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METARs and Aviation Weather Observing: Key Concepts

METAR Structure and Reporting Cadence

  • Teletype era: observations from multiple airports (e.g., Daytona Beach, Orlando, Sanford) came in sequence, one observation at a time, so each observation had to be compact. This led to a complex, compact code rather than plain-language readability. Even after modernization, the code format persists under government/FAA practices and isn’t expected to change soon.
  • Observations are issued hourly. Special (nonroutine) aviation weather reports exist in addition to the hourly METARs.
  • Specials are essentially nonroutine METARs issued between the top of the hour when certain criteria are met (e.g., visibility dropping below a threshold). They are disseminated as needed and aren’t aligned to the hour.
  • The date/time of the METAR is included in the report.

Date, Time, and Data Groups in METAR

  • The first two numbers in the data group are the day of the month. 2 (the day) is coded in the data group.
  • The observation time is often described as minutes past the hour; for example, a report that says 53 minutes past the hour corresponds to that minute value.
  • The METAR data may appear as if it is the official time, and the transcript notes that the official observation time is 12Z (12:00 UTC). The example mentions “11:53 is the official 12Z observation,” illustrating how local reads can differ from the official UTC reference. In practice, the official observation is the 12Z time.
  • The official “12Z” observation is used as the standard reference time for the METAR.
  • The METAR string can be supplemented by human observers when needed; the automated observation may be labeled with “AUTO” but human observers (e.g., contract weather observers or ATC/controllers) may add information when necessary.

Automated vs Manual Observing Systems

  • Automated observing systems are impressive, but they have limitations and cannot detect everything.
  • Some airports have human supplementers (e.g., contract weather observers) who provide additional data beyond the automated readouts.
  • At some airports, there is no contract weather observer, but air traffic controllers or tower staff may contribute extra weather information.
  • Key limitation: automated systems cannot reliably determine certain phenomena (e.g., a tornado). Human presence is needed to assess these conditions.
  • Other phenomena that may require human observation include smoke and volcanic ash deposition.
  • This is why human observers remain important alongside automated systems for comprehensive aviation weather reporting.

Winds: Direction, Speed, and Variability

  • Wind direction in METAR is always given as where the wind is coming from, not where it’s going to. This reflects historical usage and interpretation.
  • Winds can be more variable when the wind is weaker; a wind vane would swing through multiple directions if the wind is light.
  • If the wind speed is v \le 6 knots, the code may be recorded as VRB (variable wind) and then the wind speed is still provided.
  • When winds are strong yet variable, the direction can be unclear; variability increases as the speed decreases.
  • The concept of wind reporting reflects both directionality and variability, with coding designed to convey a stable direction when wind is consistent and a variable designation when it is not.

Visibility and Runway Visual Range (RVR)

  • Visibility values can appear as fractional quantities rather than representing a range; they denote a specific distance in statute miles, sometimes expressed as fractions.
  • Examples (statute miles):
    • 2 statute miles visibility
    • 0.75 statute miles visibility (three quarters)
    • 1.5 statute miles visibility (one and a half)
  • The use of a slash or fractional presentation is not necessarily a range; it is a fractional value representing the actual visibility.
  • Descriptors in METARs include abbreviations derived from various origins (e.g., some from French):
    • RA for rain, DZ for drizzle, SN for snow
    • GR for hail (the speaker notes a humorous confusion with the French pronunciation, mentioning “grail” as a misunderstanding)
    • MI for shallow fog (the instructor notes uncertainty about the origin but confirms the meaning)
  • Runway Visual Range (RVR) is used when visibility can vary by runway or when the prevailing visibility is not representative of all locations on the airfield.
    • Airports place instruments along different runways to measure RVR and capture possible differences in visibility across runways.
    • RVR helps address patchy or runway-specific visibility conditions when interpreting the overall prevailing visibility.
  • Prevailing visibility is a concept used when reporting general visibility conditions for the area, while RVR provides runway-specific detail.

Sky Conditions and Cloud Coverage (Ceilings)

  • Sky condition reporting includes terms that indicate cloud cover and ceiling levels; one point emphasized is the interpretation of sky conditions above and below certain altitudes.
  • A note about the term “meter” (as used in the transcript) suggesting that it may imply clarity above 12,000 feet and potential overcast above that altitude; the speaker cautions that this can be confusing and is an example of why familiarity with the codes matters.
  • Important reminder: the term OVC (overcast) is a standard reporting designation for sky cover.
  • The takeaway: don’t misinterpret cloud cover indicators; for example, OVC indicates overcast, while other qualifiers (FEW, SCT, BKN) indicate partial coverage.

Abbreviations, Codes, and Language Origins

  • Many abbreviations in METARs have roots in various languages, including French:
    • RA = rain, DZ = drizzle, SN = snow
    • GR = hail (the example mentions confusion due to pronunciation)
    • MI = shallow fog
  • The instructor notes that some terms may seem illogical or foreign to students and emphasizes the importance of learning these codes for exam success.

Exam Preparation and Practical Tips

  • The PowerPoint may include questions designed to test understanding of METAR components (e.g., “What obscurations/visibility were reported in the following METAR?”).
  • Actively answering example questions in the slides can serve as a heads-up for potential exam questions and facilitate retention of the material.
  • The instructor recommends practicing with the displayed questions to reinforce familiarity with METAR content and improve readiness for the exam.

Real-World Relevance and Critical Considerations

  • The reliance on automated systems is extensive, but human observation remains essential for detecting phenomena beyond automation, affecting interpretation and decision-making in aviation operations.
  • Understanding wind reporting conventions, cloud cover terminology, and visibility metrics is crucial for safe flight operations, weather briefings, and aircrew decision-making.
  • Recognizing the limitations of automated systems (e.g., tornado detection, smoke presence, volcanic ash) informs how pilots and controllers interpret METARs and advisories in real-world settings.

Quick Reference Concepts (Summary)

  • METARs are hourly wind-and-weather reports; specials are nonroutine METARs issued as needed between hours.
  • Data groups include day-of-month, time (with 12Z as the standard reference), and “AUTO” vs human augmentation.
  • Wind: report from where wind originates; VRB if wind is ≤ 6 knots; variable wind increases with weakness.
  • Visibility: reported as statute miles and can appear as fractions (e.g., 0.75, 1.5);
    Runway Visual Range (RVR) provides runway-specific visibility data.
  • Sky condition: pay attention to ceiling and OVC; some codes have origins in languages such as French.
  • Automated systems are powerful but have gaps; human observers remain essential for comprehensive aviation weather reporting.