4.1 METAR and SPECI Surface Observations: Decoding & Operations

4.1 METAR and SPECI Surface Observations: Decoding & Operations

Aviation Routine Weather Reports (METAR) and Non-Routine Aviation Special Weather Reports (SPECI) constitute the baseline surface observations governing airline operations. For the aircraft dispatcher exercising joint operational control under 14 CFR Part 121, these reports are legal documents that determine whether an aircraft can legally depart under 14 CFR 121.651, whether a takeoff alternate is mandated under 14 CFR 121.617, and whether airborne flights must enter holding or divert. Decoding every group, from Runway Visual Range (RVR) modifiers to automated remarks (RMK), is critical to maintaining operational safety and regulatory compliance.

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1. Regulatory Framework & Report Types: METAR vs. SPECI

The Aviation Routine Weather Report (METAR) is the international standard code format for hourly surface weather observations. In the United States, METAR observations are taken between 45 and 55 minutes past each hour by certified human observers or Automated Surface Observing Systems (ASOS) / Automated Weather Observing Systems (AWOS).

When meteorological conditions deteriorate or improve across critical aviation safety thresholds between scheduled hourly observations, the observing station immediately generates a Non-Routine Special Weather Report (SPECI). Dispatchers must monitor SPECIs constantly because an unscheduled worsening of weather can instantly invalidate a planned departure or force an airborne flight into immediate diversion.

Specific SPECI Trigger Criteria

Under FAA Order JO 7900.5 (Surface Weather Observing - METAR/SPECI) and Federal Meteorological Handbook No. 1 (FMH-1), an unscheduled SPECI is mandated whenever any of the following operational threshold criteria are met:

1. Wind Shift: Wind direction changes by 45 degrees or more in less than 15 minutes, with sustained wind speeds of 10 knots or greater throughout the shift.
2. Visibility Thresholds: Prevailing surface visibility decreases below or increases to equal or exceed any of the following operational benchmarks:
   • 3 statute miles (critical VFR/IFR boundary);
   • 2 statute miles;
   • 1 statute mile;
   • 1/2 statute mile (standard Category I ILS visibility minimum);
   • The lowest published standard instrument approach procedure (IAP) minimum for any operational runway at that airfield.
3. Runway Visual Range (RVR):
   • Prevailing visibility drops to 1 statute mile or less, or RVR decreases below or exceeds 6,000 feet (the threshold where automated RVR reporting begins);
   • RVR drops below or increases above 2,400 feet (standard Category I ILS touchdown minimum);
   • RVR drops below the lowest operational landing minimum for the airfield.
4. Ceiling Thresholds: Cloud bases form, drop below, or lift to equal or exceed any of the following critical dispatch and approach levels:
   • 3,000 feet (VFR / Marginal VFR boundary);
   • 1,500 feet (critical circling approach and visual arrival threshold);
   • 1,000 feet (standard IFR ceiling threshold and Part 121 alternate requirement under § 121.619);
   • 500 feet (non-precision approach MDA threshold);
   • 200 feet (standard Category I ILS decision height);
   • The lowest published IAP minimum for an active runway.
5. Sky Coverage: A cloud layer forms or increases in coverage to become a ceiling (broken BKN or overcast OVC) below 1,000 feet AGL.
6. Convective and Severe Phenomena:
   • Thunderstorm begins, intensifies, or terminates (considered ended 15 minutes after the last thunder is heard);
   • Tornado, funnel cloud, or waterspout is sighted or ceases;
   • Squall occurs (sustained wind speed increases by at least 16 knots to 22 knots or more for at least 1 minute);
   • Hail or ice pellets begin, change intensity, or end.
7. Precipitation State Changes:
   • Freezing precipitation (freezing rain -FZRA or freezing drizzle -FZDZ) begins, changes intensity, or ceases;
   • Any precipitation type changes character (e.g., liquid rain transitions to frozen snow or ice pellets).
8. Volcanic Ash: Eruption or presence of volcanic ash clouds is observed.

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2. Header, Modifiers, and Wind Group Decoding

A standard METAR string begins with administrative and meteorological identifiers:

METAR KORD 121851Z AUTO 28018G32KT 240V320 ...

