The runway state group
Winter METARs may carry an eight-figure runway state group, read against its code table digit by digit: two digits of runway designator, one of deposit type, one of extent, two of depth, two of friction or braking action. The designator 88 means all runways, 99 a repeat of the last report. Deposit runs from 0 for clear and dry through 2 wet, 4 dry snow, 5 wet snow, 6 slush, 7 ice to 8 compacted snow and 9 frozen ruts; extent codes 1, 2, 5 and 9 mean up to 10, 11 to 25, 26 to 50 and 51 to 100 per cent; the depth pair is a coded millimetre scale; and the friction pair gives a measured coefficient (28 reads 0.28) or braking action codes 91 to 95 from poor to good, with 99 for unreliable. Two whole-word entries replace the digits when they apply: CLRD for a runway cleared of deposits, and SNOCLO for an aerodrome closed by snow.
TREND and TAF: the forecasts
Two forecasts attach to the aerodrome:
TREND — a landing forecast appended to a METAR or SPECI, valid for 2 hours, written with BECMG for a lasting change, TEMPO for a temporary one, and NOSIG when nothing significant is expected.
TAF — the aerodrome forecast proper: short TAFs of 9 hours issued every 3 hours, long TAFs of 24 or 30 hours issued every 6. Inside it the same change language returns with reinforcements: BECMG and TEMPO with their time pairs, FM for a rapid, complete change of conditions, and PROB30 or PROB40 for an alternative with 30 or 40 per cent probability, the only two probabilities the code allows.
SIGMET and AIRMET
En route, the warnings come from the meteorological watch office. A SIGMET warns all aircraft of observed or expected en-route phenomena:
thunderstorms that are obscured, embedded, frequent or in squall lines, with or without hail;
tropical cyclones;
severe turbulence;
severe icing, including from freezing rain;
severe mountain waves;
heavy dust or sandstorms;
volcanic ash;
radioactive cloud.
The list is close kin to the special air-report list of the satellite lesson, with two additions of its own, tropical cyclones and radioactive cloud, now flowing back to the fleet. A SIGMET's validity is at most 4 hours, extended to 6 for volcanic ash and tropical cyclones.
An AIRMET is its low-level sibling, for flights below FL 100 (FL 150 in mountainous regions), covering hazards that matter to the light and low even when no SIGMET is in force:
moderate icing or turbulence;
widespread strong surface winds;
wide areas of poor visibility;
broken or overcast cloud below 1,000 ft;
scattered or isolated thunderstorms.
GAMET and the low-level package
For low-level flying a whole area forecast exists: the GAMET, an area forecast for low-level flights in abbreviated plain language, in two sections (the first listing the hazards en route, the second the supporting forecast detail), issued for a flight information region or its sub-areas. With AIRMET and the aerodrome messages it forms the briefing package of the light aeroplane, as the SWH and SIGMET form the airliner's.
The advisories
Two hazards earn dedicated advisory messages from dedicated centres, in both written and graphical form:
The volcanic ash advisory — gives the observed extent of an ash cloud and its forecast movement at 6, 12 and 18 hours ahead.
The tropical cyclone advisory — gives the cyclone's centre position, movement, central pressure and maximum surface wind, with forecast positions.
Both feed the SIGMETs that watch offices issue for the affected regions: the advisory is the specialist's forecast, the SIGMET the warning drawn from it.
The broadcasts
In flight, the messages come to the aircraft by voice and datalink:
VOLMET — a continuous broadcast of METAR and SPECI, often with TAF and SIGMET, for a group of aerodromes, on VHF for aircraft within range.
HF-VOLMET — the same service on high frequency across oceanic and remote regions, where one broadcast must serve half an ocean's aerodromes.
ATIS — the aerodrome's own loop: operational and meteorological information from the local report (wind, visibility and RVR, weather, cloud, temperature and dew point, QNH), issued as lettered editions so a crew can confirm which information it holds; by datalink it becomes the D-ATIS.
Briefing and the flight documents
Before flight, the MET office provides briefing and consultation, an explanation of the expected conditions in person, by telephone or through self-briefing systems, and the flight documentation:
the significant weather chart and upper wind and temperature charts of the previous lesson;
METAR and SPECI;
TAF;
SIGMET and AIRMET (GAMET for low level);
volcanic ash and tropical cyclone advisories and aerodrome warnings, as applicable.
Applied along a route, the package answers the planning questions in order: charts for the big picture, TAFs for destination and alternates, SIGMETs for what must be avoided, warnings for what awaits on the ground.
Weather in flight
Airborne, the same information keeps flowing: VOLMET and HF-VOLMET for aerodrome reports, ATIS and D-ATIS for the destination, and the flight information service, which relays SIGMETs and other warnings affecting the route, with updated reports and forecasts available on request. The habit the course has built applies here: a forecast read at breakfast is a hypothesis, and the reports flowing in en route are its test — re-plan when they disagree.
Aerodrome and wind-shear warnings
Two warning families guard the aerodrome itself:
Aerodrome warnings — from the MET office to operators and aerodrome services, for weather that endangers parked aircraft and facilities: gales, squalls, thunderstorms, hail, snow, freezing precipitation and frost.
Wind-shear warnings — these concern the flying: observed or expected shear along the approach and departure paths or on the runway, from reports, forecasts or the shear situations the wind-shear lesson catalogued. Wind-shear alerts are the automatic, sensor-generated version: anemometer arrays around the aerodrome detecting a shear now, runway by runway, typically quoting the expected gain or loss in knots on final.
The offices behind it all
The exam wants the provider named for every product. At the top sit the two world area forecast centres, London and Washington, providing the global upper-air gridded forecasts — the sets behind the previous lesson's charts, now in full:
upper winds, temperature and humidity;
direction, speed and flight level of the maximum wind;
flight level and temperature of the tropopause;
areas of cumulonimbus, icing, clear-air and in-cloud turbulence;
the geopotential altitude of flight levels.
Below them, the division of labour:
MET offices — make the aerodrome forecasts and the briefing documents;
meteorological watch offices — keep watch over their FIR and issue SIGMET and AIRMET;
aeronautical meteorological stations — make the observations behind METAR and the local reports;
volcanic ash advisory centres — forecast ash clouds;
tropical cyclone advisory centres — forecast tropical cyclones.