Ch 12 Quiz: Aviation Weather Reports — METAR, TAF, SIGMET & PIREP

METAR (Meteorological Aerodrome Report): standardised routine aerodrome observation issued every 30 or 60 minutes. A SPECI is a Special observation issued when significant change occurs between scheduled METARs.

METAR visibility 9999 = 10 km or more (prevailing visibility ≥ 10 km). It does not mean exactly 9,999 metres but rather ‘visibility is at least 10 km’. Minimum CAVOK visibility = 9999 + no cloud below 5,000 ft + no significant weather.

CAVOK (Ceiling And Visibility OK): visibility ≥ 10 km (9999), no cloud below 5,000 ft or MSA (whichever higher), no CB at any height, and no significant weather (precipitation, showers, thunderstorm, fog, etc.). Temperature is NOT a CAVOK criterion.

TAF (Terminal Aerodrome Forecast): issued 4 times daily (00, 06, 12, 18 UTC) for major airports. Validity: standard 24 hours; ICAO allows 30-hour TAFs for major international aerodromes. India DGCA TAFs: typically 24-hour validity.

TEMPO: temporary fluctuations in conditions, each event lasting < 60 minutes, and total TEMPO duration < half the forecast period. Contrast BECMG (gradual, permanent change) and PROB30/40 (probability 30% or 40% of a condition occurring).

BECMG: a gradual but permanent change in conditions expected to occur within the defined time window (e.g. BECMG 1416 = change expected to be complete between 1400 and 1600 UTC). Once complete, the new conditions persist.

METAR weather descriptors: TS = thunderstorm; SH = shower; FZ = freezing; MI = shallow; BC = patches; DR = drifting; BL = blowing; PR = partial. TS may be combined: TSRA = thunderstorm with rain; TSGR = thunderstorm with hail (GR).

QNH: altimeter subscale setting that causes the altimeter to read aerodrome elevation when on the ground. Calculated by reducing actual station pressure to MSL using the ISA lapse rate below the station. With QNH set, the altimeter reads altitude AMSL.

RVR: the maximum distance in the direction of take-off/landing that the pilot of an aircraft on the centreline can see runway markings or lights. Measured by transmissometers. Reported in the METAR as R28L/0400 (runway 28L, RVR 400 m).

SIGMET: Significant Meteorological Information. Issued by MWOs to warn of en-route hazards: CB/TS, SEV TURB, SEV ICE, MTW (mountain wave), volcanic ash, tropical cyclone, dust/sandstorm. Valid up to 4 hours (6 hours for tropical cyclones/volcanic ash).

AIRMET: Airmen’s Meteorological Information. Covers lower airspace (below FL100 or MSA+1000 ft) for: MOD ICE, MOD TURB, MOD MTW, reduced visibility/cloud ceiling below VFR minima, and surface wind/low-level shear. Less intense than SIGMET.

PIREP (UA): pilot reports submitted via ATC or ACARS. Contains: /OV (location), /TM (time), /FL (altitude), /TP (aircraft type), /SK (cloud), /WX (weather), /TA (temperature), /WV (wind), /TB (turbulence), /IC (icing), /RM (remarks). Critical for CAT/icing awareness.

METAR intensity prefixes: − = light, (none) = moderate, + = heavy. RA = rain, SN = snow. RASN = rain and snow (mixed). Therefore +RASN = heavy rain and snow together. Other modifiers: SH (showers), FZ (freezing), TS (thunderstorm).

VOLMET: continuous VHF/HF radio broadcast of aviation weather (METARs, TAFs) for selected airports, transmitted in ICAO standard format. Available on specific frequencies (e.g. London VOLMET 135.375 MHz, Delhi VOLMET 126.9 MHz). Now supplemented by D-VOLMET (datalink).

ATIS: Automatic Terminal Information Service — recorded broadcast on a dedicated VHF frequency updated hourly (or when significant changes occur), providing: aerodrome weather, NOTAM, runway in use, approach type. Each update gets a letter (Alpha, Bravo, etc.).

