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METAR is a format for reporting weather information. METAR means "aviation routine weather report" and is predominantly used by pilots in fullfilment of a part of a pre-flight weather briefing.

It is also used by meteorologists, who use aggregated METAR information to help forecast the weather. METAR reports usually come from airports. Typically, reports are generated once an hour; however, if conditions change significantly, they may be updated in special reports called SPECI's. And they generally encoded by automated sites located at airports and military bases. Some locations still use Augmented observations, which are recorded by digital sensors and encoded via software, but are watch by certified weather observers or forecasters prior to being transmitted. Even more rare actual observations taken by trained observers or forecasters actually observing and manually encoding observations prior to being transmitted.

The format was introduced 1 January 1968 internationally and has been modified several times since. However, the United States and Canada did not adopt the format until 1 July 1996.

The word METAR is from the French, "message d’observation météorologique régulière pour l’aviation," and is thought to have originated as a contraction from MÉTéorologique ("Weather") Aviation Régulière ("Routine"). The FAA may consider it to be erroneous to abbreviate METAR as METeorological Aerodrome Report. The FAA and NOAA specifically define a METAR as an "aviation routine weather report," an approximate translation of the French.

A typical METAR report contains data for the temperature, dew point, wind, precipitation, cloud cover, cloud heights, visibility, and barometric pressure. A METAR report may also contain information on precipitation amounts, lightning, and other information that would be of interest to pilots or meteorologists such as Colour States.

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 TAF.

The complement to METAR reports, reporting forecasted weather rather than current weather, are TAFs. METARs and TAFs are used in VOLMET broadcasts.

METAR code is regulated by Federal Meteorological Handbook No. 1 (FMH-1) and the World Meteorological Organization (WMO) aviation routine weather reports (FM-15 METAR) and aviation selected special weather reports (FM-16 SPECI) codes.

US metars deviate slightly from the WMO (who write the code on behalf of ICAO) FM 15-XII code. Details listed in FAA's AIM. The following is a METAR example is from Sarafovo Airport in Burgas, Bulgaria, and was taken on February 4, 2005 at 16:00 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 indicates the report came from Sarafovo Airport in Burgas, Bulgaria in continental Europe.
• 041600Z indicates the date of the month is the 4th and the time of day is 1600 Zulu time, 4:00PM Greenwich Mean Time, or 6:00PM Eastern European Time.
• 12003MPS indicates the wind direction is from 120 degrees true (east-southeast) at a speed of 3 meters per second.
• 310V290 indicates the wind direction is varying from 310 degrees true (northwest) through 120 degrees true (east-southeast) to 290 degrees true (west-northwest).
• 1400 indicates the prevailing visibility is 1400 meters.
• R04/P1500N indicates the Runway visual range (RVR) along Runway 4 is 1500 meters and not changing significantly.
• R22/P1500U indicates RVR along Runway 22 is 1500 meters and rising.
• +SN indicates snow is falling at a heavy intensity.
• BKN022 indicates a broken cloud layer at 2,200 feet above ground level (agl).
• OVC050 indicates an unbroken cloud layer (overcast) at 5,000 feet above ground level.
• M04/M07 indicates the temperature is minus 4 degrees Celsius and the dewpoint is minus 7 degrees Celsius.
• Q1020 indicates the current barometric pressure extrapolated to sea level is 1020 millibars.
• NOSIG is an example of a TREND forecast which is apended 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 runway status. Format: abcdefgh ab=runway heading, c=nature of coating (4=dry snow), d=surface covered in percent (9= 51-100% of rwy covered), ef=thickness of coating in millimeters (// stands for either not measurable or not affecting usage of rwy) gh=braking index (91=bad braking index i.e grip on rwy)
• = indicates the end of the METAR report

This METAR example is from Trenton-Mercer Airport near Trenton, New Jersey, and was taken on December 5, 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=

