Aviation Weather Observations and Forecasting Notes

Wind Direction and Magnetic Variation Principles

• True North vs. Magnetic North: Wind reported by the National Weather Service and the main tower is always relative to True North. Pilots must convert this to a magnetic direction using magnetic variation for navigation and runway alignment purposes.

• Runway Alignment: The National Weather Service does not account for runway alignment. Pilots must perform the conversion to ensure their headings and checkpoints are accurate.

• Wind Speed and Gusting Phenomena:

  • Gust Definition: To be classified as a gust, the differential between the low (lull) and the peak (high) wind speed must be at least $10\,\text{knots}$.

  • Calculation Example: If a gust is reported at $38\,\text{knots}$, the lowest wind speed was at least $28\,\text{knots}$.

  • Average Wind: In a gusting scenario, the wind group reports the average wind speed along with the peak gust value.

• Calm Wind: Reported as $00000\text{KT}$, indicating no wind activity on the property.

Visibility Determination and Equipment

• Methodology: Visibility is determined by looking at pre-measured landmarks from the tower cab. This makes the determination objective rather than subjective.

  • Beaver County Airport Examples: The approach end of Runway $28$ is marked at $\frac{1}{2}\,\text{mile}$. The approach end of Runway $10$ is marked at $\frac{3}{4}\,\text{mile}$. High-tension power lines in Ohio are used at increments of $3$, $5$, and $7\,\text{miles}$.

• Runway Visual Range (RVR): Sophisticated weather equipment at Class B airports that provides a specific distance (e.g., $1,600\,\text{feet}$ or $\frac{1}{4}\,\text{mile}$) describing how far a pilot can see down the active runway centerline. This is critical for Instrument Flight Rules (IFR) environments and is prioritized over general prevailing visibility.

• Reductions in Visibility: Observers must legally list any phenomena reducing visibility. Common abbreviations include:

  • RA: Rain

  • SN: Snow

  • TS: Thunderstorms

  • FG: Fog

  • BR: Mist (commonly referred to by students as "baby rain")

  • HZ: Haze

  • FC: Funnel Clouds

• Intensity Symbols: These symbols accompany precipitation but not obscurations (like fog or haze):

  • -: Light (e.g., $-RA$ for light rain)

  • +: Heavy (e.g., $+SN$ for heavy snow)

  • No symbol: Moderate

  • Intensity Basis: Intensity is based on the rate of accumulation in a rain gauge (e.g., $1\,\text{inch}$ per hour).

Sky Condition and Cloud Layers

• Measurement: Cloud altitudes are reported as bases in feet Above Ground Level (AGL). Observers on the ground cannot determine the "tops" of cloud layers.

• The Octal Rule (Eighths): The sky is divided into eight equal portions (like a pizza or cake). Coverage is determined by counting how many eighths are filled:

  • Few (FEW): $1/8$ to $2/8$ coverage. Example: $FEW008$ (Few clouds at $800\,\text{feet}$).

  • Scattered (SCT): $3/8$ to $4/8$ coverage. Example: $SCT012$ (Scattered clouds at $1,200\,\text{feet}$).

  • Broken (BKN): $5/8$ to $7/8$ coverage. This constitutes a "ceiling." Example: $BKN030$ (Broken clouds at $3,000\,\text{feet}$).

  • Overcast (OVC): $8/8$ coverage. Example: $OVC100$ (Overcast at $10,000\,\text{feet}$).

• Cumulative Nature: Coverage is added mathematically from bottom to top. If an observer sees $8/8$ total coverage across multiple layers, the highest layer is reported as overcast even if the observer cannot see through lower layers.

Meteorological Aerodrome Reports (METAR) Component Details

• Temperature and Dew Point: Read from digitized sensors and reported in Celsius to follow international formats.

  • Saturation: When temperature and dew point are close (low spread), saturation and cloud formation occur.

• Altimeter Setting: Reported as a four-digit number representing inches of mercury (e.g., $29.92$ reported as $2992$).

• International Variations: Most international operators (e.g., British Airways) use millibars (hectopascals) instead of inches of mercury.

• Remarks (RMK): Supplemental information including:

  • A02: Automated rain gauge.

  • SLP: Sea Level Pressure in millibars (e.g., $SLP123$).

