6.1 METAR Decoding

6.1 METAR Decoding

Quick Answer: A METAR is a routine surface weather observation for an aerodrome, typically issued about hourly (with SPECI when weather changes significantly). Read it left to right: type → ICAO station → day-time UTC (Z) → AUTO (if automated) → wind (dddssKT, G for gusts) → visibility (SM) → weather codes → sky (FEW/SCT/BKN/OVC + hundreds of feet AGL) → temperature/dewpoint → altimeter (Axxxx) → RMK. The ceiling is the lowest BKN or OVC layer. For Advanced RPAS, METARs feed go/no-go, density altitude, gust limits, and whether VLOS will survive—not a substitute for eyes on the site.

Chapter 5 gave you atmospheric physics. This chapter turns raw aviation text into decisions you can defend on a flight review and on the Advanced written exam. METAR stands for Meteorological Aerodrome Report (the traditional expansion is less important than the product’s role). It describes what is happening now (or at the observation time) at a point—the observing site—not a guarantee for a field 15 km away in a valley.

Why Advanced RPAS pilots decode METARs

You are not dispatching an airliner, but you share airspace with aircraft that use these products, and Transport Canada’s knowledge requirements expect you to:

1. Extract wind, visibility, weather, cloud, temperature, and altimeter correctly.
2. Convert UTC (Zulu) to local mission time without launching into the wrong hour.
3. Know ceiling vs “there are some clouds.”
4. Link observation to manufacturer limits, CAR 901.34 weather fitness for VLOS, and icing awareness (CAR 901.35).
5. Prefer official aviation products over a phone weather app when planning near aerodromes or in controlled airspace.

Full METAR structure (Canadian stations)

A typical Canadian METAR follows a fixed order. Groups may be omitted when not applicable, but the sequence stays predictable.

Type: METAR vs SPECI

• METAR — scheduled observation (commonly at or near the hour).
• SPECI — special observation issued when criteria are met (e.g., significant wind shift, visibility/ceiling change, onset of thunderstorms). A SPECI can be more important for a mission launch than an hour-old METAR.

Station and time (exam trap: Zulu)

CYYZ 041955Z means Toronto Pearson, day 04 of the month, 19:55 UTC. Aviation products are not local standard or daylight time. Convert deliberately:

• Eastern Standard Time = UTC − 5; Eastern Daylight = UTC − 4 (examples—confirm your site’s offset and DST).
• Launch “at 10:00 local” against a TAF/METAR window written only in Z without conversion is a classic failure mode.

AUTO

AUTO indicates an automated observation (AWOS-class sensors). Automation is excellent for wind, temperature, and many visibility/ceiling estimates, but it can miss or mis-handle some phenomena humans would remark on. Always couple AUTO METARs with your site survey eyes.

Wind: dddssKT and gusts

Example: 32012G22KT → wind from 320° true at 12 kt, gusting 22 kt. Multirotors care about gusts and shear as much as mean speed. Manufacturer “max wind 10 m/s” style limits must be compared carefully (convert units). Flying in the lee of buildings when the METAR says 12G25 is often worse than the open-field METAR implies.

Visibility

Canadian METARs commonly report visibility in statute miles (SM)—e.g., 3SM, 1/2SM, 15SM. Fractions appear for low visibility. Low SM values threaten VLOS long before you hit abstract legal theory: if you cannot see the aircraft’s orientation at range, you do not have usable visual contact.

Present weather codes (high-yield subset)

FZRA, FZDZ, and freezing fog are strong cancel cues under CAR 901.35 thinking even before formal icing-equipment analysis.

Sky cover and heights

Heights are three digits × 100 ft AGL. Examples:

• SCT015 → scattered at 1,500 ft AGL
• BKN025 → broken at 2,500 ft AGL
• OVC008 → overcast at 800 ft AGL

Ceiling (for practical VFR teaching) is the height of the lowest BKN or OVC layer. SCT020 SCT100 does not create a 2,000 ft ceiling. Exam distractors love “scattered equals ceiling” and “015 means 15 ft” or “15,000 ft.”

Cumulonimbus may appear as CB in the cloud group or remarks (e.g., BKN040CB). Treat CB as a mission-stopper in the vicinity.

Temperature, dewpoint, altimeter

• M05/M07 → temperature −5 °C, dewpoint −7 °C (M = minus).
• Small temperature–dewpoint spread → air near saturation → fog/low cloud risk.
• A3012 → altimeter 30.12 inHg. (Some international products use QNH in hPa; Canadian METARs you will drill use A inHg.)

Temperature + elevation + pressure context drive density altitude (Chapter 5): hot and high with low pressure hurts multirotor thrust and battery endurance even under blue sky.

Remarks (RMK)

Remarks may include cloud types and amounts, sea-level pressure (SLP), density altitude, sensor status, or precipitation begin/end times. Do not skip RMK when the body looks “fine” but remarks mention thunderstorms in the vicinity or missing sensors.

Worked example (step-by-step)

METAR:

METAR CYUL 151745Z AUTO 25012G20KT 5SM -SHRA BKN012 OVC030 14/12 A2978 RMK SC6 AC2 SLP085

RPAS decision sketch: Gusts to 20 kt may exceed small multirotor comfort/limits; 5 SM with showers and a 1,200 ft broken layer reduces VLOS margin and contrast; moist air and showers increase wet-electronics and (if colder) icing concern. A mapping grid to 3 km might be legal on paper only if you truly keep VLOS and stay inside manuals—professionally this is often a delay or cancel for precision work.

AWOS and LWIS notes

• AWOS (Automated Weather Observation System) — continuous automated sensors feeding METARs/SPECI-type products and voice/data broadcasts at many Canadian aerodromes.
• LWIS (Limited Weather Information System) — reduced sensor suite; fewer elements than full AWOS. Do not invent missing ceiling or weather groups.

Practical pilot habits:

1. Prefer the latest METAR/SPECI, not yesterday’s screenshot.
2. If AUTO and conditions are marginal, walk the site and check nearby stations.
3. Remember the METAR is at the sensor site, often an airport—not your farm field 8 NM downwind of a lake.

Density altitude, performance, and VLOS implications

From a METAR you can roughly grade the day:

• Hot + high elevation + low altimeter → expect high density altitude → less hover margin, shorter flights, weaker gust rejection.
• Strong G gusts → control and battery-current spikes; set hard personal limits below manufacturer absolute maxima.
• Low SM / FG / BR / FU → VLOS fails even if wind is calm.
• BKN/OVC low → flat light, lost sky texture, harder depth perception; keep clear of cloud (do not climb into the base “for a better shot”).

Legal VLOS (CAR 901.11) and weather fitness (CAR 901.34(1)) both require that the pilot or visual observer can maintain the entire flight in visual line-of-sight and within manuals. A perfect decode of 10SM FEW250 does not help if local fog sits in your valley.

Exam traps (memorize these)

1. Time is Zulu — convert to local before briefing the client.
2. Ceiling ≠ any cloud — only BKN/OVC (or VV) define ceiling in standard teaching; FEW/SCT do not.
3. Heights are hundreds of feet AGL — BKN015 = 1,500 ft, not 15 or 15,000.
4. Gusts matter — G is peak, not sustained; plan for the gust.
5. METAR is an observation, not a forecast — pair with TAF (Section 6.2) for mission windows.
6. AUTO ≠ complete human narrative — verify marginal days on site.

Bottom line: Decode every group in order, convert Zulu, identify ceiling correctly, respect gusts and visibility for multirotor VLOS, and treat the METAR as a professional input—not a magic clearance to fly.