A Meteorological Aerodrome Report (METAR) is the standardized format for communicating current weather conditions at airports to pilots and aviation personnel. This report is a crucial tool for flight planning and safety, providing details on temperature, visibility, and wind direction and speed. Since aviation operations rely heavily on magnetic orientation, confusion often arises whether the wind direction reported in the METAR refers to True North or Magnetic North. The answer lies in understanding the strict international standards governing meteorological reporting versus the practical needs of aircraft navigation.
The Definitive Answer: METAR Wind Direction Standard
The wind direction found within a METAR report is referenced to True North, also known as geographical north. This standard is set by international bodies, including the World Meteorological Organization (WMO) and the International Civil Aviation Organization (ICAO), to ensure global consistency for meteorological observations. The wind direction is indicated by the first three digits of the wind group, showing the direction from which the wind is blowing in tens of degrees.
Reporting wind direction against a fixed reference point ensures the data remains universally comparable regardless of location. Since METARs are fundamentally weather observation reports, they adhere to the conventions used for all global weather maps and forecasts. The consistent use of True North for all textual weather products, including the Terminal Aerodrome Forecast (TAF), prevents the need for continuous correction based on local magnetic variation.
Understanding True North and Magnetic North
The distinction between True North and Magnetic North is central to understanding the METAR standard. True North is the fixed, geographical reference point corresponding to the Earth’s axis of rotation, where all lines of longitude converge at the North Pole. This point never changes position, making it an absolute reference for mapping and large-scale geographic measurements, and maps based on this point are stable.
Magnetic North, in contrast, is the wandering point on the Earth’s surface where the planet’s magnetic field lines point vertically downward. This magnetic pole is not fixed; it constantly shifts due to the motion of molten metals in the Earth’s outer core. The pole can move by approximately 60 kilometers per year, meaning its position relative to any fixed point is always changing.
The angular difference between True North and Magnetic North at any given location is known as magnetic declination or magnetic variation. This variation is not consistent across the globe; it changes depending on the observer’s position. The magnitude of the difference can range from zero to double-digit degrees, making Magnetic North an unsuitable reference for standardized, global meteorological reports like the METAR.
Practical Application: Magnetic Bearings in Aviation
While METAR reports use the fixed reference of True North, most day-to-day operations for pilots and air traffic control (ATC) rely on Magnetic North. This reliance stems from the fact that most aircraft navigation instruments, such as the magnetic compass and directional gyro, are aligned with the Earth’s magnetic field. Pilots fly magnetic headings, and ATC issues instructions based on magnetic bearings.
The most visible application of this magnetic standard is the numbering of airport runways. A runway is designated by a number corresponding to its magnetic bearing, rounded to the nearest 10 degrees, with the final zero dropped. For example, a runway aligned with a magnetic heading of 357 degrees is rounded to 360 degrees, and the number painted on the pavement is ’36’.
This magnetic alignment means a pilot must perform a calculation when using the True wind direction from the METAR. To determine the effective headwind or crosswind component for a magnetic runway, the pilot must convert the True METAR wind direction to a Magnetic wind direction using the local magnetic declination. This conversion is not necessary when receiving a wind check directly from the control tower or an automated voice system like ATIS, as those spoken reports are typically converted to Magnetic North before being broadcast.