8.1 Surface Weather Observations: METAR & SPECI Decoding

Surface Weather Observations: METAR & SPECI Decoding

Surface weather observations represent the primary source of real-time meteorological data for aerodrome operations within the National Airspace System (NAS). In 1996, the Federal Aviation Administration (FAA) and the National Weather Service (NWS) transitioned United States surface aviation reporting to the standardized International Civil Aviation Organization (ICAO) code format known as METAR (Aviation Routine Weather Report, from the French Météorologique Aviation Régulière). Ground instructors must train pilots not merely to recite memorized abbreviations, but to perform comprehensive diagnostic decoding of every group within the observation—correlating surface barometric trends, dew point convergence, wind shifts, and obscurations with impending flight hazards.

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Regulatory Foundations & Observation Types

Surface observations in the United States are categorized into two operational formats based on transmission timing and atmospheric conditions:

1. Aviation Routine Weather Report (METAR): A scheduled observation recorded and transmitted every hour (the "hourly" observation). It provides an hourly snapshot of conditions at the airport surface.
2. Aviation Selected Special Weather Report (SPECI): An unscheduled report transmitted whenever significant meteorological changes occur between routine hourly reports. A SPECI contains all the standard groups of a routine METAR but reflects critical operational criteria.

Mandatory Criteria for SPECI Issuance

Under NWS and FAA reporting criteria, a SPECI must be issued immediately when any of the following operational thresholds are met or crossed:

• Wind Shift: Direction changes by 45 degrees or more in less than 15 minutes, with wind speed of 10 knots or more throughout the shift.
• Visibility: Prevailing surface visibility decreases to less than, or increases to equal or exceed, key regulatory benchmarks: 3 statute miles (the VFR/IFR boundary), 2 miles, 1 mile, or the lowest published standard instrument approach minimum for the aerodrome.
• Ceiling: The lowest cloud ceiling forms, drops below, or rises to equal or exceed: 3,000 feet (the VFR/MVFR boundary), 1,500 feet, 1,000 feet (the basic VFR minimum under 14 CFR 91.155), 500 feet, or the lowest published approach minimum.
• Sky Condition: A cloud layer appears below 1,000 feet AGL where none previously existed.
• Thunderstorms: A thunderstorm begins or ends (a thunderstorm is considered ended 15 minutes after the last thunder is heard).
• Precipitation: Hail begins or ends; freezing precipitation begins, ends, or changes intensity; or ice pellets begin, end, or change intensity.
• Squalls: Sustained wind speed increases by at least 16 knots and is sustained at 22 knots or more for at least 1 minute.
• Runway Visual Range (RVR): Prevailing visibility drops to 1 statute mile or less, or RVR decreases below or exceeds 2,400 feet (1/2 SM equivalent).

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Element-by-Element METAR & SPECI Syntax Breakdown

A standardized METAR or SPECI follows a strict sequential syntax. Each group communicates a distinct physical property of the surface environment.

1. Report Type & Station Identifier

Every observation begins with the report type (METAR or SPECI). If omitted, the report is assumed to be a routine METAR. Next is the 4-letter ICAO station identifier:

• In the 48 contiguous United States (CONUS), major domestic airports prepend a "K" to the 3-letter FAA identifier (e.g., KDFW for Dallas/Fort Worth, KORD for Chicago O'Hare, KJFK for New York JFK).
• In Alaska, identifiers begin with "PA" (e.g., PANC for Anchorage); in Hawaii, "PH" (e.g., PHNL for Honolulu); in Guam, "PG"; and in Puerto Rico, "TJ" (e.g., TJSJ for San Juan).

2. Date and Time Group

The date and time of the observation are represented as a 6-digit group ending with the letter Z (DDHHMMZ):

• The first two digits (DD) denote the day of the current calendar month.
• The next four digits (HHMM) reflect the coordinated universal time (UTC), also termed Zulu time (Z), at which the observation was completed.
• Example: 261853Z decodes as an observation recorded on the 26th day of the month at 1853 UTC.

3. Report Modifiers: AUTO vs. COR

If the report is generated without human oversight, a modifier follows the date/time group:

• AUTO: Indicates the report is fully automated from an Automated Surface Observing System (ASOS) or Automated Weather Observing System (AWOS) without human observer augmentation, validation, or manual editing.
• COR: Indicates a corrected report that replaces an earlier observation that contained an erroneous value.
• Absence of a Modifier: Signifies that the report was manually prepared by a certified weather observer or was generated by an automated station with human augmentation (an observer added remarks, clouds, or obscurations).

