How to read a SIGMET and an AIRMET

A SIGMET and an AIRMET are short coded warnings about weather that is hazardous in flight, and knowing which is which tells you at a glance how serious the threat is.

This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.

What each one warns of

A SIGMET (significant meteorological information) warns of weather phenomena that are hazardous to all aircraft: severe turbulence, severe icing, thunderstorms, volcanic ash, tropical cyclones, and similar threats. An AIRMET (airmen's meteorological information) covers less severe en-route weather that still matters, especially to lighter aircraft and to flight under visual flight rules: moderate turbulence, moderate icing, widespread areas of reduced visibility or low cloud, and strong surface winds.

Both are international products. The requirement to issue them, and the structure of the message, come from ICAO Annex 3, with the code form set out in the WMO Manual on Codes (WMO No. 306). They are issued by a designated meteorological watch office for a flight information region (FIR), and they cover the airspace, not a single aerodrome. In the United Kingdom, SIGMETs for the UK FIRs are issued by the Met Office acting as the meteorological watch office.

The phenomena codes

A SIGMET names the hazard with a short abbreviation. The common en-route phenomena, drawn from the ICAO Annex 3 template and corroborated by the NOAA/NWS Aviation Weather Center, include:

• Thunderstorms, qualified as OBSC (obscured), EMBD (embedded), FRQ (frequent), or SQL (squall line), with GR added for hail.
• SEV TURB for severe turbulence.
• SEV ICE for severe icing, with (FZRA) if it is due to freezing rain.
• SEV MTW for severe mountain waves.
• HVY DS and HVY SS for heavy dust storms and sandstorms.
• VA for volcanic ash, usually with the volcano's name.
• TC for a tropical cyclone, with its name.
• RDOACT CLD for a radioactive cloud.

An AIRMET uses the same grammar at a lower intensity, for example MOD TURB for moderate turbulence or MOD ICE for moderate icing, plus broad areas of cloud and reduced visibility.

How long they stay valid

A SIGMET is normally valid for up to 4 hours. For the two longest-lead hazards, volcanic ash and tropical cyclones, the validity extends to up to 6 hours, as ICAO Annex 3 provides and the Aviation Weather Center confirms in US practice (where the 6-hour case is described as hurricanes). ICAO Annex 3 caps AIRMET validity at no more than 4 hours; in the United States the FAA issues AIRMETs on a 6-hour schedule, so the 6-hour figure is a US convention rather than the ICAO baseline. Whichever you are reading, treat it as a moving picture: a new issue supersedes the old one, so brief from the latest.

A worked SIGMET

Here is a SIGMET in the standard ICAO format for the London FIR:

EGTT SIGMET 2 VALID 181200/181600 EGRR- EGTT LONDON FIR SEV TURB FCST WI N5130 W00100 - N5200 E00030 - N5100 E00100 - N5030 W00030 - N5130 W00100 FL250/350 MOV E 25KT WKN=

Reading it through:

• EGTT is the location indicator of the flight information region the SIGMET covers, here the London FIR.
• SIGMET 2 is the message type and sequence number: the second SIGMET issued for that FIR in the current series.
• VALID 181200/181600 is the validity period in UTC: from the 18th at 1200 until the 18th at 1600, a 4-hour SIGMET.
• EGRR- identifies the issuing meteorological watch office (the UK Met Office at Exeter).
• EGTT LONDON FIR repeats the affected region in plain form.
• SEV TURB is the phenomenon: severe turbulence.
• FCST says the phenomenon is forecast. The alternative is OBS for observed, sometimes with a time.
• WI introduces the area, bounded by the latitude and longitude points that follow, which close back to the first point to form a polygon.
• FL250/350 is the vertical extent: between flight level 250 and flight level 350.
• MOV E 25KT is the movement: moving east at 25 knots. A system that is not moving reads STNR for stationary.
• WKN is the expected intensity trend: weakening. The alternatives are INTSF (intensifying) and NC (no change).
• = marks the end of the message.

The ICAO baseline and the US additions

The ICAO SIGMET and AIRMET are the international baseline, but the United States layers extra products on top, and you should not assume they exist elsewhere. Per the FAA Aeronautical Information Manual, the FAA issues domestic AIRMETs in three named types: Sierra for instrument conditions and mountain obscuration, Tango for turbulence and strong surface winds, and Zulu for icing and freezing levels. It also publishes a G-AIRMET, the same hazards drawn as graphical snapshots at fixed times rather than as one continuous-validity bulletin, as the Aviation Weather Center describes.

The most important US-only product is the Convective SIGMET, issued for thunderstorm-related hazards (lines of thunderstorms, embedded storms, large areas of storms, and severe weather). It is a distinct American product with its own short validity, so do not read it as an ICAO SIGMET or expect it abroad. Outside the United States, thunderstorm hazards are carried in the ordinary SIGMET using the TS codes above.

Common pitfalls

• A SIGMET is an area warning, not an aerodrome report. Pair it with the METAR and TAF for your aerodromes and with the area forecast for the route.
• Mind the validity window. A SIGMET that expires before your arrival tells you nothing about your arrival; look for the reissue.
• The polygon and flight levels both matter. A severe-turbulence SIGMET at FL250/350 does not constrain a piston aircraft at 8000 ft, but a low-level icing AIRMET very much does.
• Do not assume the US products exist everywhere. Convective SIGMETs and the Sierra/Tango/Zulu naming are American.

In Pilot EFB

Pilot EFB pulls the weather for your route and shows the decoded report alongside the raw text, with the raw warning always kept and never replaced, so you can trace a plain-language summary back to the original coded groups. A briefing you have already pulled stays readable with no signal, because your device holds what you have saved; fetching a fresh SIGMET or AIRMET needs a connection. Pilot EFB is offline-first and is not a certified Electronic Flight Bag, so brief the hazardous-weather picture from your official meteorological source of record.