8.1 Surface Weather Observations (METAR & SPECI)

8.1 Surface Weather Observations (METAR & SPECI)

Quick Summary: Surface weather observations are the legal bedrock of airline operational control under 14 CFR Part 121. The METAR (Aviation Routine Weather Report) is transmitted hourly, while the SPECI (Aviation Selected Special Weather Report) is an unscheduled transmission triggered by critical operational threshold crossings (such as ceilings crossing 3,000, 1,500, 1,000, or 500 feet; visibility crossing 3, 2, 1, or 1/2 SM; freezing precipitation; thunderstorms; or wind shifts ≥ 45° with sustained speeds ≥ 10 knots). Under 14 CFR § 1.1, a ceiling is legally defined as the lowest layer aloft reported as broken (BKN), overcast (OVC), or vertical visibility (VV) into an indefinite surface-based obscuration. When Runway Visual Range (RVR) is reported for a specific runway, it serves as the controlling visibility over prevailing visibility for Part 121 takeoff and landing operations.

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1. Regulatory Framework: Surface Observations in Part 121 Dispatch

Under 14 CFR § 121.613, no aircraft dispatcher may release a flight unless the weather reports and forecasts indicate that the weather conditions at the destination airport will be at or above the authorized minimums at the estimated time of arrival (ETA). Furthermore, under 14 CFR § 121.651, flight crews are prohibited from commencing an instrument approach unless the latest weather observation shows that the airport weather conditions are at or above authorized landing minimums. In terminal operations, the METAR (Aviation Routine Weather Report) and SPECI (Aviation Selected Special Weather Report) represent the official ground truth. While forecasts (TAFs) govern preflight planning hours before departure, current surface observations govern tactical release amendments, takeoff approvals, and approach clearance authorizations.

+---------------------------------------------------------------------------------------------------+
|                         ROUTINE METAR vs. UNSCHEDULED SPECI COMPARISON                            |
+-----------------------+---------------------------------------------------------------------------+
| Characteristic        | METAR                                   | SPECI                           |
+-----------------------+-----------------------------------------+---------------------------------+
| Purpose               | Scheduled hourly baseline observation   | Unscheduled hazard notification |
| Issuance Window       | Routine: between 45 and 55 min past hr  | Immediate upon trigger event    |
| Regulatory Status     | Controlling surface observation         | Controlling surface observation |
| Human Augmentation    | Manual observer or augmented automated  | Automated or augmented observer |
+-----------------------+-----------------------------------------+---------------------------------+

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2. Element-by-Element METAR & SPECI Anatomy

A standard METAR or SPECI follows a strict standardized international syntax established by the World Meteorological Organization (WMO) and adapted by the Federal Aviation Administration (FAA) in Advisory Circular AC 00-45 and FAA Order 7900.5:

METAR KDFW 111853Z 34018G26KT 1/2SM R17C/2400FT +TSRA BR BKN008 OVC015CB 21/20 A2988 RMK AO2 SLP118 T02110200

1. Report Type & Station Identifier

• Report Type: METAR indicates a routine scheduled report; SPECI indicates an unscheduled special report.
• Station Identifier: A 4-letter International Civil Aviation Organization (ICAO) code. In the contiguous United States (CONUS), station identifiers begin with the prefix K (e.g., KDFW for Dallas/Fort Worth, KJFK for New York Kennedy). In Alaska, Hawaii, and U.S. Pacific territories, identifiers begin with P (e.g., PANC for Anchorage Ted Stevens, PHNL for Honolulu Daniel K. Inouye, PGUM for Guam Antonio B. Won Pat). Canadian stations use prefix C, and Mexican stations use prefix M.

2. Date and Time Group

• Formatted as six digits followed immediately by the letter Z (UTC / Coordinated Universal Time / Zulu):
  • First two digits: Day of the current calendar month (11).
  • Next four digits: Hour and minute of observation in UTC (1853Z = 18:53 UTC).

3. Report Modifier

• AUTO: Indicates the report originated from an automated observation station without human augmentation or manual observer validation.
• COR: Indicates a corrected report transmitted to supersede a previously transmitted observation that contained typographical or sensor errors.