• Report Type: METAR indicates a routine hourly observation; SPECI indicates a special unscheduled observation.
• Station Identifier: The 4-letter ICAO station code (KORD = Chicago O'Hare, KJFK = New York JFK, PANC = Ted Stevens Anchorage, PHNL = Daniel K. Inouye Honolulu).
• Date and Time: Six digits followed by Z representing Coordinated Universal Time (UTC). The first two digits are the day of the month (12), followed by the hour (18) and minute (51) of the observation: 121851Z.
• Modifiers:
  • AUTO: Denotes a fully automated observation without human intervention.
  • COR: Indicates a corrected observation that replaces an erroneous report previously transmitted.
  • If a certified human weather observer took or augmented the report, the modifier field is omitted.

Wind Group Decoding

The wind group encodes true direction (in tens of degrees), sustained velocity, gusts, and directional variance:

• Direction and Speed: Three digits for true direction from which the wind is blowing (referenced to true north, unlike ATC tower winds which reference magnetic north), followed by two or three digits for sustained speed in knots (KT). For example, 28018KT indicates winds from $280^\circ\text{ true}$ at 18 knots. Calm wind is encoded as 00000KT.
• Gusts: Indicated by the letter G followed by the peak gust speed in knots. In 28018G32KT, sustained winds are 18 knots with peak gusts reaching 32 knots.
• Directional Variability: If the wind direction varies by 60 degrees or more and sustained wind speed exceeds 6 knots, a variable wind group is appended (e.g., 240V320 indicates winds fluctuating between $240^\circ$ and $320^\circ$). If wind speed is 6 knots or less and direction fluctuates by 60 degrees or more, the direction is reported as VRB (e.g., VRB04KT).

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3. Prevailing Visibility and Runway Visual Range (RVR)

Visibility is reported in statute miles and fractions of statute miles, followed immediately by SM (e.g., 1/4SM, 3/4SM, 1 1/2SM, 2SM, 10SM). Prevailing visibility represents the greatest horizontal distance over which objects or lights can be seen and identified throughout at least half of the horizon circle, which need not be continuous.

Runway Visual Range (RVR) Mechanics & Formats

When prevailing visibility drops to 1 statute mile or less, or when an instrument runway equipped with transmissometers reports an RVR value of 6,000 feet or less, automated sensors report RVR in feet. RVR represents the horizontal distance a pilot looking down the runway from the touchdown zone can see the runway surface markings or the lights delineating the runway:

R04R/2400FT
R01L/P6000FT
R28/1600V4000FT
R14L/M0600FT

• Runway Designator: R followed by the two-digit runway identifier and parallel descriptor (L, C, R). R04R/ indicates Runway 04 Right.
• Visual Range: Reported in hundreds of feet. 2400FT equals 2,400 feet of visual range.
• Modifiers (Prefixes M and P):
  • M (Minus / Below minimum): The visual range is less than the lowest reportable value of the instrument (typically less than 600 feet). M0600FT indicates RVR is below 600 feet.
  • P (Plus / Above maximum): The visual range is greater than the highest reportable value of the instrument (typically greater than 6,000 feet). P6000FT indicates RVR exceeds 6,000 feet.
• Variable RVR (V): If the RVR fluctuates significantly during the 10-minute evaluation period, the minimum and maximum values are separated by a V. In R28/1600V4000FT, RVR on Runway 28 varies from 1,600 feet to 4,000 feet.

Operational Controlling Hierarchy

In commercial air carrier operations under 14 CFR Part 121, RVR is legally controlling over prevailing visibility for takeoff and landing on designated runways. If prevailing visibility is reported as $1/4\text{ SM}$ ($1{,}320\text{ ft}$), but the touchdown zone RVR for the active runway reports $2{,}400\text{ ft}$, an aircraft requiring a Category I minimum of $2{,}400\text{ RVR}$ is legally authorized to execute the approach and land. Conversely, if prevailing visibility is $1\text{ SM}$, but RVR drops to $1{,}200\text{ ft}$ on an approach requiring $1{,}800\text{ RVR}$, the approach cannot legally be initiated past the final approach fix under 14 CFR 121.651.

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4. Present Weather: Intensity, Descriptors, and Phenomena

The present weather group uses standardized two-letter abbreviations arranged in a strict structural hierarchy: [Intensity/Proximity] + [Descriptor] + [Precipitation] + [Obscuration] + [Other].