• METAR indicates that the following is a standard hourly observation.
• KTTN indicates the report came from Trenton-Mercer Airport, near the city of Trenton, New Jersey in the continental United States.
• 051853Z indicates the date of the month is the 5th and the time of day is 1853 Zulu time, 6:53PM Greenwich Mean Time, or 1:53PM Eastern Standard Time.
• 04011KT indicates the wind direction is from 40 degrees true (northeast) at a speed of 11 knots (approximately 13 statute miles per hour).
• 1/2SM indicates the prevailing visibility is 0.5 statute miles.
• VCTS indicates there is a thunderstorm in the vicinity (within 10SM, but beyond 5SM).
• SN indicates snow is falling at a moderate intensity.
• FZFG indicates the presence of freezing fog.
• BKN003 indicates a broken cloud layer at 300 feet above ground level.
• OVC010 indicates an unbroken cloud layer (overcast) at 1,000 feet above ground level.
• M02/M02 indicates the temperature is minus 2 degrees Celsius and the dewpoint is minus 2 degrees Celsius.
• A3006 indicates the altimeter setting is 30.06 inches of mercury.

Note that what follows is not part of the international format. This example is particular to the United States, a format not shared with Canada.

• RMK indicates the remarks section follows.
• AO2 indicates that the station has a automated precipitation sensor.
• TSB40 indicates the thunderstorm began 40 minutes after the top of the hour at 1840 Zulu time, 6:40PM Greenwich Mean Time, or 1:40PM Eastern Standard Time.
• SLP176 indicates the current barometric pressure extrapolated to sea level is 1017.6 millibars.
• P0002 indicates that 0.02 inches of liquid-equivalent precipitation accumulated during the last hour.
• T10171017 indicates the temperature is 29 degrees Fahrenheit (converted to minus 1.7 degrees Celsius) and the dewpoint is 29 degrees Fahrenheit (converted to minus 1.7 degrees Celsius).
• = indicates the end of the METAR report.

Meteorological data and variables
Atmospheric pressure | Baroclinity | Cloud | Convection | CAPE | CIN | Dew point | Heat index | Humidex | Humidity | Lightning | Pot T | Precipitation | Sea surface temperature | Surface solar radiation | Surface weather analysis | Temperature | Theta-e | Visibility | Vorticity | Wind chill | Water vapor | Wind

Meteorological instrumentation and equipment (Earth based)

Anemometer | Barograph | Barometer | Ceiling balloon | Ceiling projector | Ceilometer | Dark adaptor goggles | Disdrometer | Field mill | Hygrometer | Ice Accretion Indicator | LIDAR | Lightning detector | Nephelometer | Nephoscope | Pyranometer | Radiosonde | Rain gauge | Snow gauge | SODAR | Solarimeter | Sounding rocket | Stevenson screen | Sunshine recorders | Thermograph | Thermometer | Weather balloon | Weather radar | Weather vane | Windsock | Wind profiler

Meteorological Observation Systems and Weather Stations (Earth based)
Argo | Global Atmosphere Watch | Automatic weather station | Remote Automated Weather Stations (RAWS) | Automated Surface Observing System (ASOS) | Meteorological Aerodrome Report (METAR) | NEXRAD radar | Global Sea Level Observing System | SST buoys | Hurricane Hunters | Dropsonde | Weather balloon

Format specifications

• OFCM.gov - Complete documentation on the METAR format
• NOAA.gov - Information on METAR and TAF reports. Also provides a link to current metar reports and cycle files.
• WMO documentation on METAR format (PDF)

Software libraries

• Perl modules for parsing METAR reports at the CPAN website
• PhpWeather is a PHP application (with a GNU GPL licence) that parses METAR reports.
• pymetar python library for METAR fetching and parsing

Current reports

• The Met Office - METARs and TAFs for the UK (requires free login)
• Nav Canada aviation weather site - provides METARs and TAFs for Canada (and some international stations)
• MyMetar.com - weather mashup providing METARs, TAFs and radar images from a variety of sources; weather via XML too.
• AC Network Weather - Current TAFs and METARs for selected airports in Canada, the United States, and around the world