Pete

Pete has been flying aircraft for the last 20 years. He has flown everything from light piston aircraft up to heavy jets as both First Officer and Captain. He’s currently enjoying life flying the Airbus A330 for a major international airline.

A METAR (Meteorological Terminal Air Report) is a routine aviation weather report of actual observed conditions at an airport. 

METARs report actual weather observed at an airport. METARs are encoded in a standard format that can be understood internationally. There are slight differences between North American and International METARs but the format is generally the same, with different units of measurements used.

How do you decode a METAR?

Let’s take the following example of a METAR we’ve just picked up via ACARS for Chicago O’Hare International:

• KORD 021651Z 260/14kt 10SM FEW250 29/17 A2988 RMK AO2 SLP112 T02940167 $
  
  
• Station: e.g. KORD is Chicago.
  
  
• Issue time: 02165Z The date and the time the METAR was issued in UTC (also known as “Zulu” time), i.e. 021651Z means on the second day of the month, at 1651 Zulu time.
  
  
• Wind direction and speed: 260/14KT Wind direction in METARs is in degrees true (i.e. with reference to true north, as opposed to magnetic north). Wind speed is normally issued in knots, but can also be issued in “MPS” (meters per second). In this case the wind is from 260º True, at 14 knots.
  
  
• Visibility: In the U.S. visibility is measured in Statute Miles (SM) and feet. 10SM is a visibility of “10 Statute Miles or greater”).
  
  
• Cloud cover: FEW250 refers to few cloud at 25,000 feet.
  Cloud cover is measured in “Oktas” or “eighths of the sky”.
  Few (FEW) = 1-2 Oktas
  Scattered (SCT) = 3-4 Oktas 
  Broken (BKN) = 5-7 Oktas (i.e. greater than half the sky)
  Overcast (OVC) = 8 Oktas (i.e. the sky is fully obscured by cloud)
  
  
• Temperature and dew point: 29/17 The temperature is reported first (in degree celsius) followed by the dew point (the temperature at which the air would become saturated with water vapor). The dew point is also reported in degrees celsius.
  
  
• Sea level pressure: A2988 Refers to an altimeter setting in inches of mercury which will show QNH (i.e. the elevation above sea level). In this case, setting 29.88 on the altimeter when at the gate in Chicago will show 668 feet (i.e. the elevation of Chicago O’Hare).
  
  
• RMK (Remarks): The remarks section can contain a number of different items. In this case AO2 tell us this METAR was recorded by an automatic station with a precipitation discriminator. SLP112 tell us the sea level pressure in millibars – in this case 1011.2mb. (A great explanation of SLP by David Werntz can be found here). T02940167 gives us more accurate temperature and dew point information: Temperature of 29.4ºC and dew point of 16.7ºC.
  
  

METAR Decode – Definitive List

How long are METARs valid for?

METARs in the U.S. are valid for one hour and are issued hourly.

In other countries METARs are generally valid for 30 minutes and are issued twice per hour – at 20 minutes past the hour and 50 minutes past the hour.

If there are significant changes since the time a METAR was released a SPECI will be issued.

Related: Aviation SPECI Reports – A Pilot’s Guide

What does $ mean in a METAR?

The $ indicates maintenance is required on the automated system. Sometimes the RMK (remarks) section of the METAR will contain a maintenance status indicator.

METAR Maintenance Status Indicators

Aviation Flight Categories

In the U.S. METARs can be shorted to display the time of flying that is possible at an airport with respect to ceiling and visibility. The four aviation flight categories are:

1. VFR: Visibility greater than 5 miles and ceiling greater than 3000 feet.
2. Marginal VFR: Visibility between 3 and 5 miles and/or a ceiling between 1000 feet and 3000 feet.
3. IFR: Visibility of 1 mile of more but less than 3 miles and/or a ceiling of 500 feet or more but less than 1000 feet.
4. Low IFR: Visibility less than 1 mile and/or a ceiling less than 500 feet.
   
   
   
   

Trend Type Forecast

A trend type forecast, or simply a “trend” is a forecast valid for two hours appended to a METAR or SPECI.

An example of a trend type forecast: TEMPO 3SM VCSH = Temporarily visibility of 3 statute miles, with showers in the vicinity (between 5 and 10 SM of the airport).

What are the differences between METARs and TAFs?

A METAR is Meteorological Terminal Air Report (or “aviation routine weather report”) is a weather report of a actual conditions at an airport at a specific time.

A TAF (Terminal Area Forecast) is a weather forecast for a given area around the airport specified in the forecast.

Therefore the main difference between the two is that METARs are a record of actual conditions where TAFs are a prediction of future weather.

 Another difference is that METARs may include a “remarks”(RMK) section. Remarks may include more accurate temperature, sea level pressure (SLP) or runway state group (for example if the runway is contaminated by snow).

TAFs, METARs and SPECIs use a common code to describe the weather such as TS for thunderstorms or SHRA for rain showers.

Frequently Asked Questions about METARs.

Where can I get more information about aviation weather?

FAA Advisory Circular – Aviation Weather Services

Featured image: jvinals/Unsplash

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Pete

Pete has been flying aircraft for the last 20 years. He has flown everything from light piston aircraft up to heavy jets as both First Officer and Captain. He’s currently enjoying life flying the Airbus A330 for a major international airline.