Aerodrome weather messages

Is SPECIAL simply another name for SPECI?

The intended audience and sampling area help explain a difference between reports.

Aerodrome observation products

A METAR is a routine aerodrome meteorological report issued at published observation intervals, commonly half-hourly or hourly. A SPECI is a special aerodrome report issued when specified significant changes occur under the applicable reporting arrangements. Both describe an observation at a stated time; neither is a forecast simply because a pilot reads it later. The absence of a new SPECI is not a promise that every value is unchanged between observations.

MET REPORT is the local routine report and SPECIAL the local special report, intended for dissemination at the aerodrome of origin. They use an abbreviated plain-language presentation and can describe runway-representative wind, visibility and the relevant RVR zones. METAR/SPECI are coded reports for wider dissemination and use the prescribed aerodrome/prevailing quantities. SPECIAL and SPECI therefore share the idea of a significant-change report but are different products.

Read the four-letter location identifier, observation date/time DDHHMMZ and any COR, AUTO or NIL indication. Z denotes UTC; the day is the day of the month, so month rollover needs contextual date information. COR corrects a report; AUTO identifies automatic observation, and NIL indicates a missing report. An older report re-displayed by a portal is still older data. A TREND appended after the observation must be interpreted separately from what has already been observed.

What does each group in this fictional report establish?

Separate wind, prevailing visibility and the runway measurement.

A complete observation example

Fictional teaching report: METAR YUDO 211220Z 24015G25KT 200V280 1200 R27L/0650U -RA BR BKN004 OVC012 08/07 Q1002 TEMPO 0800 RA=. YUDO is the fictitious training aerodrome. The observation is on day 21 at 1220 UTC. Wind is from 240° true, mean 15 kt, maximum gust 25 kt; the extremes vary clockwise from 200° to 280°. The G25 is not a 25 kt addition to the mean. In a normal European METAR, the mean represents the preceding ten minutes, with marked-discontinuity rules where applicable.

1200 gives prevailing visibility 1,200 m. R27L/0650U gives touchdown-zone RVR for runway 27 left of 650 m with an upward tendency over the reporting interval. It does not predict improvement or supply all runway-zone values. P/M before an RVR value denote above/below the reporting limit; N/D mean no distinct tendency/downward tendency where provided. Do not confuse RVR in metres with a national report explicitly using FT.

VRB replaces a reportable mean direction under the variable-wind rules; 00000KT means calm. Winds in MPS use metres per second, and some national products use other permitted units—read the suffix. A runway number approximates a magnetic runway direction, while METAR wind is true: align references before a crosswind calculation. The later TEMPO group is a forecast, not another part of the observed visibility.

Does BKN004 give 400 ft above sea level?

Use the cloud datum and the sign convention before assessing the flight.

Weather, cloud, temperature and pressure

In the example, -RA is light rain and BR is mist. Weather groups combine an intensity or proximity qualifier, a descriptor and one or more phenomena. Thus +SHRA is heavy rain showers; FZRA is freezing rain; SHSN is snow showers; BLSN is blowing snow; MIFG is shallow fog; BC indicates patches and PR partial coverage. TS identifies a thunderstorm; VC describes the vicinity under the applicable distance convention and does not mean overhead at the observing point. Intensity is not added indiscriminately to every obscuration code.

BKN004 is broken cloud, 5–7 oktas, with base 400 ft above the aerodrome reference level; OVC012 is overcast at 1,200 ft. The lower BKN defines the cloud ceiling in this example. FEW/SCT alone do not define a cloud ceiling. CB/TCU identify operationally significant convective types. VV003 is vertical visibility 300 ft into an obscuration, while VV/// is not assessable; it is not a clear-sky code. If field elevation is 700 ft AMSL, a 400 ft cloud base is approximately 1,100 ft AMSL.

08/07 is air temperature +8 °C and dew point +7 °C. M02/M03 means −2/−3 °C, not missing data. Q1002 is QNH 1,002 hPa, reported in whole hectopascals under the rounding rule; it is not QFE and not the pressure at the field surface. National A-groups such as A2992 use hundredths of inches of mercury and must not be read as hPa. Recent-weather RE groups concern specified recent significant weather, while present-weather groups describe the current observation.

Does CAVOK mean no cloud at any height?

Operational significance and sensor coverage differ from an unrestricted clear sky.

