6.1 Decoding METAR and TAF Weather Reports

Decoding METAR and TAF Weather Reports

Quick Summary: Aviation weather reports provide real-time and predictive meteorological data for flight planning. A METAR reports actual surface conditions observed hourly, while a TAF forecasts conditions within 5 nautical miles of the aerodrome over a 12-, 24- or 30-hour validity period. For RPAS pilots, interpreting wind velocity (degrees True), prevailing visibility, weather hazards, temperature-dew point spread, and the statutory ceiling (lowest BKN or OVC layer) is essential to satisfy CAR 901.34(1) and maintain control.

Every RPAS flight in Canada requires assessing current and forecast weather. Under CAR 901.34(1), a pilot may only conduct a VLOS operation when the weather conditions at the time of flight permit (a) operation in accordance with the operating manuals applicable to the system and (b) the pilot or a visual observer to conduct the entire flight in visual line-of-sight. Note what the rule does not say: the numeric minima — ground visibility of not less than three miles and clear of cloud — sit in CAR 901.34(3) and apply to BVLOS operations. A Basic pilot stays out of cloud because you cannot see an aircraft you are inside of, not because a numeric VLOS minimum exists. Standard aeronautical reports follow ICAO codes published by Environment and Climate Change Canada and NAV CANADA.

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Surface Observations vs. Aerodrome Forecasts

Aviation reports fall into observations of existing conditions and predictive forecasts:

A SPECI is triggered when safety criteria cross key thresholds, such as a ceiling dropping below VFR minimums, visibility deteriorating, wind shifting by >= 45° with speed > 15 knots, or the onset of thunderstorms or freezing precipitation.

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Step-by-Step Anatomy of a METAR

Sample observation: METAR CYOW 211900Z 28014G24KT 15SM -RA BKN012 OVC045 09/07 A2985 RMK SC6ST2 SLP108

1. Station Identifier & Observation Time

• CYOW: 4-letter ICAO station code. Canadian aerodromes start with C (CYVR Vancouver, CYYC Calgary, CZBF Bathurst).
• 211900Z: Day of month (21) and time in Coordinated Universal Time (19:00 UTC), designated by Z (Zulu).

2. Station Modifier

• AUTO: Automated station (AWOS/LWIS) without human observer.
• COR: Corrected report; NIL: Missing report.

3. Wind Velocity and Gusts (28014G24KT)

• Direction (280): Wind from 280 degrees True.

Key Exam Rule: METAR/TAF winds are always degrees True. Verbal reports from ATC, ATIS, or CFS runway headings use degrees Magnetic.

• Speed & Gusts (14G24KT): Sustained 14 knots with gusts to 24 knots (G).
• Variability: Winds <= 3 kt with variable direction are coded VRB02KT. Winds > 3 kt varying by >= 60° append variability (e.g., 180V250). Calm is 00000KT.

4. Visibility & Runway Visual Range (15SM)

• Prevailing visibility is reported in statute miles and fractions (e.g., 15SM, 1/4SM).
• RVR: Reported when visibility drops (e.g., R06L/4000FT = Runway 06 Left visual range 4,000 feet).

5. Present Weather Codes (-RA)

Codes combine intensity (- light, moderate [no prefix], + heavy), proximity (VC in vicinity: 5–10 NM), descriptors, and phenomena.

6. Sky Condition & Statutory Ceiling Definition (BKN012 OVC045)

Cloud coverage is reported in oktas (eighths) with bases in hundreds of feet AGL:

• SKC / CLR: Sky Clear (0/8) / Clear below 10,000 ft AGL.
• FEW: 1/8 to 2/8 (not a ceiling); SCT: 3/8 to 4/8 (not a ceiling).
• BKN: Broken, 5/8 to 7/8 (forms a ceiling).
• OVC: Overcast, 8/8 (forms a ceiling).
• VV: Vertical visibility into surface obscuration (e.g., VV002 = 200 ft; forms a ceiling).

Ceiling Definition: In Canadian aviation weather reporting (TC AIM, MET chapter), the ceiling is the height above ground of the lowest layer aloft reported as broken (BKN, 5/8 to 7/8) or overcast (OVC, 8/8) — that is, the lowest layer covering more than half the sky — or, where the sky is obscured, the vertical visibility (VV) into the obscuration. FEW and SCT layers do not form a ceiling. Note that for a Basic VLOS operation the ceiling is a practical limit, not a legal minimum: Part IX sets no numeric cloud clearance for VLOS.

In BKN012 OVC045, the ceiling is 1,200 feet AGL. Under CAR 901.25, RPAS are capped at 400 ft AGL. If a ceiling is reported at BKN003 (300 ft AGL), the practical ceiling drops to below 300 ft AGL: climbing into the cloud would end visual line-of-sight and breach CAR 901.11 and CAR 901.34(1)(b).

7. Temperature, Dew Point & Spread (09/07)

• Coded in whole degrees Celsius (sub-zero prefixed by M).
• In 09/07, temperature is +9°C and dew point is +7°C (spread of 2°C).
• Spread Rule: A spread of 2°C or less indicates relative humidity >= 90%, creating an immediate hazard for radiation fog, low stratus, and rapid propeller icing near 0°C.

8. Altimeter Setting & Remarks (A2985 RMK SC6ST2 SLP108)

• A2985: Altimeter setting of 29.85 inHg.
• RMK: Cloud genera in oktas (SC6ST2 = 6/8 stratocumulus, 2/8 stratus) and sea level pressure (SLP108 = 1010.8 hPa).

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Aerodrome Forecasts: Decoding the TAF

A Terminal Aerodrome Forecast (TAF) provides predictive conditions within 5 NM of an aerodrome runway complex, valid for 24 to 30 hours and updated every 6 hours.

Sample:
TAF CYVR 211740Z 2118/2224 26012KT P6SM SCT020 BKN050
FM212100 29016G26KT P6SM -RA OVC025
TEMPO 2122/2202 3SM RA BKN010

Forecast Change Groups

• FM (From): A rapid, permanent change taking effect at the specified hour (FM212100 = 21st day at 21:00Z). Cancels and supersedes all previous forecast lines.
• BECMG (Becoming): A gradual permanent transition occurring over a 2-hour window.
• TEMPO (Temporary): Fluctuations lasting less than 1 hour in each instance and aggregating to less than half the total forecast period.
• PROB30 / PROB40 (Probability): A 30% or 40% probability of an operational hazard (e.g., thunderstorm) during the window.

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Practical Exam Scenarios & Operational Analysis

• Scenario 1: Identifying the Ceiling: Given FEW010 SCT025 BKN045 OVC080, the ceiling is 4,500 feet AGL (BKN045). FEW and SCT layers do not form a ceiling.
• Scenario 2: Low Cloud Compression: With OVC003 (300 ft AGL), an RPA cannot reach its 400 ft AGL limit without entering cloud. The pilot would lose visual line-of-sight, breaching CAR 901.11 and CAR 901.34(1)(b). The 400 ft ceiling is a maximum, not an entitlement.
• Scenario 3: Narrow Temperature Spread: A reading of 03/02 indicates high humidity and imminent fog formation; pre-flight must account for rapid visibility drops.
• Scenario 4: True vs. Magnetic Wind: A METAR reporting 36015KT means wind from 360° True. In an area with 15° East magnetic variation, this equals 345° Magnetic.