ANNEX 3 to the Convention on  International Civil Aviation

Meteorological Service for International Air Navigation

Pilots need to be informed about meteorological conditions along the routes to be flown and at their destination aerodromes.

The object of the meteorological service outlined in Annex 3 is to contribute to the safety, efficiency and regularity of air navigation. This is achieved by providing necessary meteorological information to operators, flight crew members, air traffic services units, search and rescue units, airport management and others concerned with aviation. Close liaison is essential between those supplying meteorological information and those using it. At international aerodromes the meteorological information is normally supplied to aeronautical users by a meteorological office. Suitable telecommunications facilities are made available by States to permit those aerodrome meteorological offices to supply information to air traffic services and search and rescue services. Telecommunications between the meteorological office and control towers or approach control offices should be such that the required points may normally be contacted within 15 seconds.

Aerodrome reports and forecasts are required by aeronautical users to carry out their functions. Aerodrome reports include surface wind, visibility, runway visual range, present weather, cloud, air and dew-point temperature and atmospheric pressure, and are issued either half-hourly or hourly. These reports are complemented by special reports whenever any parameter changes beyond pre-fixed limits of operational significance. Aerodrome forecasts include surface wind, visibility, weather, cloud and temperature, and are issued every three or six hours for a validity period of 9 to 24 hours. Aerodrome forecasts are kept under continuous review and amended by the meteorological office concerned, as necessary.

Landing forecasts are prepared for some international aerodromes to meet requirements of landing aircraft. They are appended to the aerodrome reports and have a validity of two hours. Landing forecasts contain expected conditions over the runway complex in regard to surface wind, visibility, weather and cloud.  To assist pilots with their flight planning, most States provide meteorological briefings which are increasingly carried
out using automated systems. Briefings comprise details of en-route weather, upper winds and upper-air temperatures, often given in the form of meteorological charts, warnings related to hazardous phenomena en-route, and reports and forecasts for the destination aerodrome and its alternates.

To provide aircraft in flight with information about significant changes in weather, meteorological watch offices are maintained. They prepare warnings of hazardous weather conditions, including thunderstorms, tropical cyclones, severe squall lines, heavy hail, severe turbulence, severe icing, mountain waves, sandstorms, duststorms and volcanic ash clouds. Moreover, these offices issue aerodrome warnings of meteorological conditions that could adversely affect aircraft or facilities on the ground: for example, warnings of expected snowstorms. They also issue warnings for wind shear for the climb-out and approach paths. Furthermore, aircraft in flight are required to report severe weather phenomena encountered en route. These reports are disseminated by the air traffic services units to all aircraft concerned.

On most international routes routine observations are made by aircraft of upper winds and temperatures. They are transmitted by aircraft in flight to provide observational data that can be used in the development of forecasts. These aircraft observations of winds and temperatures are being automated using the air-ground data link communications. As far as route forecasts are concerned, all flights require advance and accurate meteorological information so as to chart a course that will permit them to make use of the most favourable winds and conserve fuel. With rising fuel costs, thishas become increasingly important. Therefore, ICAO has implemented the World Area Forecast System (WAFS). The purpose of this system is to provide States and aviation users with standardized and high-quality forecasts on upper-air temperature, humidity and winds and on significant weather. The WAFS is based on two world area forecast centres which use the most up-to-date computers and satellite telecommunications (ISCS and SADIS) to prepare and disseminate global forecasts in digital form directly to States and users.  

During the past few years a number of incidents have occurred due to aircraft encounters with volcanic ash clouds following volcanic eruptions. In order to provide for the observation and reporting of volcanic ash clouds and the issuance of warnings to pilots and airlines, ICAO, with the assistance of other international organizations, has established an international airways volcano watch (IAVW). The corner stones of the IAVW are nine volcanic ash advisory centres which issue advisory information on volcanic ash globally, both to aviation users and meteorological offices concerned. Automated observing systems are becoming increasingly useful at aerodromes and currently are considered to meet the aeronautical requirements as far as the observation of the surface wind, visibility, runway visual range and height of the cloud base, air and dew-point temperature and atmospheric pressure are concerned. In view of the improved performance of fully automated systems, they may now be used, without any human intervention, during non-operational hours of the aerodrome.

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