L13 Aviation Weather Reports

Aviation Weather Reports

Introduction

• The aim of meteorology knowledge for pilots is to make the best of available information, recognize, and avoid hazardous weather situations.

• This lesson covers the International Meteorological Organisation and available reports for pilots before and during flights.

Meteorological Services

• SARPS Annex 3:

  • Sets standards for weather reporting and forecasting.

  • Specifies accuracy requirements and formats.

• World Meteorological Organization (WMO):

  • Ensures implementation of SARPS Annex 3 standards for aviation.

• World Area Forecasting System (WAFS):

  • Developed by ICAO in the 1980s.

  • Provides upper air forecasts.

• World Area Forecasting Centres (WAFC):

  • Two centers were established:

    • NOAA in the US.

    • UK Met Office.

• WAFCs prepare weather forecasts that include:

  • Upper winds (direction, speed, and flight level of maximum wind speed).

  • Upper-air temperature and humidity.

  • Temperature at the tropopause.

  • Areas of cumulonimbus cloud formations.

  • Areas of icing.

  • Areas of turbulence (clear-air and in-cloud).

Meteorological Organization

• To effectively use meteorological services, pilots must understand what they are requesting and receiving.

• Weather information comes in two forms:

  • Weather Report:

    • A factual statement of actual weather conditions at a specific place and time.

    • Provided by ‘aeronautical meteorological stations’.

  • Weather Forecast:

    • A prediction of the expected weather during a set period.

    • Available at various levels: charts for large areas, mixed chart/text forecasts for localized areas, and forecasts for individual airfields.

• Pilots must distinguish between a weather report (actual weather) and a forecast (prediction).

Meteorological Observations and Reports

• Various sources are used to observe weather conditions and for forecasting.

Surface Observations

• Surface Observation:

  • The most basic source of weather observation.

  • Provided to the pilot as a METAR (METeorological Actual Report), also called an 'actual'.

  • Provided by trained meteorological observers or automatic systems.

  • SPECI:

    • A special report of meteorological conditions.

    • Issued if one or more elements meet specific aviation criteria or after 10 minutes following an improvement.

Radiosonde Observations

• Radiosonde:

  • A small instrument package suspended below a helium or hydrogen-filled balloon.

  • Ascends through the atmosphere, transmitting measurements of:

    • Position

    • Altitude

    • Pressure

    • Temperature

    • Relative humidity

    • Wind speed and direction

  • The data creates an accurate vertical model of the atmosphere, aiding meteorologists in confirming predictions and forecasting.

Satellite Observations

• Satellite images are widely used in weather forecasting and available online.

• Two basic formats:

  • Visible Image:

    • Similar to a black-and-white photograph from space.

    • Thick cloud appears white, coastlines and snow-covered ground are visible.

    • Limitations:

      • Snow can be confused with thick cloud.

      • Thin cloud (cirrus) may be transparent.

      • Not available at night (dark areas near sunrise/sunset).

  • Infrared (IR) Image:

    • Based on temperature differences; unaffected by daylight.

    • Cold areas appear white, warmer areas are grey, warmest areas are black.

    • Limitations:

      • High cloud (coldest) can obscure lower levels.

      • Low cloud/fog may have similar temperatures to the surface, making it hard to distinguish.

Processing Satellite Images

• Satellite images are processed before use:

  • Latitude and longitude grid lines are added.

  • Coastlines are added.

  • Adjustments are made for daylight.

• Satellite images are most useful when used with other synoptic or significant weather information.

Weather Radar Observations

• National meteorological offices use radar systems to detect precipitation areas and intensity.

• Radar systems pinpoint active thunderstorms based on electrical activity.

• Weather radar images are available online but are often delayed (up to 30 minutes).

• Modern ATC radar filters out weather 'clutter,' limiting its usefulness for weather avoidance.

Pilot Observations

• Pilots must report observed weather (AIREPs).

• AIREP SPECIALS are required in European FIRs when:

  • Severe icing or turbulence is encountered.

  • Moderate turbulence, hail, or cumulonimbus clouds are encountered in transonic or supersonic flight.

  • Other meteorological conditions affect safety or efficiency (thunderstorm, hail, snow, freezing precipitation, wind shear, volcanic ash).

  • Requested by the Meteorological Authority.

• Private pilots should report encountered wind shear to assist other pilots.

Decoding METARS

• METARs are aviation weather reports produced by a Meteorological Watch Office (MWO).

• In the UK, they are compiled every 30 or 60 minutes at fixed times.

• A METAR consists of elements described below.

Location

• The ICAO four-letter code for the airfield.

Date/Time of Observation

• A six-digit group:

  • Day of the month (first two digits).

  • Hours and minutes in UTC (next four digits).

  • Followed by the letter Z.

AUTO

• Indicates a fully automated observation with no human intervention.

Wind

• Surface wind direction in degrees True (three digits), rounded to the nearest 10 degrees.

