METAR is a format for reporting weather information. A METAR weather report is predominantly used by aircraft pilots, and by meteorologists, who use aggregated METAR information to assist in weather forecasting.
Raw METAR is highly standardized through the International Civil Aviation Organization (ICAO), which enables it to be understood throughout most of the world.
Report names Although it is capitalized like an acronym, the origin of the word "METAR" has not been confirmed. The United Nations International Civil Aviation Organization (ICAO) refers to the Meteorological Aerodrome ReportAerodrome Meteorological Observation and Forecast Study Group (AMOFSG) at ICAO. while other sources link the acronym to METeorological Terminal Aviation Routine Weather ReportMETAR (MEteorological Terminal Aviation Routine Weather Report) Station Network at the Centre for Environmental Data Archival. Some unofficial English-language sources suggest that the acronym comes from French phrases such as ''MÉTéorologique Aviation Régulière''. However, this seems unlikely since authoritative sources in French agree that the name comes from the ICAO's "METeorological Aerodrome Report". Other organizations make no attempt to connect their terminology with the METAR acronym. For example, in its publication the Aeronautical Information Manual (AIM), the United States Federal Aviation Administration (FAA) describes the report as aviation routine weather report, while the international authority for the code form, the World Meteorological Organization (WMO), describes it as the aerodrome routine meteorological report. The National Oceanic and Atmospheric Administration (part of the United States Department of Commerce) and the United Kingdom's Met Office both employ the definition used by the FAA.
Frequencies and types
METARs typically come from airports or other permanent weather observation stations. Reports are generated once an hour or half-hour at most stations, but if conditions change significantly at a staffed location, a report known as a special (SPECI) may be issued. Some stations make regular reports more frequently, such as Pierce County Airport (ICAO code: KPLU) which issues reports three times per hour. In addition to METARs and SPECIs, ASOS One-Minute Observations (OMO) are updated once a minute. OMOs can be in various formats, including the METAR format.
Some METARs are encoded by automated airport weather stations located at airports, military bases, and other sites. Some locations still use augmented observations, which are recorded by digital sensors, encoded via software, and then reviewed by certified weather observers or forecasters prior to being transmitted. Observations may also be taken by trained observers or forecasters who manually observe and encode their observations prior to transmission. In the United States, prior to mid-1990s, most observations were made manually, but today the vast majority are automated or augmented observations.
History The METAR format was introduced internationally on 1 January 1968, and has been modified a number of times since. North American countries continued to use a Surface Aviation Observation (SAO) for current weather conditions until 1 June 1996, when this report was replaced with an approved variant of the METAR agreed upon in a 1989 Geneva agreement. The WMO's publication No. 782 "Aerodrome Reports and Forecasts" contains the base METAR code as adopted by the WMO member countries.
Digital dissemination METAR and TAF data are distributed globally through aeronautical fixed telecommunication networks and are also made available via internet services. They are consumed by airline operations centres, automated briefing systems and a wide range of general aviation tools, including web-based briefing portals, electronic flight bags and mobile applications that display weather overlays on maps and provide route-based summaries of observed and forecast conditions.
Information contained in a METAR A typical METAR contains data for the airport identifier, time of observation, wind direction and speed, visibility, current weather phenomena such as precipitation, cloud cover and heights, temperature, dew point, and barometric pressure. This information forms the body of the report, consisting a maximum of 11 groups of information. A METAR may also contain information on precipitation amounts, lightning, and other information that would be of interest to pilots or meteorologists such as a pilot report or PIREP, colour states and runway visual range (RVR). These may be provided in coded or plain language and appended to the end of the METAR as remarks.
In addition, a short period forecast called a TREND may be added at the end of the METAR covering likely changes in weather conditions in the two hours following the observation. These are in the same format as a terminal aerodrome forecast (TAF).
The complement to METARs, reporting forecast weather rather than current weather, are TAFs. METARs and TAFs are used in VOLMET broadcasts.
Cloud reporting Cloud coverage is reported by the number of "oktas" (eighths) of the sky that is occupied by cloud. Automated substation substitutes time averaging of sensor data gathered during 30-minute period prior to reporting.
This is reported as:
+ Cloud coverage codes Abbreviation Meaning SKC "No cloud/Sky clear" used worldwide but in North America is used to indicate a human generated reportSky Condition Group NsNsNshshshs or VVhshshs or SKC Department of Atmospheric Sciences at Texas A&M University NCD "Nil Cloud detected" automated METAR station has not detected any cloud, either due to a lack of it, or due to an error in the sensors CLR "No clouds below (US) or (Canada)", used mainly within North America and indicates a station that is at least partly automated NSC "No (nil) significant cloud", i.e., none below and no TCU or CB. Not used in North America. FEW "Few" = 1–2 oktas SCT "Scattered" = 3–4 oktas BKN "Broken" = 5–7 oktas OVC "Overcast" = 8 oktas, i.e., full cloud coverage TCU Towering cumulus cloud, e.g., SCT016TCU CB Cumulonimbus cloud, e.g., FEW015CB VV "Vertical visibility" = clouds cannot be seen because of fog or heavy precipitation, so vertical visibility is given instead.
