In April 2010, a volcano in Iceland with a name almost no broadcaster could pronounce, Eyjafjallajokull, shut down most of European aviation for the better part of a week. More than 100,000 flights were canceled. Millions of passengers were stranded. No aircraft was damaged and no one was hurt, and that was exactly the point. The system saw the hazard coming and kept aircraft on the ground rather than in an ash cloud that can melt jet engines from the inside.
The information behind those decisions did not appear by magic. It came from the quiet global machinery defined by Annex 3.
Annex 3 to the Convention on International Civil Aviation governs the Meteorological Service for International Air Navigation. It is the rulebook for the weather information that flight crews, airline operations staff, and air traffic services depend on every minute of every day. It is also unusual among the Annexes, because ICAO does not write it alone. Annex 3 is maintained jointly with the World Meteorological Organization (WMO), the UN agency for weather and climate, so that aviation's requirements and the world's meteorological infrastructure stay in step. The codes a pilot reads in Riyadh are the codes a forecaster issues in London, because the two bodies agreed on them together.
The reports that start at the aerodrome
The most familiar products of Annex 3 are the routine weather reports issued at airports. The METAR, the routine aerodrome weather report, is the regular snapshot of conditions at an airport: surface wind, visibility, present weather, cloud, temperature, dew point, and the altimeter setting. It is issued on a fixed schedule, usually every half hour or hour. Where it matters, it also carries runway visual range (RVR), how far a pilot can see down the runway in fog, and recent weather of operational significance. Every pilot learns to decode it. Its terse, abbreviated format is deliberately international, so the message survives any language barrier.
When conditions change sharply between routine reports, a sudden drop in visibility, a wind shift, the onset of a thunderstorm, a SPECI, the special aerodrome report, is issued out of cycle to flag it. METAR is the heartbeat. SPECI is the alarm between beats. Together they give the operational world a continuously updated picture of what the weather is actually doing on the ground. Once issued they are archived, building the climatological record that informs everything from runway design to seasonal scheduling.
Forecasts: what the weather will do
Observation tells you the present. Aviation lives on the future, because a flight dispatched now lands hours from now in conditions that do not yet exist. Annex 3 prescribes a layered set of forecasts to cover that gap.
- The TAF (Terminal Aerodrome Forecast) is the airport forecast, a structured prediction of conditions at an aerodrome over a defined period, typically up to 24 or 30 hours. It is the backbone of flight planning and fuel decisions.
- The TREND forecast is a short landing forecast appended to a report, describing expected changes over the next two hours: what the runway will be like as you arrive.
- Take-off forecasts describe conditions expected at the runway around departure.
- Area and route forecasts cover the airspace and the journey in between, not just the endpoints.
The point of the layering is that different decisions need different horizons. The dispatcher planning fuel needs the TAF. The captain on short final needs the TREND. A forecast is also a living document: when the actual weather diverges from what a TAF predicted, it is amended and reissued, because a forecast everyone has stopped trusting is worse than none at all. Annex 3 makes sure each product exists, is kept current, and means the same thing everywhere it is used.
Figure 1 — Different decisions need different horizons
Different decisions need different horizons. The dispatcher lives on the TAF; the captain on short final lives on the TREND.The hazard warnings
Routine reports and forecasts handle the expected. A separate family of messages handles the dangerous. These are the warnings that can change a flight plan in minutes.
A SIGMET (Significant Meteorological Information) warns of weather hazardous to all aircraft: thunderstorms, severe turbulence, severe icing, volcanic ash, tropical cyclones. An AIRMET covers phenomena significant mainly to lower-level and smaller-aircraft operations, a step below SIGMET in severity. At the airport itself, aerodrome warnings alert ground staff and operators to conditions that could damage parked aircraft or facilities, and wind shear warnings and alerts flag the sudden wind changes near the runway that are among the most dangerous things a transport aircraft can meet close to the ground.
A METAR tells you what the sky is doing. A SIGMET tells you to change your plan.