4. Wind Group: True North vs. Magnetic North

The wind group consists of five or six characters (plus gust and variability indicators) ending in KT (knots):

DDD SS (G GG) KT

• Direction (DDD): Reported as three digits representing the direction from which the wind is blowing, rounded to the nearest 10 degrees, referenced to True North.
• Speed (SS): Two or three digits indicating the sustained wind speed in knots.
• Gusts (G): If peak instantaneous wind gusts exceed the sustained speed by 10 knots or more, the letter G is appended, followed by the peak gust speed in knots (e.g., 18015G25KT = wind from 180° true at 15 knots, gusting to 25 knots).
• Calm Wind: Encoded as 00000KT when the wind speed is less than 1 knot.
• Variable Wind (VRB): Encoded as VRB followed by the speed when the wind direction is variable and speed is 6 knots or less (e.g., VRB04KT), or during intense convective activity.
• Directional Variability (V): If the wind speed is greater than 6 knots and the direction fluctuates by 60 degrees or more, a variable group is appended (e.g., 18016KT 140V220 = wind from 180° true at 16 knots, fluctuating between 140° and 220°).

The Golden True vs. Magnetic Rule: A fundamental tenet tested on the AGI knowledge exam is: "If you read it in print, it is True; if you hear it spoken on the radio, it is Magnetic." Written METARs, TAFs, winds aloft forecasts, and chart legends reference True North. Conversely, verbal transmissions—such as ATIS broadcasts, tower wind checks, and ASOS/AWOS radio voice broadcasts—are automatically corrected for local magnetic variation and broadcast relative to Magnetic North so pilots can directly align wind vectors with magnetic runway designations.

5. Prevailing Visibility & Runway Visual Range (RVR)

Prevailing visibility is the greatest horizontal distance over which objects can be seen and identified through at least 50% of the horizon circle. It is reported in statute miles (SM) and fractions of a mile:

• Whole numbers and fractions are formatted with a space (e.g., 1 1/2SM, 2 1/2SM, 10SM).
• A prefix M designates "less than" the lowest readable threshold (e.g., M1/4SM = prevailing visibility less than 1/4 statute mile).

Runway Visual Range (RVR): When prevailing visibility drops to 1 statute mile or less, or runway visibility is instrumentally monitored, RVR is reported. RVR represents the horizontal distance down the runway a pilot on the center line can see the runway surface markings or lights:

• Format: R[runway designator]/[range]FT.
• Example 1: R04R/4500FT = Runway 04 Right RVR is 4,500 feet.
• Example 2: R18/2000V4000FT = Runway 18 RVR fluctuates between 2,000 feet and 4,000 feet.
• Prefixes: M denotes less than the minimum instrument threshold (e.g., M0600FT = less than 600 feet); P denotes greater than the maximum measurable limit (e.g., P6000FT = greater than 6,000 feet).

6. Present Weather Phenomena

The present weather group is assembled from qualifiers (intensity or proximity), descriptors, and weather phenomena (precipitation, obscurations, or other):

Coding Examples: +TSRA = Heavy thunderstorm with rain; -FZRA = Light freezing rain; VCSH = Rain showers 5 to 10 miles from the station; BLSN = Blowing snow.

7. Sky Condition & Ceiling Determination

Cloud coverage is reported in octas (eighths) of the celestial dome, followed by the base height in hundreds of feet Above Ground Level (AGL):

• SKC (Clear): Clear skies reported by a manual observer.
• CLR (Clear below 12,000): Automated system detects no clouds below 12,000 feet AGL.
• FEW (Few): 1/8 to 2/8 cloud coverage.
• SCT (Scattered): 3/8 to 4/8 cloud coverage.
• BKN (Broken): 5/8 to 7/8 cloud coverage (Constitutes a ceiling!).
• OVC (Overcast): 8/8 full cloud coverage (Constitutes a ceiling!).
• VV (Vertical Visibility): Vertical visibility into an indefinite surface-based obscuration, followed by hundreds of feet AGL (e.g., VV002 = indefinite ceiling at 200 feet AGL).

Convective Clouds: Significant convective clouds are explicitly appended to the layer height: CB for Cumulonimbus (e.g., BKN030CB) and TCU for Towering Cumulus (e.g., SCT025TCU).

Ceiling Definition (14 CFR 1.1): Under federal regulations, a ceiling is strictly defined as the height above the surface of the lowest layer of clouds or obscuring phenomena reported as Broken (BKN), Overcast (OVC), or Vertical Visibility (VV). Layers reported as FEW or SCT do not constitute a ceiling. For example, if a report indicates FEW010 SCT025 BKN060 OVC100, the official ceiling is 6,000 feet AGL (BKN060).