The following codes identify the cloud types used in the 8/nnn part of RMK.
+ WMO codes for cloud types Code Low clouds Middle clouds High clouds 0nonenonenone 1 Cumulus(fair weather) Altostratus(thin) Cirrus(filaments) 2 Cumulus(towering) Altostratus(thick) Cirrus(dense) 3 Cumulonimbus(no anvil) Altocumulus(thin) Cirrus(often with cumulonimbus) 4 Stratocumulus(from cumulus) Altocumulus(patchy) Cirrus(thickening) 5 Stratocumulus(not cumulus) Altocumulus(thickening) Cirrus / cirrostratus(low in sky) 6 Stratus or Fractostratus(fair) Altocumulus(from cumulus) Cirrus / cirrostratus(hi in sky) 7 Fractocumulus / fractostratus(bad weather) Altocumulus(with altocumulus,altostratus, nimbostratus) Cirrostratus(entire sky) 8 Cumulus and stratocumulus Altocumulus(with turrets) Cirrostratus(partial) 9 Cumulonimbus(thunderstorm) Altocumulus(chaotic) Cirrocumulus orCirrocumulus / cirrus / cirrostratus / not valid above overcast above overcast
Wind reporting Wind observation measures the horizontal vector component of the wind, which includes both direction and speed. These are determined by evaluating the measurement over a 2-minute period.
The wind direction is coded with the first three digits in tens of degrees relative to the true north. If wind speed is less than or equal to , the wind direction will be displayed as variable or "VRB". If the wind speed is greater than 6 knots, but the direction varies more than 60° in the past 2 minutes, METAR will report the range of wind direction. For example, 21010KT 180V240 suggests the wind was variable from 180° to 240° at 10 knots.
Immediately after the wind direction is the wind speed, coded in two or three digits measured in knots, km/h or m/s. If during past 10 minutes, the weather station detects more than between minimum and maximum windspeed, METAR determines a wind gust exists and reports the maximum instantaneous windspeed.
If the air is motionless, the wind will be reported as calm and coded as 00000KT.
Visibility and runway visual range Visibility measures the atmospheric opacity. It is the greatest distance where at least half of the horizon circle can be seen from the surface.
Runway visual range (RVR) is an instrument-derived measurement that suggests the horizontal distance an observer may see down the runway. In the US, for stations with RVR reporting capacity, this information is omitted from the METAR unless the visibility is at or below , or the designated instrument runway's RVR is at or below . RVR of up to four designated runways may be reported, depending on the country.
Regulations and conventions METAR code is regulated by the World Meteorological Organization in consort with the International Civil Aviation Organization. In the United States, the code is given authority (with some US national differences from the WMO/ICAO model) under the Federal Meteorological Handbook No. 1 (FMH-1), which paved the way for the US Air Force Manual 15-111 on surface weather observations, being the authoritative document for the US Armed Forces. A very similar code form to the METAR is the SPECI. Both codes are defined at the technical regulation level in WMO Technical Regulation No. 49, Vol II, which is copied over to the WMO Manual No. 306 and to ICAO Annex III.
Although the general format of METARs is a global standard, the specific fields used within that format vary somewhat between general international usage and usage within North America. Note that there may be minor differences between countries using the international codes as there are between those using the North American conventions — ICAO allows member countries to modify METAR code for use in their particular countries, as long as ICAO is notified.
Examples
The two examples which follow illustrate the primary differences between the international and the North American METAR variations.
International METAR codes The following is an example METAR from Burgas Airport in Burgas, Bulgaria. It was taken on 4 February 2005 at 16:00 Coordinated Universal Time (UTC).
METAR LBBG 041600Z 12012MPS 090V150 1400 R04/P1500N R22/P1500U +SN BKN022 OVC050 M04/M07 Q1020 NOSIG 8849//91=
METAR indicates that the following is a standard hourly observation. LBBG is the ICAO airport code for Burgas Airport. 041600Z indicates the time of the observation. It is the day of the month (04) followed by the time of day (1600 Zulu time, which equals 4:00 pm Greenwich Mean Time or 6:00 pm local time). 12012MPS indicates the wind direction is from 120° (east-southeast) at a speed of . Speed measurements can be in knots (abbreviated KT) or metres per second (abbreviated MPS). 090V150 indicates the wind direction is varying from 90° true (east) to 150° true (south-southeast). 1400 indicates the prevailing visibility is . R04/P1500N indicates the Runway Visual Range (RVR) along runway 04 is and not changing significantly. R22/P1500U indicates RVR along runway 22 is and rising. +SN indicates snow is falling at a heavy intensity. If any precipitation begins with a minus or plus (-/+), it's either light or heavy. BKN022 indicates a broken (over half the sky) cloud layer with its base at above ground level (AGL). The lowest "BKN" or "OVC" layer specifies the cloud ceiling. OVC050 indicates an unbroken cloud layer (overcast) with its base at above ground level (AGL). M04/M07 indicates the temperature is and the dew point is . An M in front of the number indicates that the temperature/dew point is below zero Celsius. Q1020 indicates the current altimeter setting (in QNH) is .