What these warnings do not do is make the decision. Annex 3 draws a deliberate line: the meteorological service provides the hazard information, and the operator and pilot-in-command decide what to do with it. A volcanic-ash SIGMET does not close an airspace by itself. It gives the people who can close it the evidence to act. That separation between information and authority runs through the whole Annex.
The system above the system
Local reports and forecasts only work because they sit on top of a global and regional architecture, and Annex 3 defines that too. This is the part most passengers never imagine exists.
The World Area Forecast System (WAFS) provides standardized global forecasts of upper-air wind, temperature, and significant weather for flight planning. They are produced by two World Area Forecast Centers (WAFC), one in London and one in Washington, whose outputs are designed to be consistent, so an airline gets the same picture whichever it draws from, and the second can back up the first if one goes offline. For specific hazards, ICAO designates specialist centers:
- Volcanic Ash Advisory Centers (VAAC) track and forecast the movement of volcanic ash clouds and issue the advisories that drive closures and reroutes.
- Tropical Cyclone Advisory Centers (TCAC) issue advisory information on tropical cyclones affecting flight.
- Space weather advisory centers, a more recent addition, warn of solar activity that can disrupt communications, navigation, and radiation levels on high-latitude routes.
If you think of it like a content-delivery network, the WAFCs are the origin servers producing the authoritative global product, and the specialist advisory centers are dedicated services for the hazards that need expert, focused handling. States consume and redistribute these products rather than each trying to model the whole planet independently, which would be wasteful for large states and impossible for small ones.
Who does the work on the ground
Annex 3 assigns the day-to-day duties to two key offices. The meteorological watch office (MWO) keeps watch over a defined area, typically aligned with a flight information region, and is the unit that issues SIGMET and AIRMET for that airspace. The aerodrome meteorological office serves a specific airport, producing its observations, the TAF, and the TREND, and briefing operators and crews. The division mirrors how aviation itself is organized: one eye on the wide airspace, one on the runway.
Figure 2 — The system above the system
The system above the system. Two world forecast centers and the specialist advisory centers feed every region, airport, and cockpit.Charts and upper-air data
Not all weather information is text. Significant weather (SIGWX) charts depict hazards, fronts, jet streams, areas of turbulence, thunderstorms, and the freezing level, across a region in a single graphical view. Upper-air wind and temperature data feed both the charts and the flight-planning systems that compute the most efficient and economical route through the winds aloft. Picking up a tailwind or dodging a headwind across an ocean is worth real money and real fuel, and it starts with this data.
How it reaches the cockpit
All of this would be inert if it stayed on a forecaster's screen. Before every flight, the crew receives a meteorological briefing. Historically it was a printed flight folder of charts and coded reports. Now it is increasingly a self-briefing through an electronic system that pulls the relevant METARs, TAFs, SIGMETs, and charts for the specific route. Annex 3 defines what that briefing must contain and the obligation to supply it, so that the captain signing for a flight has, in front of them, the same authoritative picture the dispatcher used to plan it.
Quality, and the move to digital
Weather information is only useful if it is trustworthy and timely, so Annex 3 requires quality management of meteorological services. The providers must run their operations under a managed quality system, not as best effort. Just as important is the shift in how the information moves. For decades, aviation weather traveled as the compact alphanumeric codes designed for teleprinters. Annex 3 has been steering the world toward digital, machine-readable exchange through the ICAO Meteorological Information Exchange Model (IWXXM), an XML-based format that lets systems ingest, filter, and display weather automatically rather than relying on a human to decode a string. The METAR a pilot reads is not going away, but underneath it the data increasingly flows as structured digital information that an automated flight-planning system can act on without a person in the loop.
How it connects, and how states implement it
Annex 3 does not stand alone. It feeds Annex 11 (Air Traffic Services), because controllers need the weather picture to manage traffic, and it is tied to Annex 15 (Aeronautical Information Services), the broader information supply chain that delivers data to the cockpit. Weather is one stream in a larger river of operational information.
