A METAR transmits weather information and forecasts, which are key elements for air traffic management.
It is well known that there is a close relationship between meteorology and aeronautics. For an airport, having correct, complete and up-to-date weather information is vital for its operations, as its operating procedures vary according to the prevailing weather.
Therefore, all airports have their own weather station, which provides ATC with information on the meteorological values existing at the airport so that this information is distributed from ATC to the other airport stakeholders for whom this information is relevant (mainly pilots and Operations). Based on these values, the pilot acts accordingly, following the procedures established for this purpose, or Operations decides the runway configuration to be used.
This weather information is transmitted via a code known as a METAR (METeorological Aerodrome Report) consisting of a series of letters and numbers that provide the information in an abbreviated and standardised form. These codes, which date back to 1968, came into full use in aviation in the late 1980s and are accepted internationally - although in North America they are used with slight differences.
The information contained in an International METAR code always follows the following order: Location - Date and Time - Wind - Visibility - phenomena - clouds - temperature - pressure, so it is easy to understand once a few are read:
Place
ICAO code of the airport for which the report is issued.
Date and Time
It consists of a code of six numbers followed by the letter Z. The first two indicate the date of the METAR code, while the next four indicate the time at which the METAR was published. The letter "Z" indicates that the time is "Zulu", i.e. the time corresponding to the Greenwich Meridian, known as Coordinated Universal Time (UTC).
Wind
The METAR code provides both the direction with respect to geographical north and the average wind speed expressed in knots.
After the code indicating the wind direction and average wind speed there may be another code of the form XXXVYYYY which indicates that the wind direction varies between XXX and YYYY.
Visibility
Indicates the horizontal visibility expressed in metres. If the visibility is greater than 10km, 9999 is indicated. If the visibility is less than 2000m, the existing RVR should be indicated.
Meteorological Phenomena
The existence or not of significant meteorological phenomena is indicated by the acronyms that can be seen in the graph on the right:
Clouds
The METAR code indicates the cloudiness expressed in octas of overcast sky, indicating the height of the cloud cover. For this purpose, the sky above the airport is divided into eight parts and, depending on how many eighths of the sky are overcast, the following is indicated:
FEW 1 to 2 octas of overcast sky.
SCT (Scattered) 3 to 4 octas of overcast sky.
BKN (Broken) 5 to 7 octas of overcast sky.
OVC (Overcast) 8 octas overcast (Totally overcast)
The height of the cloud cover is given in hundreds of feet (ft), as is the flight level.
If there is a total absence of clouds, significant weather phenomena and a visibility of 10km or more, the abbreviations CAVOK (Ceiling and Visibility OK)
Temperature
It consists of a code of four numbers. The first two indicate the temperature at the airport and the remaining two indicate the dew temperature, both expressed in degrees Celsius.
Pressure
It consists of a Q followed by a number (usually four digits) indicating the QNH at the airport, i.e. the pressure at sea level deduced from the pressure at the airport, expressed in hectoPascals.
Finally, if the code "NOSIG" appears after the METAR code, it indicates that no significant change in weather conditions is expected within two hours of the time it was published in METAR.
As the proof of the pudding is in the eating, we incorporate a couple of examples to better understand how the METAR code works:
LEMG 280800Z 30002KT CAVOK 13/09 Q1020 NOSIG
Place: Malaga Airport (LEMG)
Day: 28
Time: 08:00 UTC
Wind direction and wind speed: 300º and 2kt
Clouds and Visibility: CAVOK conditions (no clouds, weather phenomena and visibility above 10km).
Temperature: 13º C
Dew Temperature: 9º.
Pressure: 1020hPa
No significant changes for the next two hours.
EGLL 251720Z 35004KT 320V020 9999 BKN042 OVC049 11/04 Q1013
Place: London-Heathrow Airport (EGLL)
Day: 25
Time: 17:20 UTC
Wind direction and wind speed: 350º and 4kt
Wind variable between 320º and 20º with respect to Geographic North.
Visibility: Greater than 10km
Clouds: Broken sky ("Broken") at 4200ft and overcast ("OVC") at 4900ft
Temperature: 11º C
Dew Temperature: 4º.
Pressure: 1013hPa
To put all this into practice, I invite the reader to decode the following METAR codes:
LEMD 270530Z 11004KT 9999 FEW050 05/02 Q1025 NOSIG
GCXO 280500Z 18004KT 150V230 5000 HZ 17/07 Q1019 NOSIG
EHAM 271825Z 13008KT 9000 -RADZ FEW008 SCT010 BKN012 07/06 Q1005
It may seem overly complicated, but to help users, there are sites such as Metar Reader which are used to decode any METAR.
