7.1 Aviation Routine Weather Reports (METAR) & SPECI

Aviation Routine Weather Reports (METAR) & SPECI

Quick Answer: A METAR is an hourly scheduled surface weather observation containing true wind velocity, visibility, Runway Visual Range (RVR), weather phenomena, sky conditions, temperature/dew point, altimeter setting, and coded remarks. A SPECI is an unscheduled special report issued immediately when critical parameters change significantly (e.g., ceiling or visibility crossing IFR/VFR thresholds, wind shifts $\ge 45^\circ$ at $\ge 10\text{ kt}$, or thunderstorm onset). The legal ceiling is defined as the height AGL of the lowest broken (BKN) or overcast (OVC) layer, or vertical visibility (VV) into a surface-based obscuration. Wind direction in METARs is always True North.

Accurate interpretation of surface weather reports is fundamental to single-pilot instrument flight. Operating under Instrument Flight Rules (IFR) requires pilots to continuously compare real-time surface conditions against published approach minimums, takeoff minimums, and regulatory alternate requirements. The international standard format for surface weather reporting is the METAR (Meteorological Aerodrome Report) and its unscheduled counterpart, the SPECI (Special Report).

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METAR Structure & Standard Syntax

METAR reports follow a strictly defined, sequential block format established by the World Meteorological Organization (WMO) and adapted for United States domestic operations under FAA Advisory Circular AC 00-45 (Aviation Weather Services).

+---------------------------------------------------------------------------------------------------------+
|                                 STANDARD METAR SYNTAX & FIELD ORDER                                     |
|                                                                                                         |
|  METAR  KORD  181952Z  AUTO  24015G25KT  180V280  3/4SM  R28C/2400FT  +TSRA  BR  BKN008CB  OVC015       |
|  [Type] [ID]   [Time]  [Mod]    [Wind]    [Var]    [Vis]    [RVR]      [Present Wx]    [Sky Condition]  |
|                                                                                                         |
|  18/16  A2988  RMK  AO2  PK WND 25032/45  SLP115  T01830161                                             |
|  [Temp] [Altim]              [Remarks & Diagnostic Sensor Codes]                                        |
+---------------------------------------------------------------------------------------------------------+

1. Report Type & Station Identifier

• Report Type: METAR indicates a scheduled hourly observation (taken between H+45 and H+55). SPECI indicates an unscheduled special observation issued due to significant weather changes.
• Station Identifier: A 4-character ICAO identifier. In the contiguous United States, stations begin with the prefix K (e.g., KJFK, KORD, KDEN). Alaskan stations use PA, Hawaiian stations use PH, and Canadian stations use C.

2. Date and Time Group

• Formatted as DDHHMMZ where DD is the day of the month, HHMM is the time of observation in Universal Coordinated Time (UTC / Zulu), and Z denotes Zulu time (e.g., 181952Z = 18th day of the month at 1952 UTC).

3. Report Modifiers

• AUTO: Identifies an automated observation generated by an Automated Surface Observing System (ASOS) or Automated Weather Observing System (AWOS) without human augmentation or observer logging.
• COR: Indicates a corrected report transmitted to replace an earlier erroneous report (e.g., METAR KDFW 181955Z COR...).

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Wind Group Decoding Rules

Wind data in METAR reports is averaged over a 2-minute period (or 10-minute period internationally).

$\text{Format: } \mathbf{dddff(Ggg)KT}$

+-----------------------------------------------------------------------+
|                      METAR WIND ENCODING RULES                        |
|                                                                       |
|   1. Direction: 3 digits representing TRUE NORTH (not magnetic)       |
|   2. Speed: 2 or 3 digits in KNOTS (KT)                               |
|   3. Gusts: Letter 'G' followed by peak gust speed (e.g., G25KT)      |
|   4. Variable Direction (>6 kt): e.g., 180V240 (varies 180° to 240°)  |
|   5. Light and Variable: VRB03KT (used when speed <= 6 knots)         |
|   6. Calm Wind: 00000KT (wind speed < 3 knots with no direction)      |
+-----------------------------------------------------------------------+

Critical Aviation Rule: Wind direction in spoken ATC communications (tower, ATIS, ground) is given relative to Magnetic North to align with runway magnetic headings. Wind direction in printed text products (METAR, TAF, Winds Aloft FB) is always referenced to True North. "If you read it, it's True; if you hear it, it's Magnetic."