States meet their Annex 3 obligations through their national meteorological arrangements. In Saudi Arabia, the General Authority of Civil Aviation (GACA) works with the national meteorological service to provide aviation weather. In the United States, the National Weather Service's Aviation Weather Center issues the SIGMETs, forecasts, and products for US airspace. Across Europe, national meteorological services provide aviation weather under a harmonized framework coordinated with the wider European air-navigation system. The labels differ. The products, METAR, TAF, SIGMET, are the same by design, which is the entire purpose of having an Annex.
Where the gaps still are
For all its maturity, aviation meteorology has honest weak points, and they tend to show up exactly where the stakes are highest.
- Timeliness and consistency of warnings. A SIGMET issued late, or differing across a boundary because two watch offices saw the same storm differently, undercuts the value of the warning. Harmonizing how and when hazards are flagged remains a live effort.
- Coverage over remote and oceanic regions. Observations are sparse where there are no airports and few aircraft to report, over oceans and unpopulated terrain. So the forecast quality in exactly the areas with the least margin for error can be the weakest.
- The transition to digital. Moving the whole world from legacy codes to IWXXM is uneven. Until everyone is on the digital format, systems must straddle both, and the benefits of automation are only partly realized.
Go back to that Icelandic volcano. The week it grounded Europe was later criticized as over-cautious, and the response reshaped how ash concentration thresholds and no-fly zones are defined, so that future eruptions could be managed with more precision and fewer blanket closures. That arc, a hazard, a hard decision made on imperfect information, and a refinement of the system afterward, is Annex 3 in miniature. The weather will always be the one part of the operation that no regulator can control. The most aviation can do is make sure that everyone, everywhere, is looking at the same honest picture of it before they decide whether to fly.
Glossary
Aerodrome Meteorological Office
The office serving a specific airport: it produces the observations, the TAF and the TREND, and briefs operators and crews.
AIRMET
A warning of weather significant mainly to lower-level and smaller-aircraft operations, a step below SIGMET in severity.
ICAO Meteorological Information Exchange Model (IWXXM)
The XML-based digital format for exchanging aviation weather so that systems can process it automatically.
METAR
The routine aerodrome weather report: wind, visibility, present weather, cloud, temperature, dew point and altimeter setting, issued every 30 to 60 minutes.
Meteorological Watch Office (MWO)
The office keeping watch over a defined area, typically a flight information region. It issues the SIGMET and AIRMET for that airspace.
Runway Visual Range (RVR)
How far a pilot can see along the runway, reported when visibility is reduced.
SIGMET
Significant Meteorological Information: a warning of weather hazardous to all aircraft, such as thunderstorms, severe turbulence, severe icing, volcanic ash or tropical cyclones.
Significant Weather (SIGWX) Chart
A graphical chart showing hazards such as fronts, jet streams, turbulence and thunderstorms across a region.
SPECI
A special aerodrome report issued between routine METARs when conditions change sharply.
TAF (Terminal Aerodrome Forecast)
The structured forecast for an aerodrome, typically covering the next 24 to 30 hours. The backbone of flight planning and fuel decisions.
TREND Forecast
A short landing forecast appended to a report, covering expected changes over the next two hours.
Tropical Cyclone Advisory Center (TCAC)
A designated center issuing advisory information on tropical cyclones affecting flight.
Volcanic Ash Advisory Center (VAAC)
A designated center that tracks and forecasts volcanic ash clouds and issues the advisories behind closures and reroutes.
World Area Forecast System (WAFS)
The global system providing standardized upper-air and significant weather forecasts, produced by the two World Area Forecast Centers in London and Washington.
Sources
- Convention on International Civil Aviation (ICAO Doc 7300), Article 37.
- ICAO, Annex 3 to the Convention on International Civil Aviation, Meteorological Service for International Air Navigation.
- World Meteorological Organization, Technical Regulations, Volume II, Meteorological Service for International Air Navigation (published jointly with ICAO Annex 3).
- ICAO Doc 8896, Manual of Aeronautical Meteorological Practice.
- United States National Weather Service, Aviation Weather Center, operational products for US airspace.