METAR

METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting.

Raw METAR is the most popular format in the world for the transmission of weather data. It is highly standardized through International Civil Aviation Organization (ICAO), which allows it to be understood throughout most of the world.

Origin

METARs typically come from airports or permanent weather observation stations. Reports are generated once an hour, but if conditions change significantly, a report known as a special (SPECI) may be issued. 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.

History

The METAR format was introduced 1 January 1968 internationally 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 World Meteorological Organization's (WMO) publication No. 782 "Aerodrome Reports and Forecasts" contains the base METAR code as adopted by the WMO member countries.^[1]

Naming

The United States Federal Aviation Administration (FAA) lays down the definition in its publication the Aeronautical Information Manual as aviation routine weather report^[2] while the international authority for the code form, the WMO, holds the definition to be 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. METAR is also known as Meteorological Terminal Aviation Routine Weather Report or Meteorological Aerodrome Report.

Information contained in a METAR

A typical METAR contains data for the temperature, dew point, wind speed and direction, precipitation, cloud cover and heights, visibility, and barometric pressure. 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).

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.

Regulation

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 itself has paved the way for the US Air Force Manual 15-111^[3] 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.

METAR conventions

Although the general format of METAR reports 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. The two examples which follow illustrate the primary differences between the two METAR variations.^[4][5]

Example METAR codes

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 12003MPS 310V290 1400 R04/P1500N R22/P1500U +SN BKN022 OVC050 M04/M07 Q1020 NOSIG 9949//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 (the 4th) followed by the time of day (1600 Zulu time, which equals 4:00 pm Greenwich Mean Time).
• 12003MPS indicates the wind direction is from 120° (east-southeast) at a speed of 3 MPS (5.8 KT; 6.7 mph; 11 km/h). Speed measurements can vary from knots (KT) or meters/second (MPS).
• 310V290 indicates the wind direction is varying from 310° true (northwest) to 290° true (west-northwest).
• 1400 indicates the prevailing visibility is 1,400 m (4,600 ft).
• R04/P1500N indicates the Runway Visual Range (RVR) along runway 04 is 1,500 m (4,900 ft) and not changing significantly.
• R22/P1500U indicates RVR along runway 22 is 1,500 m (4,900 ft) 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 2,200 ft (670 m) 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 5,000 ft (1,500 m) above ground level.
• M04/M07 indicates the temperature is −4 °C (25 °F) and the dewpoint is −7 °C (19 °F). An M in front of the number indicates that the temperature/dew point is below zero (0) Celsius.
• Q1020 indicates the current altimeter setting (QNH) is 1,020 hPa (30.12 inHg).
• NOSIG is an example of a TREND forecast which is appended to METARs at stations while a forecaster is on watch. NOSIG means that no significant change is expected to the reported conditions within the next 2 hours.
• 9949//91 indicates the condition of the runway:
  • 99 indicates either a specific runway (e.g. 25 = Rwy 25 or 25L; adding 50 will indicate Right Runway) or all the airport's runways ("99"). Some locations will report the runway using 3 characters (e.g. 25L)
  • 4 means the runway is coated with dry snow
  • 9 means 51% to 100% of the runway is covered
  • // means the thickness of the coating was either not measurable or not affecting usage of the runway
  • 91 means the braking index is bad, in other words the tires have bad grip on the runway
• CAVOK is an abbreviation for Ceiling And Visibility OKay, indicating no cloud below 5,000 ft (1,500 m) or the highest minimum sector altitude and no cumulonimbus or towering cumulus at any level, a visibility of 10 km (6 mi) or more and no significant weather.^[6]
• = indicates the end of the METAR report

North American METAR codes

North American METARs deviate from the WMO (who write the code on behalf of ICAO) FM 15-XII code. Details are listed in the FAA's Aeronautical Information Manual (AIM), but the non-compliant elements are mostly based on the use of non-standard units of measurement. This METAR example is from Trenton-Mercer Airport near Trenton, New Jersey, and was taken on 5 December 2003 at 18:53 UTC.

