked bumps and jolts and people would normally say "encountering an airstream". In meteorology, we call it "turbulence". When encountering turbulence, pilot would promptly switch on the seat-belt sign and passengers should follow the instruction to fasten the seat belt to protect their own safety. Under severe turbulence situation, an aircraft may experience intense vibration. It may also suffer from a momentary loss of control. Hence, pilots will normally avoid flying into the turbulence regions.

Turbulence is usually caused by convective weather, especially in the vicinity of thunderstorms. As aircrafts are generally equipped with weather radar which is capable of detecting clouds and weather associated with convection, pilots normally have sufficient time to diverge the aircraft to avoid impact. Even in case that there is no way to perform avoidance actions, pilots could still be able to alert in time the passengers to fasten the seat belt for safety.

Figure 1

Figure 1      Turbulence encountered by an aircraft in clear air area is named Clear Air Turbulence.

Turbulence, however, may also happen under clear sky. In the aviation community, this kind of turbulence is called "Clear Air Turbulence", or simply "CAT". As the literal meaning tells, CAT happens in regions of fine weather. As there is no rain water, CAT is not detectable by the radar weather onboard. If an aircraft so unfortunately has flown into a CAT region, passengers and crew members might get injured in sudden bumps and jolts. Around noon on 18 February this year, an inbound flight from San Francisco encountered severe turbulence when flying over Russia. There were reports that some passengers were lifted from the seats and bumped onto the overhead lockers or hit by falling luggage, causing 12 injuries. Fortunately, the aircraft was not damaged and landed safely at the Hong Kong International Airport (HKIA) at dusk.

Figure 2

Figure 2      Using computer simulation, we can forecast the location and development of jet stream in the
upper air (red colour curves). This figure indicates a jet stream occurred over Russia on 18 Feb 2014.
An aircraft experienced severe turbulence when flying (black colour straight line) near the jet stream.
(Source: World Area Forecast Centre - London).

Typical CAT often happens high in the sky at an altitude of 20,000 feet (i.e. 6 kilometres) or above. It is usually accompanied with high wind (known as "jetstream"), or regions of abrupt wind speed or direction changes (such as interface of cold and warm air masses, regions with airstream accelerating/decelerating or turning). CAT can also be caused by terrain, particularly waves triggered by airflow passing over high mountains. Taking the Hong Kong Flight Information Region (HKFIR) as an example, the frequency of aircraft encountering CAT was around 15 days per year on average, among which one day would be severe CAT. CAT mostly occurs during winter months from December to February. Turbulence associated with CAT constitutes as large as 10% of the total number of turbulence reports. Due to its hazardous characteristics, we should never overlook CAT. But, how can the threat of CAT be minimized?

In order to assess the chance of CAT and the location of its occurrence within HKFIR so as to issue timely alert to the pilot for making avoidance, the Observatory's aviation forecasters make reference to the observational data collected by weather balloons, meteorological satellites and the "CAT index" generated by computer-based numerical weather prediction models, taking also into consideration of the pilot reports. The forecasters would also outline regions with possible turbulence on significant weather charts for pilots' attention. Before take-off, all pilots should have received a flight document containing, inter alia, detailed weather forecasts pertinent to the flight route. As such, pilots would be on the alert of the regions with possible CAT along the route and take special precaution when they fly through those regions.

To monitor CAT in the vicinity of the airport at Chek Lap Kok, aviation forecasters rely mostly on LIDAR (Light Detection And Ranging). Simply speaking, LIDAR utilizes infrared laser beam to directly probe the wind distribution in clear air to detect any CAT occurrence. The LIDAR is indispensable to the windshear and turbulence alerting services at the HKIA. Since LIDAR is so effective in detecting CAT, if on one day in the future all aircrafts can be equipped with LIDAR onboard, CAT can be detected inflight. Thus, pilots can respond swiftly and alert following aircrafts so that the injuries due to CAT could be significantly reduced. As of the present reality, one should better "fasten the seat belt" for safety whenever onboard!

A Video on "CAT" (in Cantonese only) can be found in the Observatory's "Cool Met Stuff" on YouTube.

Statistics on the occurrence and background information about "CAT" in the HKFIR are available on the Aviation Weather Services webpage of the Observatory.

P.W. Li and P. Cheung

Last revision date: <28 Apr 2014>