4. Wind Group & Wind Direction Variations

• True Wind Direction: The first three digits represent the wind direction in degrees True rounded to the nearest 10 degrees (from 010° to 360°). 000 indicates calm winds.
• Sustained Speed & Units: The next two or three digits represent sustained wind speed in knots (KT). Calm winds are encoded as 00000KT.
• Gusts: When the peak wind speed exceeds the sustained speed by 10 knots or more, the letter G follows the sustained speed, followed by the peak gust speed (e.g., 34018G26KT = wind from 340° True at 18 knots, gusting to 26 knots).
• Variable Winds:
  • If wind speed is 6 knots or less and direction fluctuates, it is encoded as VRB followed by speed (e.g., VRB03KT).
  • If wind speed exceeds 6 knots and the direction fluctuates by 60 degrees or more, a variable direction group is appended (e.g., 34018G26KT 310V020 = wind varying between 310° and 020° True).

5. Prevailing Visibility

• Reported in Statute Miles (SM) and fractions of statute miles (e.g., 1/4SM, 1/2SM, 3/4SM, 1 1/2SM, 2SM, 10SM).
• The prefix M denotes a visibility less than the lowest reportable sensor value (e.g., M1/4SM indicates prevailing visibility less than 1/4 statute mile).
• Prevailing Visibility Defined: The greatest visibility reached or exceeded through at least half of the celestial horizon (180 degrees), which need not be continuous.

6. Runway Visual Range (RVR)

• RVR represents the horizontal distance a pilot looking down the runway from the touchdown threshold can see the runway surface markings or lights delineating the runway centerline.
• Instrumentation: Measured electronically using transmissometers or forward-scatter optical meters installed adjacent to the runway touchdown, midpoint, and rollout zones.
• Format: R[runway designation]/[range]FT (e.g., R17C/2400FT = Runway 17 Center has an RVR of 2,400 feet).
• Variable RVR: When RVR fluctuates during the 10-minute assessment period, the lowest and highest values are separated by V (e.g., R04R/4500V6000FT).
• Prefix Values:
  • M: Value is below the minimum sensor capability (e.g., R28L/M0600FT = RVR less than 600 feet).
  • P: Value is above the maximum sensor threshold (e.g., R09/P6000FT = RVR greater than 6,000 feet).

[!IMPORTANT] Controlling Status of RVR under 14 CFR Part 121: Under FAA regulations and air carrier Operations Specifications (OpSpecs C052 and C078), whenever RVR is reported for a specific runway, RVR is legally controlling over prevailing visibility for all takeoff and landing operations conducted on that runway! Prevailing visibility may be 1/4 SM, but if Runway 17C reports RVR 2400 FT and the approach chart requires RVR 1800 FT, the flight may legally shoot the approach and land.

7. Present Weather Phenomena & Descriptor Matrix

Present weather is constructed from a combination of qualifiers, descriptors, precipitation, and obscurations:

