Aerodrome Weather Information and ATIS: METAR Wind Direction True vs Magnetic

090-03-01Pablo Asensio Martínez2026-04-012 min

Wind Reporting Rules

A crucial distinction exists in how wind direction is reported in aviation:

  • Written Information (Maps, METAR, TAF): Wind direction is referenced to TRUE North.
    • Rule of thumb: "If you read it, it's True."
  • Spoken Information (ATC, ATIS): Wind direction is referenced to MAGNETIC North.
    • Rule of thumb: "If you hear it, it's Magnetic."
    • Reason: Pilots use magnetic compasses/headings for takeoff and landing, so ATC converts the wind to magnetic to align with the runway, which is also numbered magnetically.

ATIS (Automatic Terminal Information Service)

ATIS is the automatic provision of current, routine information to arriving and departing aircraft.

  • Voice-ATIS: Continuous voice broadcast, usually on a discrete VHF frequency (or VOR voice channel).
  • D-ATIS: Provision of ATIS via data link (e.g., ACARS).

Content and Codes

ATIS messages follow a specific structure (Aerodrome, Designator, Time, Runway, Weather, etc.).

  • Example Decoding: "TWY E3 Southbound CLSD due WIP" $\rightarrow$ Taxiway E3 southbound closed due to Work In Progress.

VOLMET

VOLMET provides meteorological information (METAR, SPECI, TAF, SIGMET) for multiple aerodromes to aircraft in flight.

  • VHF VOLMET: Normally continuous broadcasts.
  • HF VOLMET: Scheduled broadcasts (used for long-range/oceanic coverage).
  • D-VOLMET: Provision via data link.

Meteorological Codes (METAR/SPECI)

CAVOK (Ceiling And Visibility OK)

Used when distinct conditions occur simultaneously:

  • Visibility: 10 km or more.
  • Clouds: No clouds of operational significance (No clouds below 5000 ft or MSA, no CB/TCU).
  • Weather: No significant weather phenomena.

RVR (Runway Visual Range)

Format: R(Runway)/(Value)(Tendency)

  • P: Plus (Greater than). E.g., P2000 > 2000m.
  • M: Minus (Less than). E.g., M0150 < 150m.
  • U: Upward (Increasing) tendency.
  • D: Downward (Decreasing) tendency.
  • N: No distinct tendency.
  • V: Variable (e.g., 1100V1500).

Other Definitions

  • Ceiling: The height of the base of the lowest cloud layer covering more than half the sky (BKN or OVC).
  • Wind Shear (WS): Reported if affecting takeoff/approach paths options (e.g., WS TKOF RWY20 or WS ALL RWY).

Air-Reports (AIREP)

Special Air-Reports (AIREP SPECIAL)

Pilots must make a special air-report whenever the following conditions are encountered:

  1. Moderate or Severe Turbulence.
  2. Moderate or Severe Icing.
  3. Severe Mountain Wave.
  4. Thunderstorms (with/without hail, obscured, embedded, widespread, or in squall lines).
  5. Heavy Duststorm/Sandstorm.
  6. Volcanic Ash or Pre-eruption activity.
  7. Braking Action: If the braking action encountered is different (better or worse) than reported.

Routine Air-Reports

  • Link with Data Link (ADS-C): Aircraft equipped with ADS-C are exempt from making verbal routine meteorological reports (automation handles it).
  • Reporting Intervals (if required):
    • En-route: Every 15 minutes.
    • Climb-out: Every 30 seconds for the first 10 minutes.