SH (shower): convective precipitation. Combined with RA (rain), SN (snow), GR (hail/graupel ≥ 5mm), GS (small hail/snow pellets), PE (ice pellets). SHRA = shower of rain. Shower differs from RA in being convective and shorter-duration.

00000KT = calm wind. Direction 000° is used when speed is zero (calm). Speed is 0 kt. This is the ICAO standard for wind speeds below 1 kt. Distinguishable from VRB03KT which indicates a measurable but variable-direction wind.

T/Td in METAR: temperature/dew point in °C. M prefix = minus (below zero). M02/M05 = T = −2°C, Td = −5°C. T − Td = 3°C → relatively dry air. A T/Td spread of 2°C or less indicates high humidity and fog/low cloud risk.

GR: hail (grêle in French) — solid precipitation of ice ≥ 5 mm diameter. GS: small hail or snow pellets (< 5 mm). TSGRM = thunderstorm with heavy hail (most dangerous aircraft hazard in the precipitation category). Always reported in METAR when observed.

NOSIG (No Significant change): a 2-hour trend forecast appended to a METAR. Indicates no significant change in visibility (<1500 m), cloud ceiling (<1000 ft), weather (TS, FZ, SH) is expected within the next 2 hours. NOSIG replaces the BECMG/TEMPO trend if no change expected.

BR: mist. ICAO weather code from French ‘brume’. Visibility 1,000–5,000 m. Relative humidity ≥ 95%. Distinct from: FG (fog < 1,000 m), HZ (haze, dry particles), FU (smoke), DU (dust). BR is a common cause of reduced visibility in warm humid coastal areas.

QNH reporting: metric regions (Europe, India, most of world) use hPa (millibars). US, Canada, Russia use in Hg (inches of mercury). Conversion: 1 in Hg = 33.86 hPa. Pilots must use the correct setting for their altimeter scale.

TAF validity: typically from the issuance time to 24 or 30 hours later. The TAF covers FROM the start of validity (often rounded to the next 00/06/12/18 UTC). FM0600 means the first conditions apply from 0600 UTC. BECMG periods are within the overall validity window.

TAF WS group: low-level wind shear (non-convective). Format: WS(height)/(wind at that height)KT. Example: WS020/27050KT = wind shear exists, wind at 2,000 ft AGL is 270°/50 kt. The runway surface wind is reported separately; the WS warns of the shear layer.

India VOLMET: Mumbai VOLMET broadcasts on 119.2 MHz (VHF) and 5.634 MHz (HF). Delhi VOLMET: 126.9 MHz. International VOLMET: London 135.375 MHz. VOLMET provides continuous METAR/TAF broadcasts for specified airports.

ATIS update: new code (next letter of phonetic alphabet) issued when: new METAR, ceiling/visibility changes, runway in use changes, significant NOTAMs, QNH change. Pilot reads back the ATIS identifier to ATC to confirm they have current information.

GRIB2 (General Regularly distributed Information in Binary): WMO standard format for gridded numerical weather data. Contains: NWP forecast fields, analysis fields, ensemble data. Used by WAFS for SIGWX BUFR data distribution and by airlines for computerised flight planning.

D-VOLMET: digital (datalink) equivalent of voice VOLMET. Provides METAR, TAF, SIGMET, and ATIS data to aircraft via ACARS, HFDL, SATCOM, or VDL datalink. Allows aircraft to receive current weather at destination/alternates at any range without radio reception limits.

SNOWTAM: special NOTAM format issued when runway/taxiway surfaces are contaminated by snow, slush, ice, or standing water. Provides: runway designator, cleared width, contamination type and depth, braking action, friction coefficient, and time of last observation.

INTER (Australia/some states): equivalent to ICAO TEMPO. Indicates fluctuations in conditions expected to last < 60 minutes each, total duration < half the forecast period. Australia uses both INTER and TEMPO in TAFs, with slight different conventions.

FEW010CB: few (1–2 oktas) Cumulonimbus at 1,000 ft. The CB designation is critically important — even 1 okta of CB at any height is a significant aviation hazard requiring consideration. Reported separately from other cloud layers, always noted by ATC in weather broadcasts.