METAR

Raw METAR is the most common format in the world for the transmission of observational weather data.

[citation needed] It is highly standardized through the International Civil Aviation Organization (ICAO), which enables it to be understood throughout most of the world.

[5] In addition to METARs and SPECIs, ASOS One-Minute Observations (OMO) are updated once a minute.

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.

782 "Aerodrome Reports and Forecasts" contains the base METAR code as adopted by the WMO member countries.

[6] 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.

[4]: 6 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.

Automated substation substitutes time averaging of sensor data gathered during 30-minute period prior to reporting.

[4]: 24–10 The wind direction is coded with the first three digits in tens of degrees relative to the true north.

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.

[4]: 5 The two examples which follow illustrate the primary differences between the international and the North American METAR variations.

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.

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

In Canada, RMK is followed by a description of the cloud layers and opacities, in eighths (oktas).

Combinations of two precipitation types are accepted; e.g., RASN (rain and snow mixed), SHGSSN etc.