CAVOK, NSC and unavailable data

CAVOK replaces the relevant visibility, RVR, present-weather and cloud groups when all required conditions are met: visibility at least 10 km with no separate lowest visibility to report, no cloud of operational significance, and no significant weather as defined for the report. For example, prevailing visibility of 15 km with an 800 m sector requiring a minimum-visibility report cannot be shortened to CAVOK. Operationally significant cloud includes cloud below 5,000 ft or the highest minimum sector altitude, whichever is greater, and CB or TCU at any height. Consequently CAVOK is incompatible with a CB overhead even if its base is high, and does not exclude higher non-significant cloud.

NSC indicates no cloud of operational significance when CAVOK is inappropriate; it is not a visibility forecast. NCD in an automatic report means no cloud detected within the system’s capability, not that the whole sky is proved cloud-free. Slashes identify unavailable elements under the code rules; VV/// means vertical visibility cannot be assessed. A missing RVR group cannot simply be replaced with the prevailing visibility as if both had the same meaning.

The conventional coded values are categories with resolution: 9999 means visibility at least 10 km, 0000 less than 50 m, and 00000KT calm. Read AUTO and the station’s observation arrangements alongside the data. A report of no detected convection, or absence of an otherwise optional group, must not overrule a current warning, the aircraft’s observation or evidence outside the sensor footprint.

Does NOSIG mean there is no significant weather now?

TREND describes expected significant changes during a short period after the observation.

TREND groups

A TREND landing forecast, where supplied, is appended to a METAR/SPECI or relevant local report and covers the two hours following the observation time. NOSIG means no significant change is forecast under the criteria; adverse conditions already present can persist with NOSIG. A TREND is not attached to every aerodrome report, and absence of one is not a favourable forecast.

BECMG describes an expected change reaching the stated conditions within the indicated period; TEMPO describes temporary fluctuations. TREND time indicators use hours/minutes UTC: FM from, TL until, and AT at a specified time. These are not the complete six-digit FMDDHHMM groups used to start a new TAF forecast segment. Elements not specified in the change group retain the relevant underlying conditions, but when cloud changes significantly all cloud groups are restated, including unchanged layers. PROB is not used in an EASA/ICAO TREND.

For the fictional 1220Z METAR, TEMPO 0800 RA means temporary visibility 800 m in rain during the TREND period ending 1420Z, unless a narrower time interval is supplied. It does not change the 1220Z observed visibility of 1,200 m. BECMG TL1300 3000 NSW would forecast the specified improvement completed by 1300Z; NSW ends significant weather in that forecast context, not every cloud layer. Use the actual time and qualifiers rather than assuming a change occurs exactly when you read the report.

Which part gives the issue time and which gives the valid period?

The initial prevailing conditions and subsequent changes form one dated UTC forecast.

TAF structure and scope

A TAF is an aerodrome forecast, not an observed report. It starts with its type, location, date/time of origin and forecast-validity interval, followed by wind, prevailing visibility, significant weather and cloud. Under the European rule, routine validity is 9, 24 or 30 hours, with a competent-authority provision for aerodromes operating less than nine hours; the published service gives the actual issue schedule. TAF are filed not earlier than one hour before the start of validity. AMD identifies an amendment, COR a correction, and CNL cancellation. Always find the latest applicable version and its own valid interval.

Fictional example: TAF YUDO 211100Z 2112/2212 24012KT 9999 SCT020 BECMG 2114/2116 4000 RA BKN008 TEMPO 2116/2120 1200 +RA BKN004 PROB30 TEMPO 2117/2119 0600 FG VV002 FM212100 28008KT 9999 SCT025=. It was issued on day 21 at 1100Z and is valid from day 21 at 1200Z until day 22 at 1200Z. The initial forecast has wind 240° true at 12 kt, visibility at least 10 km and scattered cloud at 2,000 ft.

A TAF uses the aerodrome cloud-base datum and wind/visibility conventions appropriate to a forecast; it does not forecast every transient runway value. Cloud and temperature-change uncertainty, local terrain and the forecast’s operational accuracy matter. No TAF group by itself establishes that a particular aircraft/operator’s approach, alternate or take-off minima are met. Decode first, then apply the actual planning rules and time window for the operation.

What can be expected at 1700Z in the example TAF?

A change interval describes timing uncertainty; it is not automatically an average condition.