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Visibility & Runway Visual Range (RVR)

Prevailing Visibility

Prevailing visibility is the greatest horizontal distance over which objects can be seen and identified throughout at least half of the horizon circle. It is reported in statute miles (SM) and common fractions (e.g., 1/4SM, 3/4SM, 1 1/2SM, 2SM, 10SM). If prevailing visibility exceeds 10 statute miles, it is coded as 10SM (or omitted as 9999 in international METARs).

Runway Visual Range (RVR)

RVR is the maximum horizontal distance down a runway that a pilot in a moving aircraft on the centerline can see runway surface markings or the lights delineating the runway. RVR is measured electronically by transmissometers positioned along the touchdown, midpoint, and rollout zones.

• Reporting Threshold: RVR is reported only when prevailing visibility is 1 statute mile or less, or the RVR for a designated instrument runway is 6,000 feet or less.
• Syntax: R[Runway]/[Visual Range]FT
  • R32L/2400FT: Runway 32 Left visual range is 2,400 feet.
  • R04R/P6000FT: Prefix P indicates Plus / greater than the maximum instrument measurable value (exceeds 6,000 feet).
  • R18/M0600FT: Prefix M indicates Minus / less than the lowest measurable value (less than 600 feet).
  • R01C/1600V3000FT: Variable RVR fluctuating between 1,600 and 3,000 feet.

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Weather Phenomena Matrix

Present weather is constructed from a systematic matrix combining Intensity/Proximity, Descriptor, and Phenomenon (Precipitation, Obscuration, or Other).

+---------------------------------------------------------------------------------------------+
|                             PRESENT WEATHER DECODING MATRIX                                 |
|                                                                                             |
|  INTENSITY / PROXIMITY    DESCRIPTOR           PRECIPITATION       OBSCURATION     OTHER    |
|  -  Light                 TS  Thunderstorm     DZ  Drizzle         FG  Fog (<5/8SM) SQ Squall|
|     Moderate (no sign)    SH  Showers          RA  Rain            BR  Mist (>=5/8) FC Funnel|
|  +  Heavy                 FZ  Freezing         SN  Snow            HZ  Haze         SS Sand  |
|  VC In Vicinity (5-10 SM) BL  Blowing          PL  Ice Pellets     FU  Smoke        DU Dust  |
|                           DR  Low Drifting     GR  Hail (>=1/4")   DU  Dust         VA Ash   |
|                           MI  Shallow          GS  Small Hail/Snow SA  Sand                  |
|                           BC  Patches          SG  Snow Grains     PY  Spray                 |
|                           PR  Partial          IC  Ice Crystals                              |
+---------------------------------------------------------------------------------------------+

Essential Decoding Rules & Distinctions

1. Fog (FG) vs Mist (BR): Obscuration is coded as FG only when visibility is less than 5/8 statute mile. When visibility is between 5/8 SM and 6 SM, water droplet obscuration is coded as BR (baby rain / mist).
2. Freezing Rain (FZRA) vs Ice Pellets (PL): FZRA consists of supercooled liquid rain freezing on impact with airframes or ground structures. PL consists of transparent or translucent pellets of ice that have already frozen solid in a sub-freezing layer aloft before reaching the surface. The presence of ice pellets always indicates freezing rain aloft.
3. Vicinity (VC): Denotes weather phenomena occurring between 5 and 10 statute miles from the airport center (e.g., VCTS = Thunderstorm in vicinity; VCSH = Showers in vicinity).

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Sky Condition & The Aviation Ceiling

Sky cover is observed in eighths (octas) of the celestial dome from ground level to the top of the atmosphere.