+TSRA BR
-FZRA
BLSN
VCSH

1. Intensity and Proximity Qualifiers

• -: Light intensity
• No symbol: Moderate intensity
• +: Heavy intensity
• VC: In the vicinity (weather observed between 5 and 10 statute miles from the aerodrome center)

2. Descriptors

• MI: Shallow
• PR: Partial
• BC: Patches
• DR: Low drifting (below 6 feet AGL)
• BL: Blowing (6 feet or more AGL)
• SH: Showers
• TS: Thunderstorm
• FZ: Freezing (supercooled water droplets that freeze upon contact with aircraft surfaces)

3. Precipitation Phenomena

• DZ: Drizzle (liquid droplet diameter $< 0.5\text{ mm}$)
• RA: Rain (liquid droplet diameter $\ge 0.5\text{ mm}$)
• SN: Snow
• SG: Snow grains
• IC: Ice crystals (diamond dust)
• PL: Ice pellets (translucent frozen raindrops; indicates freezing rain aloft)
• GR: Hail (diameter $\ge 5\text{ mm}$ or $1/4\text{ inch}$)
• GS: Small hail or snow pellets (diameter $< 5\text{ mm}$)
• UP: Unknown precipitation (automated stations only)

4. Obscurations

• BR: Mist (visibility $\ge 5/8\text{ SM}$ up to $6\text{ SM}$; relative humidity $\ge 80\%$)
• FG: Fog (visibility $< 5/8\text{ SM}$; relative humidity near $100\%$)
• FU: Smoke
• VA: Volcanic ash. This is a serious engine and airframe hazard requiring current avoidance assessment; the code alone does not establish a legally prohibited-airspace designation.

5. Other Phenomena

• PO: Well-developed dust/sand whirls
• SQ: Squall
• FC: Funnel cloud (tornado or waterspout)
• SS: Sandstorm
• DS: Duststorm

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5. Sky Condition, Ceiling Definition & Temperature/Altimeter Groups

The sky condition group reports cloud coverage in eighths (octas) and the height of cloud bases in hundreds of feet above ground level (AGL):

• SKC or CLR: Clear (0/8 coverage; CLR indicates no clouds detected below 12,000 ft by automated ceilometer)
• FEW: Few (1/8 to 2/8 coverage)
• SCT: Scattered (3/8 to 4/8 coverage)
• BKN: Broken (5/8 to 7/8 coverage)
• OVC: Overcast (8/8 coverage)
• VV: Vertical Visibility into an indefinite ceiling (e.g., VV002 = indefinite ceiling with vertical visibility of 200 feet due to surface-based fog or heavy blowing snow)

The Legal Definition of a Ceiling

Under 14 CFR 1.1 and standard FAA dispatch practice, a ceiling is explicitly defined as:

The height above the Earth's surface of the lowest layer of clouds reported as Broken (BKN), Overcast (OVC), or Vertical Visibility (VV) into a surface-based obscuration.

Layers reported as FEW or SCT do not constitute a ceiling. For example, in FEW008 SCT015 BKN025 OVC060, the ceiling is $2{,}500\text{ ft AGL}$.

Convective Cloud Additions

Only two convective cloud types are appended to sky condition groups in civil METARs:

• CB: Cumulonimbus (e.g., BKN030CB)
• TCU: Towering Cumulus (e.g., SCT025TCU)

Temperature and Dewpoint Group

Encoded as two-digit integers in degrees Celsius, separated by a solidus (/). Negative temperatures are preceded by M (Minus):

• 18/12: Temperature $+18^\circ\text{C}$, dewpoint $+12^\circ\text{C}$.
• 02/M01: Temperature $+2^\circ\text{C}$, dewpoint $-1^\circ\text{C}$.
• M04/M08: Temperature $-4^\circ\text{C}$, dewpoint $-8^\circ\text{C}$.

A close temperature-dewpoint spread ($3^\circ\text{C}$ or less) indicates high relative humidity ($> 80\%$), warning the dispatcher of imminent fog formation, low ceilings, and severe structural icing if liquid precipitation occurs near or below freezing.

Altimeter Group

The altimeter group begins with the letter A followed by four digits representing barometric pressure in inches of mercury (inHg) to hundredths:

• A2992: $29.92\text{ inHg}$ (standard sea-level datum)
• A3014: $30.14\text{ inHg}$
• A2885: $28.85\text{ inHg}$ (deep cyclone, raising lowest usable flight levels under 14 CFR 91.121)

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6. The Remarks Section (RMK): Operational Intelligence

The remarks section contains critical diagnostic data for the professional dispatcher:

• Automated Station Type:
  • AO1: Automated station without a precipitation discriminator (cannot distinguish between rain and snow).
  • AO2: Automated station equipped with an optical precipitation discriminator (distinguishes rain, snow, drizzle, and freezing precipitation).
• Maintenance Indicator ($): Appears at the end of the remarks string (e.g., RMK AO2 ... $). It indicates an automated sensor has failed internal diagnostic tests and requires maintenance. The observation remains legally valid for flight planning and dispatch unless NOTAMs declare the sensor unserviceable.
• Peak Wind: PK WND dddff/hhmm. Reports the peak instantaneous wind direction (ddd), speed in knots (ff), and minute past the hour (hhmm) if peak gusts exceeded 25 knots (e.g., PK WND 31048/1942 = peak wind $310^\circ$ at 48 knots at 1942Z).
• Wind Shift: WSHFT hhmm. Reports the time a wind shift occurred (e.g., WSHFT 1735), frequently appended with FROPA to signify a frontal passage.
• Sea Level Pressure: SLPppp. Sea level pressure in tenths of hectopascals (hPa / mb). To decode, prepend a 9 or 10, whichever brings the value closer to $1000\text{ hPa}$:
  • SLP132 $\rightarrow 1013.2\text{ hPa}$
  • SLP985 $\rightarrow 998.5\text{ hPa}$
  • SLP045 $\rightarrow 1004.5\text{ hPa}$
• Exact Hourly Temperature and Dewpoint (T-group): An eight-digit code providing temperatures to the tenth of a degree Celsius: T sTTT sddd. The sign digit s is 0 for positive and 1 for negative:
  • T00440011: Temperature $+4.4^\circ\text{C}$, dewpoint $+1.1^\circ\text{C}$.
  • T10281056: Temperature $-2.8^\circ\text{C}$, dewpoint $-5.6^\circ\text{C}$.
• Lightning Frequency and Location:
  • Frequency: OCNL (occasional, $< 1\text{ flash/min}$), FRQ (frequent, 1 to 6 flashes/min), CNS (continuous, $> 6\text{ flashes/min}$).
  • Type: CG (cloud-to-ground), IC (in-cloud), CC (cloud-to-cloud), CA (cloud-to-air).
  • Direction/Distance: LTG CG DSNT NE (cloud-to-ground lightning distant northeast).
• Pressure Tendency: PRESRR (pressure rising rapidly, $\ge 0.06\text{ inHg/hr}$), PRESFR (pressure falling rapidly).

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7. Practical Decoding Walkthroughs

Scenario Alpha: Winter Arrival at Minneapolis (KMSP)

METAR KMSP 241553Z 33022G38KT 1/2SM R12R/2200FT -SN BLSN FZFG BKN008 OVC018 M03/M05 A2944 RMK AO2 PK WND 32042/1538 WSHFT 1520 SLP978 T10331050 $

• Station and Time: Minneapolis-St. Paul, 24th day at 1553 UTC.
• Winds: From $330^\circ\text{ true}$ at 22 knots, gusting to 38 knots.
• Visibility & RVR: Prevailing visibility $1/2\text{ statute mile}$. Runway 12R visual range is $2{,}200\text{ feet}$.
• Weather: Light snow (-SN), blowing snow (BLSN), and freezing fog (FZFG).
• Clouds: Broken layer at 800 feet AGL (BKN008, the ceiling), overcast layer at 1,800 feet AGL (OVC018).
• Temp/Altimeter: Temperature $-3^\circ\text{C}$, dewpoint $-5^\circ\text{C}$. Altimeter setting $29.44\text{ inHg}$.
• Remarks: Station with precipitation discriminator (AO2). Peak wind from $320^\circ$ at 42 knots recorded at 1538Z. Wind shift occurred at 1520Z. Sea level pressure is $997.8\text{ hPa}$. Exact temperature is $-3.3^\circ\text{C}$, exact dewpoint is $-5.0^\circ\text{C}$. Maintenance flag ($) is active.

Scenario Bravo: Convective Event at Dallas-Fort Worth (KDFW)

SPECI KDFW 032115Z 18032G54KT 140V240 1 1/2SM +TSRA BKN012CB OVC035 24/22 A2968 RMK AO2 PK WND 17058/2112 WSHFT 2105 FROPA LTG CNS CG DSNT N AND NE PRESRR

• Type: SPECI (unscheduled special report).
• Winds: From $180^\circ$ at 32 knots, gusting to 54 knots, variable between $140^\circ$ and $240^\circ$.
• Visibility & Weather: $1 1/2\text{ SM}$ in heavy thunderstorm with rain (+TSRA).
• Clouds: Broken cumulonimbus at 1,200 feet AGL (BKN012CB, ceiling), overcast at 3,500 feet AGL.
• Temp/Dewpoint/Altimeter: $24^\circ\text{C} / 22^\circ\text{C}$, altimeter $29.68\text{ inHg}$.
• Remarks: Peak wind $170^\circ$ at 58 knots at 2112Z. Wind shift at 2105Z associated with frontal passage (FROPA). Continuous cloud-to-ground lightning distant north and northeast. Pressure rising rapidly (PRESRR).

