SIGMET
Definition
A SIGMET is a weather advisory issued by meteorological authorities to warn pilots and air traffic services of significant and potentially hazardous weather phenomena that could affect the safety of aircraft in flight. Unlike METARs and TAFs, which are routine reports and forecasts, SIGMETs are event-driven and concern dangerous, often fast-developing conditions.
Purpose
- Warn of severe weather that poses a direct threat to flight safety
- Support ATC rerouting and flow management
- Help pilots avoid hazardous areas in preflight planning or enroute
When a SIGMET Is Issued
| Weather Phenomenon | Description |
|---|---|
| Thunderstorms | Widespread or embedded TS, squall lines, or TS with hail |
| Severe Turbulence | Beyond moderate levels; includes mountain wave activity |
| Severe Icing | Accumulation beyond aircraft systems’ ability to manage |
| Volcanic Ash (VA) | Ash clouds from eruptions; highly dangerous to jet engines |
| Tropical Cyclones | Hurricanes, typhoons, or named storms |
| Dust Storms / Sandstorms | Causing visibility < 1,000 m |
| Radioactive Clouds | From nuclear incidents (rare) |
SIGMETs are issued for specific Flight Information Regions (FIRs) and are time-limited (usually valid for 4 hours).
Format and Structure
SIGMETs follow ICAO standardized format and typically include:
- Header and issuing authority
- Time of validity (UTC)
- Affected FIR or airspace
- Weather phenomenon (type, intensity, movement)
- Vertical extent (altitudes affected)
- Geographical area (coordinates or reference points)
Example SIGMET (decoded):
EHAA SIGMET 3 VALID 081000/081400 EHDB–
EHAA AMSTERDAM FIR
SEV TURB OBS AT 1000Z BTN FL100 AND FL200 MOV E 30KT
WKN INTSF.
Translation:
- Amsterdam FIR
- Severe turbulence observed at 10:00 UTC between FL100 and FL200
- moving east at 30 knots
- expected to weaken in intensity
SIGMET vs. AIRMET
| Feature | SIGMET | AIRMET |
|---|---|---|
| Severity | Severe or hazardous | Moderate |
| Frequency | Irregular, event-driven | More routine |
| Impact | Significant risk to aircraft | Operationally relevant, but less critical |
| Audience | All aircraft | Light aircraft especially |
Where to Access SIGMETs
Phraseology Examples
- “SIGMET in effect for Vienna FIR: severe icing between FL080 and FL160, embedded thunderstorms, avoid area west of Salzburg”
- “Radar vectors to avoid SIGMET area”
- “Request deviation left of track due to SIGMET turbulence forecast”