+---------------------------------------------------------------------------------------------------+
|                             PRESENT WEATHER DECODING MATRIX                                        |
+--------------------+---------------------+---------------------+----------------------------------+
| Category           | Code                | Meaning             | Operational Context              |
+--------------------+---------------------+---------------------+----------------------------------+
| Qualifiers         | -                   | Light intensity     | Minimal operational degradation  |
| (Intensity)        | (None)              | Moderate intensity  | Standard approach impact         |
|                    | +                   | Heavy intensity     | Severe runway contamination risk |
|                    | VC                  | Vicinity (5-10 SM)  | Weather off-airport boundary     |
+--------------------+---------------------+---------------------+----------------------------------+
| Descriptors        | TS                  | Thunderstorm        | Convection, wind shear, microburst|
|                    | SH                  | Showers             | Rapid convective fluctuation     |
|                    | FZ                  | Freezing            | Supercooled water; structural ice|
|                    | BL                  | Blowing             | Lifted by wind to >= 6 ft AGL    |
|                    | DR                  | Low Drifting        | Lifted by wind to < 6 ft AGL     |
|                    | MI                  | Shallow             | Ground-hugging fog layer < 6 ft  |
|                    | BC                  | Patches             | Intermittent fog patches         |
|                    | PR                  | Partial             | Obscuring substantial aerodrome  |
+--------------------+---------------------+---------------------+----------------------------------+
| Precipitation      | RA                  | Rain                | Liquid precipitation             |
|                    | SN                  | Snow                | Frozen crystalline precipitation |
|                    | DZ                  | Drizzle             | Very small liquid drops (<0.5mm) |
|                    | GR                  | Hail (>= 1/4 inch)  | Severe structural damage hazard  |
|                    | GS                  | Small Hail (<1/4 in)| Snow pellets / soft hail         |
|                    | PL                  | Ice Pellets         | Freezing rain aloft indicator    |
|                    | SG                  | Snow Grains         | Very small white opaque grains   |
|                    | IC                  | Ice Crystals        | Diamond dust in sub-zero air     |
|                    | UP                  | Unknown Precip      | Automated sensor unclassified    |
+--------------------+---------------------+---------------------+----------------------------------+
| Obscurations       | FG                  | Fog (Vis < 5/8 SM)  | Surface obscuration (RH ~100%)   |
|                    | BR                  | Mist (Vis 5/8-6 SM) | Light obscuration (RH > 80%)     |
|                    | HZ                  | Haze                | Fine dry particulates aloft      |
|                    | FU                  | Smoke               | Industrial or wildfire particulates|
|                    | DU                  | Widespread Dust     | Suspended fine dust particles    |
|                    | SA                  | Sand                | Blowing sand particles           |
|                    | VA                  | Volcanic Ash        | Severe jet engine shutdown hazard|
|                    | PY                  | Spray               | Wind-driven water droplets       |
+--------------------+---------------------+---------------------+----------------------------------+
| Other              | SQ                  | Squall              | Sudden wind surge >= 16 kts to 22|
|                    | FC                  | Funnel Cloud/Tornado| Extreme vortex hazard            |
|                    | SS / DS             | Sandstorm/Duststorm | Severe visibility restriction    |
+--------------------+---------------------+---------------------+----------------------------------+

8. Sky Condition & Legal Ceiling Definition

Cloud layers are reported using coverage codes in eighths (oktas) of the celestial dome, followed by the base altitude in hundreds of feet AGL:

• SKC (Sky Clear): Manual station observation of clear skies (0/8 coverage).
• CLR (Clear): Automated station reporting no clouds detected below 12,000 feet AGL.
• FEW (Few): Trace to 2/8 coverage (0 to 2 oktas).
• SCT (Scattered): 3/8 to 4/8 coverage (3 to 4 oktas).
• BKN (Broken): 5/8 to 7/8 coverage (5 to 7 oktas).
• OVC (Overcast): 8/8 full celestial coverage (8 oktas).
• VV (Vertical Visibility): Indefinite ceiling caused by a surface-based obscuration (e.g., fog, heavy snow). Encoded as VV plus base in hundreds of feet (e.g., VV002 = indefinite ceiling at 200 feet AGL).

[!WARNING] The Regulatory Definition of Ceiling (14 CFR § 1.1): Under 14 CFR § 1.1, a ceiling is legally defined as: "The height above the Earth's surface of the lowest layer of clouds or obscuring phenomena that is reported as 'broken', 'overcast', or 'vertical visibility'." Layers reported as FEW or SCT never constitute a ceiling! If a METAR reports FEW005 SCT010 BKN025, the legal ceiling is 2,500 feet AGL (BKN025), not 500 or 1,000 feet.

• Convective Descriptors: When significant convective clouds are present, only two descriptors may be attached to the layer:
  • CB (Cumulonimbus): Thunderstorm clouds characterized by severe vertical development (e.g., OVC015CB).
  • TCU (Towering Cumulus): Vigorous convective updrafts preceding thunderstorm formation (e.g., SCT025TCU).