8. Temperature and Dew Point Group

Reported in whole degrees Celsius separated by a solidus (/). Negative values are prefixed with M (minus):

• Example: 04/M01 = Temperature 4°C, Dew point -1°C.
• Example: M05/M12 = Temperature -5°C, Dew point -12°C.

Operational Significance of the Spread: When the temperature-dew point spread narrows to 2°C (4°F) or less, relative humidity is roughly 85 to 90% or higher. Ground instructors emphasize that this condition creates high probability for radiation fog, low stratus clouds, and severe induction carburetor icing—even in ambient temperatures as warm as 21°C (70°F) when relative humidity is high.

9. Altimeter Setting

Prefixed by the letter A, followed by four digits representing barometric pressure in inches of mercury (inHg) to the hundredth (e.g., A2992 = 29.92 inHg; A3014 = 30.14 inHg). Pilots enter this value into the altimeter's Kollsman window to read true indicated altitude above mean sea level (MSL).

10. Remarks Group (RMK)

The remarks group begins with RMK and contains essential diagnostic observations:

• Station Type:
  • AO1: Automated station without a precipitation discriminator (cannot differentiate rain from snow).
  • AO2: Automated station with a precipitation discriminator sensor.
• Sea Level Pressure (SLP): SLP followed by three digits representing pressure in tenths of a hectopascal (millibar). If the three-digit number is 500 or higher, prepend a 9; if below 500, prepend a 10:
  • SLP124 = 1012.4 hPa.
  • SLP982 = 998.2 hPa.
• Precise Temperature & Dew Point (T-Group): T followed by 8 digits. The 1st digit denotes temperature sign (0 = positive, 1 = negative); digits 2-4 denote temperature in tenths of a degree Celsius. The 5th digit denotes dew point sign (0 = positive, 1 = negative); digits 6-8 denote dew point in tenths of a degree:
  • T00421012 = Temperature +4.2°C, Dew point -1.2°C.
  • T10281055 = Temperature -2.8°C, Dew point -5.5°C.
• Peak Wind: PK WND [direction][speed]/[time] indicates peak instantaneous wind over 25 knots (e.g., PK WND 28045/1952 = peak wind from 280° at 45 knots at 1952Z).
• Wind Shift: WSHFT [time] indicates a rapid wind direction change (e.g., WSHFT 1940 = wind shift occurred at 1940Z).
• Lightning: Decodes type and location: LTG + type (CG cloud-to-ground, IC in-cloud, CC cloud-to-cloud) + direction (e.g., LTGICCG DSNT NW = lightning in-cloud and cloud-to-ground distant northwest).
• Precipitation Timing: RAB18E33 = Rain began at 18 minutes past the hour, ended at 33 minutes past.

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Comprehensive METAR Walkthrough Scenario

Consider the following operational observation:

SPECI KOKC 261955Z 18020G32KT 140V220 1 1/2SM +TSRA BR BKN015CB OVC035 22/20 A2984 RMK AO2 PK WND 19036/1948 WSHFT 1940 LTGICCG ALQDS SLP098 T02220200

1. SPECI KOKC: Special unscheduled weather report for Will Rogers World Airport, Oklahoma City.
2. 261955Z: 26th day of the month at 1955 UTC. There is no AUTO modifier, so an observer took or augmented the report, which is why the cumulonimbus layer and lightning types can be reported.
3. 18020G32KT 140V220: Wind from 180° True at 20 knots, gusting to 32 knots, fluctuating between 140° and 220°.
4. 1 1/2SM: Prevailing surface visibility is 1.5 statute miles (IFR visibility).
5. +TSRA BR: Heavy thunderstorm with rain, and mist obscuring visibility.
6. BKN015CB OVC035: Lowest broken cloud ceiling at 1,500 feet AGL with Cumulonimbus, overcast layer at 3,500 feet AGL.
7. 22/20: Surface temperature 22°C, dew point 20°C (spread is 2°C, relative humidity about 88%).
8. A2984: Altimeter setting 29.84 inHg.
9. RMK AO2 ...: Station with precipitation discriminator; peak wind from 190° at 36 knots at 1948Z; wind shift at 1940Z; lightning in-cloud and cloud-to-ground all quadrants (ALQDS); sea level pressure 1009.8 hPa; exact temperature +22.2°C, dew point +20.0°C.