METAR KTTN 051853Z 04011KT 1/2SM VCTS SN FZFG BKN003 OVC010 M02/M02 A3006 RMK AO2 TSB40 SLP176 P0002 T10171017=^[7]

• METAR indicates that the following is a standard hourly observation.
• KTTN is the ICAO identifier for the Trenton-Mercer Airport.
• 051853Z indicates the day of the month is the 5th and the time of day is 1853 Zulu/UTC, 6:53PM GMT, or 1:53PM Eastern Standard Time.
• 04011KT indicates the wind is from 040° true (north east) at 11 knots (20 km/h; 13 mph). In the United States, the wind direction must have a 60° or greater variance for variable wind direction to be reported and the wind speed must be 7 knots (13 km/h; 8.1 mph) or higher.
• 1/2SM indicates the prevailing visibility is ¹⁄₂ mi (800 m).
• VCTS indicates a thunderstorm (TS) in the vicinity (VC), which means from 5–10 mi (8–16 km).
• SN indicates snow is falling at a moderate intensity; a preceding plus or minus sign (+/-) indicates heavy or light precipitation.
• FZFG indicates the presence of freezing fog.
• BKN003 OVC010 indicates a broken (5/8 to 7/8 of the sky covered) cloud layer at 300 ft (91 m) above ground level (AGL) and an overcast (8/8 of the sky covered) layer at 1,000 ft (300 m).
• M02/M02 indicates the temperature is −2 °C (28 °F) and the dewpoint is −2 °C (28 °F). An M in front of the number indicates that the temperature/dew point is below zero (0) Celsius.
• A3006 indicates the altimeter setting is 30.06 inHg (1,018 hPa).
• RMK indicates the remarks section follows.

Note that what follows are not part of standard observations outside of the United States and can vary significantly.

• AO2 indicates that the station is automated with a rain/snow precipitation sensor. Stations that aren't equipped with a rain/snow sensor are designated AO1.^[8]
• TSB40 indicates the thunderstorm began at 40 minutes past the hour at 1840 Zulu/UTC, 6:40PM GMT, or 1:40PM Eastern Standard Time.
• SLP176 indicates the current barometric pressure extrapolated to sea level is 1,017.6 hPa (30.05 inHg).
• P0002 indicates that 0.02 inches (0.5 mm) of liquid-equivalent precipitation accumulated during the last hour.
• T10171017 is a breakdown of the temperature and dew point in 8 digits separated into two groups of four. the first four digits (1017) describe the temperature. The first digit (1) designates above or below zero Celsius (0=above zero 1=below zero). The next three digits in the group '017' is the temperature in degrees and tenths of a degree Celsius, −1.7 °C (28.94 °F). The last four digits '1017' is the same as the first group but for dew point, −1.7 °C (28.94 °F). Note: ASOS software, as of this update, uses whole degrees in °F to compute the °C values in this group.^{[citation needed]}
• = indicates the end of the METAR report.

In Canada, RMK is followed by a description of the cloud layers and opacities, in eighths (oktas). For example, CU5 would indicate a cumulus layer with 5/8ths opacity.^[9]

Cloud reporting

Cloud coverage is reported by the number of 'oktas' (eighths) of the sky that is occupied by cloud.

This is reported as:^[10]

SKC = 'No cloud/Sky clear' used worldwide but in North America is used to indicate a human generated report^[11][12]

CLR = 'No clouds below 12,000 ft (3,700 m) (US) or 10,000 ft (3,000 m) (Canada)' used mainly within North America and indicates a station that is at least partly automated^[11][12]

NSC = 'No (nil) significant cloud' (i.e. none below 5,000 ft (1,500 m) 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 (ie full cloud coverage)

Flight categories

METARs can be expressed concisely using so-called aviation flight categories, which indicates what classes of flight can operate at each airport by referring to the visibility and ceiling in each METAR. Four categories are used:^[13]

US METAR abbreviations

The following are METAR abbreviations used in the United States, however some are used worldwide:^[4]

METAR and TAF Abbreviations and Acronyms:

See also

• Surface weather observation
• SYNOP
• CLIMAT
• TAF

References

External links

Decoding

• METAR Study Guide approved by the National Weather Services Directorate of Environment Canada
• Wunderground METAR Tutorial
• METARreader.com online decoder

Format specifications

• Meteorological Service For International Air Navigation - July 2007 (PDF format see Table A3-2)
• OFCM.gov - Complete documentation on the METAR format
• Aviation Weather Services AC 00-45F
• NOAA.gov - Information on METAR and TAF reports. Also provides a link to current metar reports and cycle files.
• WMO documentation on METAR format

Software libraries

• Perl modules for parsing METAR reports at the CPAN website
• PhpWeather is a PHP application (with a GNU General Public License) that parses METAR reports.
• pymetar Python library for METAR fetching and parsing
• python-metar a Python package for parsing METAR reports

Current reports

• Selection of worldwide METAR reports from the US NOAA
• List of Stations in NOAA database. Use CTRL+F to search for a station. Input 4 letter ICAO identifier to Worldwide METAR Data Access from the US NOAA link.
• CheckWX Raw and decoded METARs, METAR cycles, trends and graphs for locations worldwide.

Current and historical reports

• Wunderground searchable by location, can view historical METARs by location.