9. Temperature & Dew Point Group

• Expressed in whole degrees Celsius, separated by a solidus / (e.g., 21/20 = OAT 21°C, Dew Point 20°C).
• Sub-zero (negative) temperatures are prefixed with M (e.g., M03/M07 = OAT -3°C, Dew Point -7°C).
• Operational Importance of Dew Point Spread:
  • Spread ≤ 3°C (5°F): Indicates high relative humidity (typically > 85%). Prime conditions for the formation of radiation fog, advection fog, low stratus, and carburetor or engine induction icing.
  • Sub-zero spread with liquid precipitation: Signals severe freezing rain or freezing drizzle hazards.

10. Altimeter Setting

• Prefixed with the letter A, followed by four digits representing inches of mercury (inHg) to hundredths (e.g., A2988 = 29.88 inHg).
• In international METARs (ICAO format outside North America), this is prefixed with Q and reported in whole hectopascals / millibars (e.g., Q1012 = 1012 hPa).

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3. Advanced Remarks Decoding (RMK)

The remarks (RMK) section contains vital meteorological details generated by automated sensors and human weather observers:

Automated Station Types: AO1 vs. AO2

• AO1: Automated observation station without a precipitation discriminator. The sensor cannot distinguish liquid from solid precipitation (e.g., rain vs. snow).
• AO2: Automated observation station with an optical precipitation discriminator. The sensor can differentiate between rain, snow, drizzle, and ice pellets.

Sea Level Pressure (SLP)

• Encoded as SLP followed by three digits representing sea level pressure in tenths of hectopascals (mb):
  • If the first digit is 5, 6, 7, 8, or 9, prefix with 9 (e.g., SLP982 = 998.2 hPa).
  • If the first digit is 0, 1, 2, 3, or 4, prefix with 10 (e.g., SLP118 = 1011.8 hPa; SLP012 = 1001.2 hPa).

Precipitation Accumulation Groups

• P0000: Hourly liquid precipitation accumulation in hundredths of an inch (e.g., P0018 = 0.18 inches of precipitation in the past hour).
• 60000: 3- or 6-hour precipitation accumulation (e.g., 60045 = 0.45 inches in the past 6 hours).
• 70000: 24-hour precipitation accumulation (e.g., 70120 = 1.20 inches in the past 24 hours).

Precise Temperature and Dew Point (T-Group)

Automated stations encode exact temperature and dew point to tenths of a degree Celsius in an 8-digit format: TsTTTsdd:

• T: Designates the temperature group.
• s: Sign bit for temperature (0 = positive, 1 = negative).
• TTT: Temperature in tenths of °C.
• s: Sign bit for dew point (0 = positive, 1 = negative).
• ddd: Dew point in tenths of °C.
• Example: T02110200 decodes to Outside Air Temperature +21.1°C, Dew Point +20.0°C.
• Example: T10441012 decodes to Outside Air Temperature -4.4°C, Dew Point -1.2°C.

Lightning Remarks

• Format: [Frequency] LTG[Type] [Location]
• Frequency:
  • OCNL (Occasional): Less than 1 flash per minute.
  • FRQ (Frequent): 1 to 6 flashes per minute.
  • CNS (Continuous): More than 6 flashes per minute.
• Lightning Types:
  • CG: Cloud-to-ground lightning.
  • IC: In-cloud lightning.
  • CC: Cloud-to-cloud lightning.
  • CA: Cloud-to-air lightning.
• Locations:
  • OHD: Overhead (within 5 SM).
  • VC: In the vicinity (5 to 10 SM).
  • DSNT: Distant (beyond 10 SM).
  • Compass sectors: N, SW, ALQDS (all quadrants).
  • Example: FRQ LTGICCG OHD = Frequent in-cloud and cloud-to-ground lightning overhead.

Operational Pressure & Wind Remarks

• PK WND 34038/1842: Peak wind from 340° at 38 knots occurred at 18:42 UTC.
• WSHFT 1825: Wind shift occurred at 18:25 UTC.
• CIG 006V012: Ceiling is fluctuating between 600 and 1,200 feet AGL.
• PRESRR / PRESFR: Pressure rising rapidly / falling rapidly.

