A Meteorological Aerodrome Report, or METAR, is the standardized global format for reporting current weather conditions at airports and permanent weather observation stations. This coded report is a fundamental tool used by flight crews, air traffic controllers, and flight dispatchers for aviation safety and planning. A METAR provides a concise snapshot of the weather at a specific location, including details on wind, visibility, temperature, and atmospheric pressure.
The Standard Hourly Schedule
The routine issuance of a METAR follows a predictable schedule. At most major airports and observation sites, a new METAR is generated once every hour. This routine weather report is known as an “Aviation Routine Weather Report” and is considered valid for the hour it covers.
The observation is typically taken a few minutes before the top of the hour, often between 5 to 10 minutes prior to the hour mark. For example, the report for the 1800 hour might be generated at 1753 or 1755 Coordinated Universal Time (UTC). The time of the observation is always included in the report using the Zulu (Z) designator, which is the aviation term for UTC.
Triggers for Special Updates
While the hourly schedule provides a regular baseline, rapidly changing weather conditions mandate an immediate, unscheduled update known as a SPECI (Special weather report). A SPECI is issued when specific, tightly defined meteorological criteria are met that could impact flight operations and safety. These criteria must be triggered by a significant change in one or more weather elements.
One of the most common triggers is a substantial change in visibility, such as when it drops below or rises above thresholds like three, two, or one statute mile. Similarly, a SPECI is required if the cloud ceiling crosses defined limits, particularly 1,500 feet above ground level, or other specified heights. The wind can also trigger an update if a wind shift occurs by 45 degrees or more in direction, and the speed is 10 knots or greater.
Furthermore, the onset or cessation of a thunderstorm, hail, or any type of freezing precipitation requires an immediate SPECI. A sudden change in the intensity of precipitation, such as rain or snow shifting from light to heavy, also mandates an unscheduled report.
How METAR Data is Collected and Shared
The information contained within a METAR report is gathered primarily through sophisticated automated weather systems located at the airport. The two main types of systems are the Automated Surface Observing System (ASOS) and the Automated Weather Observing System (AWOS). These systems use an array of sensors to continuously monitor conditions, including temperature, wind speed and direction, cloud height, and visibility.
ASOS and AWOS systems generate observations as frequently as every minute, providing a near real-time stream of data. The standardized METAR report, however, is a coded summary of this minute-by-minute data, formatted for international use. At many locations, human weather observers supplement or augment the automated data, especially during complex weather events that sensors cannot fully interpret.
Once the METAR is finalized, whether fully automated or human-augmented, it is disseminated rapidly through the global meteorological network. This process ensures the report is immediately available to flight dispatchers, air traffic controllers, and pilots across the world for timely flight planning and operational decision-making.