The Legal Definition of a Ceiling

Under 14 CFR § 1.1 and FAA instrument flight procedures:

$\mathbf{Ceiling} = \text{The height AGL of the lowest layer of clouds reported as } \mathbf{BKN}\text{, } \mathbf{OVC}\text{, or } \mathbf{VV}$

• Layer heights are reported in hundreds of feet Above Ground Level (AGL) using three digits:
  • FEW015 = Few clouds at 1,500 ft AGL (Not a ceiling).
  • SCT025 = Scattered clouds at 2,500 ft AGL (Not a ceiling).
  • BKN040 = Broken ceiling at 4,000 ft AGL (Ceiling is 4,000 ft AGL).
  • OVC080 = Overcast layer at 8,000 ft AGL.
• Convective Cloud Types: Significant convective clouds are appended to the layer height:
  • CB: Cumulonimbus (e.g., BKN020CB).
  • TCU: Towering Cumulus (e.g., SCT035TCU).
• Indefinite Ceiling (VV): When fog, heavy snow, or blowing dust completely obscures the sky from the surface, an indefinite ceiling is reported with vertical visibility into the obscuration (e.g., VV002 = Indefinite ceiling with 200 feet vertical visibility).

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Temperature, Dew Point & Altimeter Group

• Temperature / Dew Point (18/16 or M02/M05): Reported in whole degrees Celsius. Sub-zero temperatures are prefixed with M (Minus). When the temperature/dew point spread narrows to $2^\circ\text{C}$ ($4^\circ\text{F}$) or less, relative humidity reaches nearly 100%, creating high probability of fog, low stratus, or carburetor/induction icing.
• Altimeter Setting (A2992): Encodes the current barometric pressure setting in inches of mercury (in Hg) prefixed by A. A2992 represents $29.92\text{ in Hg}$; A3012 represents $30.12\text{ in Hg}$.

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Coded Remarks (RMK) & Automated Diagnostics

Remarks provide high-resolution meteorological and engineering data:

+---------------------------------------------------------------------------------------------+
|                              COMMON METAR REMARKS & CODES                                   |
|                                                                                             |
|  Code             Meaning & Translation                                                     |
|  AO1 / AO2        Automated station without (AO1) or with (AO2) precipitation discriminator |
|  PK WND 28045/15  Peak wind from 280° at 45 knots occurred at 15 minutes past the hour      |
|  WSHFT 1715       Wind shift occurred at 1715 UTC                                           |
|  PRESRR / PRESFR  Barometric pressure rising rapidly (PRESRR) or falling rapidly (PRESFR)   |
|  SLP115           Sea Level Pressure: 1011.5 mb (add 10 or 9, place decimal before last digit)|
|  T01830161        Exact Temp/Dew Point: Temp = +18.3°C, Dew Point = +16.1°C (0=pos, 1=neg)  |
|  T10221056        Exact Temp/Dew Point: Temp = -2.2°C, Dew Point = -5.6°C                   |
|  P0002            Precipitation accumulation: 0.02 inches in the past hour                  |
|  FZRANO / TSNO    Freezing rain sensor (FZRANO) or thunderstorm sensor (TSNO) inoperative  |
+---------------------------------------------------------------------------------------------+

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SPECI Issuance Criteria

A SPECI is issued immediately whenever any of the following operational triggers occur between scheduled hourly METARs:

1. Ceiling Thresholds: Ceiling forms or drops below, or increases to exceed:
   • $3,000\text{ ft}$ (VFR/MVFR boundary)
   • $1,500\text{ ft}$
   • $1,000\text{ ft}$ (MVFR/IFR boundary)
   • $500\text{ ft}$ (IFR/LIFR boundary)
   • The lowest published IFR approach landing minimum for the airport.
2. Visibility Thresholds: Prevailing visibility decreases below, or increases to exceed: $3\text{ SM}$, $2\text{ SM}$, $1\text{ SM}$, or $1/2\text{ SM}$.
3. Wind Shift: Direction changes by $45^\circ$ or more in less than 15 minutes, accompanied by sustained wind speeds of 10 knots or more.
4. Thunderstorm: A thunderstorm begins or ends.
5. Precipitation Changes: Initiation or cessation of freezing rain, freezing drizzle, ice pellets, snow, or hail.
6. Severe Convective Phenomena: Onset of tornadoes, funnel clouds, waterspouts, or squalls.