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4. Mandatory SPECI Issuance Criteria

Under FAA observation standards (FAA Order 7900.5), a SPECI must be generated immediately when any of the following operational thresholds are crossed:

+---------------------------------------------------------------------------------------------------+
|                             MANDATORY SPECI ISSUANCE CRITERIA                                      |
+-----------------------+---------------------------------------------------------------------------+
| Meteorological Factor | Mandatory Trigger Criteria                                                |
+-----------------------+---------------------------------------------------------------------------+
| Ceiling               | Decreases to or increases to/above: 3,000 ft, 1,500 ft, 1,000 ft, 500 ft, |
|                       | or the lowest published precision/non-precision instrument minimum.      |
| Sky Cover             | A cloud layer forms below 1,000 feet AGL where none previously existed.   |
| Visibility            | Decreases to or increases to/above: 3 SM, 2 SM, 1 SM, or 1/2 SM, or the   |
|                       | lowest published precision/non-precision landing visibility minimum.     |
| Wind Shift            | Wind direction shifts by >= 45 degrees in less than 15 minutes with       |
|                       | sustained wind speeds of >= 10 knots.                                     |
| Thunderstorms         | A thunderstorm begins, intensifies to severe, or ends.                    |
| Freezing Precip       | Freezing rain (FZRA) or freezing drizzle (FZDZ) begins, ends, or changes  |
|                       | intensity (any transition between light, moderate, and heavy).            |
| Hail & Ice Pellets    | Hail (GR), small hail (GS), or ice pellets (PL) begins or ends.           |
| Squalls               | A squall occurs (wind surges by >= 16 kts to sustained >= 22 kts for 1m).|
| Volcanic Eruptions    | Volcanic ash eruption or ash cloud is observed.                           |
| Severe Phenomena      | Tornado, funnel cloud, or waterspout is sighted.                          |
+-----------------------+---------------------------------------------------------------------------+

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5. Practical Dispatch Analysis & Operational Traps

Comprehensive METAR Decoding Scenario

METAR KORD 111451Z 02014G24KT 1/4SM R10C/1800FT +SN FZFG VV002 M04/M05 A2972 RMK AO2 PK WND 03032/1422 SLP068 P0004 T10441050

1. Station & Time: Chicago O'Hare (KORD), 11th day of the month at 14:51 UTC (111451Z).
2. Wind: From 020° True at 14 knots with gusts to 24 knots (02014G24KT).
3. Prevailing Visibility: 1/4 Statute Mile (1/4SM).
4. Runway Visual Range: Runway 10 Center reports 1,800 feet RVR (R10C/1800FT).
5. Weather & Obscuration: Heavy snow (+SN) and freezing fog (FZFG).
6. Sky Condition & Ceiling: Indefinite ceiling with vertical visibility of 200 feet AGL (VV002). This establishes a legal ceiling of 200 feet AGL.
7. Temperature/Dew Point: Outside Air Temperature -4°C, Dew Point -5°C (M04/M05). Tight 1°C spread confirms near-saturated surface air.
8. Altimeter Setting: 29.72 inHg (A2972). Under 14 CFR § 91.121, this sub-29.92 setting elevates the lowest usable flight level to FL 190.
9. Remarks Breakdown:
   • AO2: Automated station with precipitation discriminator.
   • PK WND 03032/1422: Peak wind was 030° at 32 knots at 14:22 UTC.
   • SLP068: Sea level pressure is 1006.8 hPa.
   • P0004: 0.04 inches of liquid water equivalent precipitation in the past hour.
   • T10441050: Exact OAT is -4.4°C, exact dew point is -5.0°C.

Operational Dispatch Evaluation

• Approach Clearance: For a flight planning an ILS Category I approach to Runway 10C requiring RVR 1800 FT, the flight is legal to commence the approach because the controlling RVR (1,800 ft) satisfies the minimum, even though prevailing visibility is 1/4 SM (1,320 ft).
• Takeoff Clearance: Under standard Part 121 takeoff minimums (1 SM or 5,000 RVR for 2-engine aircraft without OpSpecs authorization), departure is prohibited without authorized low-visibility takeoff (LVTO) OpSpecs approval down to 500 RVR.