Meteorological Codes & the Pre-Flight Briefing Service

Part I (ix) Meteorological codes, Pre-Flight briefing service and their usages

Weather decisions are made by crews of every nationality, and the information often crosses a noisy radio or telex link. A free-text English forecast would be slow, ambiguous and easy to garble. So the world adopted a fixed-order, internationally standardised code: every report follows the same sequence of groups, so a pilot in Mumbai or Madrid reads it identically and a single transposed letter is obvious. Master the order of the slots and any report decodes itself.

Reports describe weather that is happening: METAR (routine, observed) and SPECI (special, off-schedule observation).
Forecasts describe weather that will happen: TAF (aerodrome forecast) and the appended TREND (a 2-hour landing forecast).

A METAR is the routine observed weather at an aerodrome, issued at fixed intervals (commonly every 30 or 60 minutes). A SPECI uses the same format but is issued off-schedule when something changes significantly. Both follow the same fixed order.

From the METAR above: lowest broken (operational ceiling) is BKN090 = 9000 ft; visibility 8000 m; light rain; temp/dewpoint 32/26 — a 6 °C spread, so humid but not yet fog.
Why temp/dewpoint matters: when the dewpoint closes on the temperature, the air is near saturation and fog/low cloud becomes likely. A 1–2 °C spread on a cooling evening is a warning sign for the destination.

FEW 1–2 oktas · SCT (scattered) 3–4 · BKN (broken) 5–7 · OVC (overcast) 8. Heights are in hundreds of feet above aerodrome (020 = 2000 ft). NSC = nil significant cloud; SKC/NCD = sky clear / no cloud detected.

When visibility is low, an RVR group appears, e.g. R27/0600 = runway 27, RVR 600 m. Runway Visual Range is the distance over which a pilot on the centreline can see the runway markings or lights — measured by transmissometers, and the governing visibility for low-visibility approaches (full RT detail in Chapter 21).

A TAF is the forecast for an aerodrome over a validity period, written in the same vocabulary as a METAR but with change groups that say "and then it will become…". A TAF begins with its issue time and the validity period (e.g. 2412/2512 = from the 24th 1200Z to the 25th 1200Z).

ATIS — Automatic Terminal Information Service: a continuous broadcast of one aerodrome's weather, runway-in-use and operational information, identified by a letter (Information "Alpha", "Bravo"…), so the frequency isn't tied up reading it to every aircraft.
VOLMET — a scheduled broadcast of en-route meteorological information (METARs/TAFs/SIGMETs) for several aerodromes, for aircraft in flight, on HF and/or VHF.

You listen to ATIS before first contact and read back the letter ("…with Information Charlie") so the controller knows you have the current weather and runway. It saves a congested frequency from reading the same numbers a hundred times an hour — and confirms you and ATC share the same picture.

Used when all of these hold simultaneously: visibility 10 km or more; no cloud below 5000 ft (or below the highest minimum sector altitude, whichever is greater) and no CB or TCU at any level; and no significant weather at or near the aerodrome. When CAVOK applies, the visibility, cloud and weather groups are replaced by the single word.

Before a flight a pilot must gather the weather and the NOTAMs, study the procedures, and file a flight plan. The service that makes this possible is the pre-flight briefing service, run by the Aeronautical Information Service at the aerodrome, increasingly through self-briefing terminals and online portals.

At the AIS/briefing office (or via self-briefing terminals / the online flight-planning portal) a pilot obtains: the relevant METARs and TAFs; SIGMET/AIRMET/GAMET warnings; the route NOTAMs packaged as a PIB (Pre-flight Information Bulletin); charts and AIP data; and the means to file the ATS flight plan.

1. Obtain the weather — METAR/TAF for departure, destination and alternates; SIGMETs en route.
2. Obtain the PIB — NOTAMs affecting the route and aerodromes.
3. Study the AIP/charts for procedures, facilities and frequencies.
4. File the flight plan with ATS.
5. Make the go / no-go decision against the operating minima and fuel.