BECMG, TEMPO, FM and inheritance

BECMG 2114/2116 forecasts the specified change at an unspecified time between 1400Z and 1600Z on day 21. Once established, the new stated conditions become prevailing until superseded. Unchanged elements are inherited, except that all cloud groups are restated when cloud changes significantly. A BECMG interval is at most four hours. Do not assume the transition waits until the end, occurs at the midpoint, or is a smooth linear improvement/deterioration. In the example, by 1600Z the prevailing visibility/weather/cloud are 4,000 m, rain, BKN008; the 240°/12 kt wind continues because it was not changed.

TEMPO 2116/2120 forecasts temporary fluctuations during 1600–2000Z: each episode is less than an hour and their aggregate occupies less than half the period. It does not mean the conditions persist continuously through all four hours or occur at a predictable clock time. In this example the temporary conditions are 1,200 m in heavy rain with BKN004; the prevailing background remains outside those episodes.

FM212100 starts a new prevailing forecast segment from day 21 at 2100Z. The wind, visibility, weather and cloud for that segment replace the previous segment’s prevailing set as appropriate; do not carry an earlier adverse layer through an explicitly complete new segment. Here it gives 280°/8 kt, at least 10 km, nil significant weather and SCT025. Actual planning treatment of transitions and temporary conditions is governed by the applicable operational rules, not by a display’s choice of a single green/red label.

PROB is a chance, not a fraction of time

PROB30 and PROB40 describe 30% and 40% probability of the specified alternative conditions during their period. PROB30 TEMPO combines that probability with temporary-fluctuation timing; it does not mean fog covers exactly 30% of the aerodrome, lasts 30% of every hour, or that the pilot may ignore it. In the example, 1700–1900Z has a 30% probability of temporary 600 m fog with vertical visibility 200 ft, on top of the applicable background forecast.

Probabilities below the coding threshold may be omitted; omission is not a universal zero-risk statement. At 1700Z, consider the prevailing 4 km rain/BKN008, the TEMPO heavy-rain deterioration, and the PROB30 TEMPO fog branch. Do not add the probability numbers or average the visibility distances. Aircraft/operator planning rules determine which conditions must be allowed for; the meteorological decoder’s job is to preserve all relevant branches and times.

What did R27/590593 mean in the old reporting system?

Historical reference exercise: this six-character state suffix is not a current RCR or a present-day runway condition code.

Decode in fixed groups

The legacy state group may appear as R27/590593: runway 27, deposit type 5, contamination extent 9, depth 05 and friction/braking indicator 93. Using the historical code table, that means wet snow, 51–100% coverage, 5 mm depth and medium braking action. The final 93 is a qualitative code, not a friction coefficient of 0.93. The same suffix can be embedded in older eight-digit formats with a numerical runway designator; old regional conventions such as adding 50 for the right parallel runway must be read under that edition. The historical designator 88 can refer to all runways, while 99 repeats the previous information when no new report is available; neither is an actual runway heading.

Use the correct historical coverage codes 1, 2, 5 and 9, not a simple sequence 1, 2, 3, 4. Interpret each position with its own table: 92 in depth means 10 cm, while 92 in braking means medium/poor. A slash denotes unavailable/not reported information according to the relevant field. Historic special groups include CLRD when previously reported contamination has cleared, and SNOCLO for closure due to extreme snow deposits under the old code. Missing information must not be converted to a dry, open runway.

Can you translate an old braking code directly to today’s RWYCC?

The Global Reporting Format has a different assessment and communication structure.

Current RCR/SNOWTAM context

The old METAR runway-state group was removed from the European reporting framework with the transition to the Global Reporting Format in 2021. Current runway surface-condition information is assessed and communicated through the Runway Condition Report and applicable dissemination, including SNOWTAM, ATIS and ATS. The 2020 EASA learning-objective baseline still names the historical group, which is why the preceding exercise is explicitly labelled historical.

A current RCR describes conditions by runway thirds, including runway condition codes (RWYCC) and relevant contaminant, coverage and depth information under the current assessment rules. SNOWTAM is an aeronautical information message conveying the required runway-condition information; it is not a forecast and does not replace the latest ATS/ATIS update or the aircraft’s own report of worse-than-reported braking. Time, direction/third ordering, assessment and subsequent changes matter.

Do not map a historical qualitative braking code or a measured ground-friction coefficient directly onto one universal current RWYCC or aircraft landing-performance correction. Use the current RCR/RCAM, approved aircraft performance information and operator procedures. This lesson establishes the reporting distinction; it does not certify an approach, calculate a landing distance or give permission to use an outdated observation.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.