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8. Operational Application to Part 121 Dispatch & Takeoff Minimums

Takeoff Minimums (14 CFR 121.651 & OpSpecs C056/C078)

Under standard Part 121 regulations, takeoff minimums are:

• Aircraft with 1 or 2 engines: $1\text{ statute mile}$ prevailing visibility or $5{,}000\text{ RVR}$.
• Aircraft with 3 or more engines: $1/2\text{ statute mile}$ prevailing visibility or $2{,}400\text{ RVR}$.

Some carriers hold lower-than-standard takeoff authority. Apply the actual issued authorization, runway aids, controlling RVR reports, aircraft capability, and crew qualification; do not assume every Part 121 carrier or every runway permits a 500-foot-RVR departure.

Controlling Weather Concept

Under 14 CFR 121.651, visibility (or RVR) is the sole controlling factor for departure and landing. Unless a published departure procedure explicitly mandates a ceiling for obstacle climb clearance, ceiling is not controlling for takeoff. If the METAR reports an indefinite ceiling of 100 feet (VV001) with $1/2\text{ SM}$ visibility ($2{,}400\text{ RVR}$), a twin-engine transport jet authorized for standard takeoff minimums can legally take off despite the 100-foot ceiling.

Takeoff Alternate Rule (14 CFR 121.617)

If the current METAR at the departure airport reports weather conditions below the lowest authorized Category I landing minimum for that airfield, the flight cannot return for an immediate landing in the event of an engine failure on takeoff. In this situation, 14 CFR 121.617 mandates that the dispatcher list a takeoff alternate on the dispatch release:

• Two-engine aircraft: Takeoff alternate must be located within 1 hour normal cruising speed in still air with one engine inoperative.
• Three- or four-engine aircraft: Takeoff alternate must be located within 2 hours normal cruising speed in still air with one engine inoperative.

Destination Release Rule (14 CFR 121.613)

A common dispatch trap is assuming a flight cannot be released if the destination airport's current METAR is below landing minimums. Under 14 CFR 121.613, dispatch release depends on the weather reports and forecasts indicating that conditions will be at or above minimums at the estimated time of arrival (ETA). Therefore, an air carrier flight may legally depart for a destination currently reporting zero-zero fog, provided the Terminal Aerodrome Forecast (TAF) predicts the fog will lift above landing minimums prior to the aircraft's arrival.

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9. Practical Traps & Operational Scenarios

Practical Trap 1: The Ceiling Fallacy on Takeoff

Candidates frequently assume an aircraft cannot depart if the ceiling is below the approach minimum. Under Part 121, takeoff minimums are established purely in visibility or RVR terms (unless a specific Standard Instrument Departure / SID specifies a minimum ceiling for obstacle avoidance). A dispatcher who cancels or delays a flight due to a 200-foot ceiling when RVR is $5{,}000\text{ ft}$ and no ceiling restriction exists causes an unnecessary and costly operational delay.

Practical Trap 2: The Maintenance Indicator ($)

Encountering $ in a METAR remark does not invalidate the report. It is an automated notification to maintenance technicians that a sensor diagnostic threshold was reached. The report remains legally valid for dispatch unless a NOTAM formally supersedes the observation.

Operational Scenario: Takeoff Alternate Assignment

A Boeing 737 is scheduled to depart Chicago Midway (KMDW). The METAR reads:

METAR KMDW 151152Z 04014KT 1/4SM R04R/1400FT FG VV002 01/01 A3002 RMK AO2

The lowest landing minimum for Runway 04R is RVR 1,800 ft (Category I). Because current weather ($1{,}400\text{ RVR}$) is below landing minimums, the aircraft cannot land back at Midway if an engine fails after $V_1$. The carrier's OpSpecs authorize lower-than-standard takeoff down to $1{,}000\text{ RVR}$, so the departure is legal. However, under 14 CFR 121.617, the dispatcher must designate a takeoff alternate located within 1 hour single-engine cruise distance (such as Milwaukee KMKE or Indianapolis KIND), where weather meets derived alternate minimums.

Official References

Source check: 2026-10-12. 2026 FAA Aviation Weather Handbook; Current NWS TAF policy (October 2024); AWC current products; Dispatcher weather information § 121.601. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.