SIGMET & AIRMET Reference
SIGMETs and AIRMETs are urgent weather advisories issued to alert pilots to hazardous conditions en route. Understanding how to read and apply these advisories is essential for safe flight planning.
SIGMET — SIGnificant METeorological Information
A SIGMET is an ICAO-defined weather advisory warning of meteorological conditions along routes that are hazardous to all aircraft — not just light aircraft. SIGMETs are issued by Meteorological Watch Offices (MWOs) responsible for each Flight Information Region (FIR) or Upper Information Region (UIR).
SIGMETs are among the highest-priority weather products available to pilots. They supplement NOTAMs and METARs and should be checked as part of every pre-flight weather briefing for any route longer than local circuits.
SIGMET = Hazardous to ALL aircraft. Unlike AIRMETs (which target light aircraft), SIGMETs describe conditions severe enough to affect jet transports, turboprops, and all other aircraft. Encountering a SIGMET-level phenomenon without appropriate equipment or authority is potentially catastrophic.
SIGMET Phenomena — ICAO
ICAO Annex 3 defines the weather phenomena that warrant SIGMET issuance. The following codes appear in SIGMET text:
Embedded (EMBD), frequent (FRQ), or squall line (SQL) thunderstorms. TSGR indicates hail-producing storms.
Hurricane, typhoon, or tropical storm. Includes name, current centre position, and movement.
Turbulence of severe or extreme intensity not associated with thunderstorms. Includes non-convective CAT.
Most severe turbulence category — aircraft may be practically uncontrollable. Structural damage risk.
Severe structural icing not associated with thunderstorms. De-icing systems unable to cope.
Severe or extreme turbulence associated with mountain wave activity. Includes rotor zone.
Dense dust or sand storms causing severe visibility reduction and potential engine/airframe damage.
Volcanic ash cloud hazardous to aircraft. Includes source volcano, ash cloud height, top, and movement. Critical for all turbojet/turbofan operations.
Radioactive material cloud affecting en-route airspace. Issued following nuclear events.
SIGMET Format (ICAO) and Decoded Example
The standard ICAO SIGMET format is used internationally. Key fields include: issuing station, advisory number, valid time window, issuing FIR, phenomenon, observed or forecast time, affected area, altitude band, movement, and trend.
Plain English decode: "Shanwick Oceanic FIR — Severe Turbulence observed at 11:30 UTC within a polygon at approximately 55–58°N 15–30°W, between FL270 and FL390, moving northeast at 15 knots and weakening. Advisory valid until 16:00 UTC."
Common trend qualifiers:
WKN— Weakening (intensity decreasing)INTSF— IntensifyingSTNR— StationaryNC— No change expected
US SIGMET Types
The United States uses specific designators for different SIGMET and AIRMET products, issued by the Aviation Weather Center (AWC):
Non-Convective SIGMET
Severe turbulence not associated with thunderstorms, severe icing, low-level wind shear. Equivalent to ICAO SIGMET for non-convective phenomena.
AIRMET
Below SIGMET threshold. Moderate icing, moderate turbulence, low visibility — significant mainly for light aircraft. See AIRMET section below.
Convective SIGMET
Severe or embedded thunderstorms, lines of thunderstorms >60 nm, severe/extreme turbulence within a convective system, or any tornado. Issued hourly + special.
Volcanic Ash SIGMET
Volcanic ash cloud. Coordinates, altitude, source, and movement. Critical — ash causes flameout in all jet engines. Avoid by wide margin.
Convective SIGMET (WC): In the US, a Convective SIGMET is automatically issued for any tornado, embedded thunderstorm, line of storms exceeding 60 nm in length, area of storms affecting >3,000 sq mi with at least moderate turbulence/icing/low-level wind shear, or hail >3/4 inch. Valid for 2 hours; issued at H+55 each hour plus as needed.
