12.metar-decoding. METAR Decoding
A METAR (Aviation Routine Weather Report) is the standard observation of current surface weather conditions at an airport. Routine METARs are issued hourly, typically between 55 minutes past the hour and the top of the hour. A SPECI is a special, unscheduled report issued when conditions change significantly between routine observations (for example, a wind shift of 45° or more at 10 knots or greater, visibility dropping below 3 SM, or the onset of thunderstorms).
METARs use a fixed sequence of coded groups. Reading them in order eliminates confusion:
- Type of report — METAR or SPECI.
- Station identifier — the four-letter ICAO code (e.g., KORD, KDEN). U.S. stations begin with K.
- Date/time — six digits followed by Z. The first two digits are the day of the month; the next four are the time in UTC (Zulu). Example: 121753Z = the 12th day at 1753Z.
- Modifier — AUTO indicates a fully automated observation with no human input; COR indicates a corrected report.
- Wind — five or six digits followed by KT. The first three digits are the direction (true north, in tens of degrees); the next two or three are the speed in knots. Gusts are appended with G. VRB indicates variable direction (used at speeds ≤ 6 knots). 00000KT means calm. Example: 24015G25KT = wind from 240° at 15 knots, gusting to 25.
- Variable wind — when direction varies 60° or more at speeds > 6 knots, a group like 210V270 follows the wind group.
- Visibility — prevailing visibility in statute miles, ending in SM (e.g., 10SM, 1 1/2SM, M1/4SM where M = less than).
- Runway visual range (RVR) — included when prevailing visibility is 1 SM or less, or RVR is 6,000 ft or less. Format: R<runway>/<value>FT, e.g., R28L/2600FT.
- Weather phenomena — coded with intensity, descriptor, and type. Intensity: "-" light, no symbol moderate, "+" heavy. Common codes:
- Descriptors: TS (thunderstorm), SH (showers), FZ (freezing), BL (blowing), DR (drifting), MI (shallow), BC (patches).
- Precipitation: RA (rain), SN (snow), DZ (drizzle), PL (ice pellets), GR (hail), GS (small hail/snow pellets), IC (ice crystals).
- Obscurations: FG (fog), BR (mist, vis 5/8–6 SM), HZ (haze), FU (smoke), VA (volcanic ash), DU (dust).
- Other: SQ (squalls), FC (funnel cloud), +FC (tornado/waterspout), SS (sandstorm). Example: +TSRA = heavy thunderstorm with rain. VCSH = showers in the vicinity.
- Sky condition — coverage and height (AGL, in hundreds of feet). SKC/CLR (clear), FEW (1–2/8), SCT (3–4/8), BKN (5–7/8), OVC (8/8). VV indicates an indefinite ceiling/vertical visibility (e.g., VV004 = 400 ft vertical visibility). A ceiling is the lowest BKN, OVC, or VV layer. Example: BKN025 OVC080 = broken at 2,500 ft, overcast at 8,000 ft (ceiling 2,500).
- Temperature/dew point — two-digit Celsius values separated by a slash. M denotes negative. 14/09 = 14°C / 9°C. M02/M05 = −2°C / −5°C. The temperature–dew point spread is a quick indicator of fog/cloud potential.
- Altimeter — A followed by four digits: inches of mercury with an implied decimal (A2992 = 29.92 in Hg).
- Remarks (RMK) — supplemental information: automated station type (AO1 has no precip discriminator; AO2 does), sea-level pressure (SLPxxx, where SLP182 = 1018.2 hPa), hourly precipitation, peak winds, runway conditions, and so on.
Worked example:
METAR KDEN 121753Z 24015G25KT 210V270 5SM -RA BR BKN025 OVC080 14/09 A2992 RMK AO2 SLP132
Decoded: Routine report, Denver, 12th at 1753Z. Wind from 240° at 15 kt gusting 25, varying 210°–270°. Visibility 5 SM in light rain and mist. Broken ceiling at 2,500 ft, overcast at 8,000 ft. Temperature 14°C, dew point 9°C. Altimeter 29.92. Automated station with precipitation discriminator; sea-level pressure 1013.2 hPa.
METARs are foundational to preflight planning under 14 CFR 91.103, which requires the pilot in command to become familiar with all available information concerning the flight, including weather reports and forecasts. Always cross-check the METAR with TAFs, PIREPs, and area forecasts to build a complete picture.