  • T-Group: Precise temperature and dew point to the tenth of a degree (e.g., $T02780150$).

  • **Dollar Sign ()**: Indicates that a weather sensor is malfunctioning and requires maintenance.\n\n# METAR Case Studies and Examples\n\n* **Bangor, Maine (BGR)**: Wind 170^{\circ}$at$6\,\text{knots}$. Visibility$10\,\text{SM}$. FEW clouds at$4,000\,\text{feet}$. Temp$28^{\circ}\text{C}$, Dew Point$15^{\circ}\text{C}$. Altimeter$29.94.\n* **Williamsport, Pennsylvania (IPT)**: Wind calm. Visibility 10\,\text{SM}$. SCT clouds at$4,200\,\text{feet}$. Temp$27^{\circ}\text{C}$, Dew Point$18^{\circ}\text{C}$. Altimeter$30.10$. Maintenance ($) symbol present.

• Charlottesville, Virginia (CHO): Wind $060^{\circ}$ at $6\,\text{knots}$. Visibility $4\,\text{SM}$. Moderate rain and mist. Layers: SCT at $1,400\,\text{feet}$, BKN at $2,400\,\text{feet}$, OVC at $4,800\,\text{feet}$. Temp and Dew Point both $24^{\circ}\text{C}$ (saturated).

• Huntington, West Virginia (HTS): Visibility $1.5\,\text{SM}$ due to heavy rain and thunderstorms ($+TSRA$). Lightning noted in remarks to the Southeast and Southwest.

• Anchorage, Alaska (ANC): Peak wind noted in remarks as $140^{\circ}$ at $28\,\text{knots}$ occurring at $1712\,\text{Zulu}$.

Pilot Reports (PIREPs) and Air Reports (AIREPs)

• Function: Actual weather conditions reported by pilots mid-flight. Essential for identifying "unforecast" weather, cloud tops, turbulence, and icing.

• Reporting Channels: Pilots communicate via the active ATC frequency (Cleveland Center, etc.) or Flight Service on $122.2\,\text{MHz}$. High-end aircraft (Delta, United) use automated data links like Starlink.

• Format Categories:

  • UA: Routine PIREP.

  • UUA: Urgent PIREP (e.g., severe icing, extreme turbulence, volcanic ash).

• Required Fields:

  • Location: Reference to a station or GPS coordinates.

  • Time: Always in Zulu ($UTC$).

  • Altitude: Flight level or altitude AGL/MSL.

  • Aircraft Type: Essential for context (e.g., $C172$ vs. $B737$ respond differently to turbulence).

• Icing and Turbulence: Reported as negative, light, moderate, or severe.

  • Rime Ice: Milky, opaque ice formed in clouds.

• AIREPs: Specific to over-water flights (e.g., Atlantic or Pacific routes). Since there is no radar, these utilize Latitude and Longitude for positioning.

Terminal Aerodrome Forecasts (TAF)

• Difference from METAR: A METAR is an observation (actual); a TAF is a forecast (predicted).

• Coverage and Issuance:

  • 24-Hour TAFs: For part-time airports (e.g., Beaver County).

  • 30-Hour TAFs: For major 24-hour airports (e.g., Chicago O'Hare).

  • Updates: Issued four times daily.

• Conditional Statements:

  • TEMPO: Temporary fluctuations lasting less than one hour.

  • PROB40: A $40\%$ probability of a weather phenomenon (e.g., thunderstorms).

  • BECMG: A gradual change in weather conditions.

  • P6SM: Visibility forecast to be greater than $6\,\text{statute miles}$.

Weather Radar and Centers

• NextRad/Doppler Radar: Measures reflectivity based on the size and type of water droplets or ice crystals.

  • Color Coding: Pale green (light rain) to Red (heavy precipitation/storms).

  • Bow Echoes: Curved radar lines indicating strong thunderstorm anvils and wind.

• Center Weather Advisory (CWA): Supplemental weather warnings issued by meteorologists at Air Route Traffic Control Centers (ARTCC) to fill gaps in National Weather Service reports.

• Flight Information Services (FIS): Subscription or broadcast-based digital weather data displays (e.g., ADS-B) provided directly to the cockpit.