• Windspeed (two digits, exceptionally three), averaged over the ten-minute period preceding the observation.

• Units: KT (knots), KMH (kilometers per hour), or MPS (meters per second).

• Maximum gust speed (prefixed with a G) in knots if it exceeds the mean speed by 10 kt or more.

• Calm: '00000' followed by the units' abbreviation.

• Variable wind direction: 'VRB' followed by speed and unit.

• Wind direction variation of 60° or more during the 10-minute period is indicated.

Horizontal Visibility

• Reported as the prevailing visibility and the minimum visibility under certain conditions.

• Prevailing visibility: visibility within at least half the horizon circle or half of the aerodrome surface.

• If visibility in one direction is less than 1500 m or less than 50% of the prevailing visibility, the lowest visibility observed is also reported with its direction.

• Recorded in meters (m):

  • Rounded down to the nearest 50 m when visibility is 800 m or less.

  • Rounded to the nearest 100 m when visibility is greater than 800 m but less than or equal to 5000 m.

• Expressed in kilometers (km) to the nearest 1 km when visibility is greater than 5000 m.

• '9999' indicates visibility of 10 km or more.

• '0000' indicates visibility of less than 50 m.

RVR

• Runway Visual Range (RVR) always includes the prefix R, the runway designator, a diagonal, and the touchdown zone RVR in meters.

• RVR is reported when horizontal visibility or RVR is less than 1500 m.

Cloud

• A six-character group reports cloud amount and level.

• Cloud amount (oktas):

  • FEW: 1 to 2 oktas.

  • SCT (scattered): 3 to 4 oktas.

  • BKN (broken): 5 to 7 oktas.

  • OVC (overcast): 8 oktas.

• The last three digits indicate the height of the cloud base in hundreds of feet above aerodrome level.

• Cloud types reported: CB (Cumulonimbus) or TCU (Towering Cumulus).

• 'NSC' (No significant cloud) is reported if:

  • No cloud below 5000 ft or below the highest minimum sector altitude (whichever is higher).

  • No towering cumulus or cumulonimbus.

• Sky obscured is coded as VV followed by the vertical visibility in hundreds of feet. 'VV///' if vertical visibility cannot be assessed.

Cavok

• Pronounced "Kav-O-Kay", this term replaces visibility, RVR, weather, and cloud groups when:

  • Visibility is 10 km or more.

  • No cloud below 5000 ft or below the highest Minimum Sector Altitude.

  • No towering cumulus or cumulonimbus.

  • No significant weather phenomena at or near the aerodrome.

QNH

• QNH is rounded down to the nearest whole millibar and reported as a four-digit group preceded by the letter Q.

• If QNH is less than 1000 mbs, the first digit will be 0.

• When reported in inches of mercury, the pressure is prefixed by 'A'.

Recent Weather

• Operationally significant weather observed since the previous observation, but not at the time of the report.

• The present weather code is used, preceded by the indicator RE.

Trend

• A forecast of significant changes in conditions during the two hours after the observation time.

• Abbreviations: BECMG (becoming) or TEMPO (temporary).

• May be followed by a time group (hours and minutes UTC) preceded by FM (from), TL (until), or AT (at).

Automatic METARS

• Increasingly generated by automatic observing systems without human input.

• The code 'AUTO' is inserted between the date/time group and the wind group.

• 'NDV' (no directional variation reported) is appended to the prevailing visibility.

• Weather sensor issues:

  • If present weather cannot be detected: '//'.

  • If the sensor cannot determine the state of precipitation: ‘UP' (unidentified precipitation) or ‘FZUP' (freezing unidentified precipitation).

  • If no weather is detected, no present weather group is reported.

• The inability to detect towering cumulus or cumulonimbus is indicated by '///' after each cloud group.

• 'NCD' (No Cloud Detected) indicates that there are no clouds present below 5000 ft.

• Recent unidentified precipitation (‘REUP') is reported if moderate or heavy unidentified precipitation has ceased or decreased in intensity since being reported in the last routine report.

• A failure of measuring equipment is indicated by '////' for visibility and '/////////' for cloud.

VOLMET

• Once flying, the pilot's observations are the best tool for assessing weather.

• Compare heading against track and groundspeed against planned speed to assess winds aloft.

• The altimeter setting alerts the pilot to pressure differences.

• Stronger winds or lower pressure than predicted are warning signs.

• VOLMET:

  • A recorded broadcast giving METARs for a group of about ten airports.

  • Available throughout Europe.

  • Details are in flight guides and official publications (AIP).

• VOLMET reports may include a 'TREND,' which is any expected change in conditions during the two-hour period following the METAR's time.

ATIS

• ATIS (Automatic Terminal Information Service):

  • Individual airfields transmit arrival and departure information (current METAR and other relevant information).

  • Broadcast on a dedicated VHF frequency or a nearby VOR navigational beacon.

  • Useful for en route or alternate airfields.

  • Should be checked at the destination or departure airfield where it exists.