AIRMET — AIRmen's METeorological Information
An AIRMET describes weather conditions that are below the SIGMET threshold but significant to light aircraft, student pilots, and single-engine operations. AIRMETs are issued pre-flight and updated regularly; pilots should check AIRMETs for the route even if no SIGMETs are current.
ICAO AIRMETs cover the same basic phenomena as SIGMETs but at lower intensity thresholds — moderate turbulence, moderate icing, mountain wave, and reduced visibility. Internationally, AIRMETs are issued for conditions that affect flight safety but fall below SIGMET-level severity.
US AIRMET Categories
AIRMET S — IFR Conditions & Mountain Obscuration
Ceiling below 1,000 ft AGL and/or visibility below 3 statute miles affecting more than 50% of the area. Also covers terrain obscured by cloud, precipitation, or fog.
AIRMET T — Turbulence
Moderate turbulence, sustained surface winds of 30 kt or greater, or non-convective low-level wind shear (below 2,000 ft AGL). Does not include severe turbulence (SIGMET level).
AIRMET Z — Icing & Freezing Level
Moderate icing (not associated with thunderstorms) and freezing level height information. Does not cover severe icing (SIGMET level).
AIRMET Example Decoded
Validity Periods and Issuance
| Product | Validity | Issuance | Notes |
|---|---|---|---|
| SIGMET (standard) | Up to 4 hours | As required | Issued when phenomenon observed or forecast |
| SIGMET (VA / TC) | Up to 6 hours | As required | Extended validity for volcanic ash and tropical cyclone tracks |
| Convective SIGMET (US) | 2 hours | H+55 each hour + special | May be updated more frequently in active convective periods |
| AIRMET (US) | 6 hours | 6 times daily (00, 06, 12, 18, 00, 06Z cycles) | Overnight AIRMETs may be valid up to 12 hours |
| AIRMET (ICAO international) | Up to 4 hours | As required | Same format as SIGMET but for lower-threshold phenomena |
SIGMET supersedes AIRMET: If a SIGMET is issued for the same area and phenomenon as a current AIRMET, the SIGMET takes precedence. The AIRMET may be cancelled or allowed to expire. Always check for the most recent advisory — a cancelled SIGMET does not mean conditions have improved unless an explicit cancellation or replacement is issued.
Pilot Actions
Pre-Flight Briefing
- Check all SIGMETs and AIRMETs for the entire route, destination, and alternate — not just the departure area
- Check validity times — an AIRMET issued several hours ago may be near expiry; look for the most recent products
- Cross-reference with METARs, TAFs, and PIREPs for the same area to assess actual conditions versus forecast
- For international flights, check both ICAO SIGMETs for each FIR and any domestic supplements
- Volcanic ash SIGMETs — treat as absolute no-go for all aircraft. Any ash cloud penetration risks engine flameout and severe structural damage
En-Route
- Request SIGMET and AIRMET status from ATC when operating en route, especially when deviating from filed route
- PIREPs from preceding aircraft supplement official advisories and provide real-time conditions
- Report any deviation for weather to ATC — assists in issuing updated SIGMETs and providing separation to following aircraft
In-Flight SIGMET Encounter
- Thunderstorms (TS/Cb): Deviate around. Minimum 20 nm lateral clearance from storm cell. Never penetrate.
- Severe turbulence: Slow to Va, maintain attitude, advise ATC, request level change
- Severe icing: Exit conditions immediately — reduce altitude, turn back, activate all available de-ice systems. Declare emergency if accumulation is severe
- Volcanic ash: If inadvertently encountered — retard throttles immediately, turn 180°, descend to below ash cloud if terrain permits, declare MAYDAY, do not delay
Reporting Duty
File a PIREP when you encounter phenomena that warrant a SIGMET or AIRMET. In many jurisdictions this is a duty under flight regulations. Accurate, timely PIREPs from other pilots are often the only real-time confirmation of forecast hazardous weather, and are directly used to issue, update, or cancel SIGMETs. Conversely, negative PIREPs (NEG TURB, NEG ICE) in areas with active advisories are equally valuable to ATC and forecasters.