12.airmets-sigmets. AIRMETs and SIGMETs
AIRMETs (Airmen's Meteorological Information) and SIGMETs (Significant Meteorological Information) are in-flight advisories issued by the Aviation Weather Center (AWC) to alert pilots to weather phenomena that may affect the safety of flight. They are inflight supplements to the Area Forecast (FA) and are amended as conditions warrant.
AIRMETs
AIRMETs concern weather of less severity than that covered by SIGMETs. They are advisories of significant weather that may be hazardous to aircraft having limited capability because of lack of equipment, instrumentation, or pilot qualifications — particularly light, single-engine aircraft and VFR pilots. However, the weather described in AIRMETs is of operational interest to all aircraft.
AIRMETs are issued every 6 hours, with unscheduled updates and corrections issued as necessary. A scheduled AIRMET Bulletin contains any combination of three AIRMETs, each identified by an alphanumeric designator:
- AIRMET Sierra (S) — IFR conditions and/or mountain obscuration. IFR criteria are ceilings less than 1,000 feet and/or visibility less than 3 statute miles affecting over 50% of the area at one time.
- AIRMET Tango (T) — Turbulence, sustained surface winds of 30 knots or greater, and/or non-convective low-level wind shear.
- AIRMET Zulu (Z) — Icing (moderate airframe icing) and freezing levels.
AIRMETs are valid for 6 hours and use plain-language abbreviations identifying the affected area by VOR/navaid bounds.
SIGMETs
SIGMETs advise of weather, other than convective activity, that is potentially hazardous to all aircraft. They are unscheduled and issued as needed. A SIGMET is valid for 4 hours, but when the SIGMET relates to hurricanes it is valid for 6 hours. SIGMETs are issued for the following non-convective phenomena when they are expected to occur or are occurring over an area of at least 3,000 square miles:
- Severe or extreme turbulence not associated with thunderstorms
- Severe icing not associated with thunderstorms
- Widespread dust storms, sandstorms, or volcanic ash lowering surface and/or in-flight visibility to less than 3 statute miles
- Volcanic eruption
In the conterminous United States, SIGMETs are identified by an alphabetic designator from November through Yankee (excluding Sierra and Tango, which are reserved for AIRMETs). The first issuance of a SIGMET is labeled as UWS (Urgent Weather SIGMET).
Convective SIGMETs
Convective SIGMETs (WST) are issued for hazardous convective weather that affects the safety of every flight. They are issued for the eastern (E), central (C), and western (W) United States. Issuance criteria include:
- Severe thunderstorms with surface winds 50 knots or greater, hail at the surface ≥ 3/4 inch in diameter, and/or tornadoes
- Embedded thunderstorms
- Lines of thunderstorms
- Thunderstorms producing precipitation greater than or equal to heavy intensity affecting 40% or more of an area at least 3,000 square miles
Convective SIGMETs are issued hourly at H+55, are valid for 2 hours, and automatically include implications of severe or greater turbulence, severe icing, and low-level wind shear. A Convective SIGMET may be issued for any convective situation that the forecaster feels is hazardous.
Reading the Advisories
A typical AIRMET begins with a header identifying the issuing center, type, valid time, and affected area, followed by a plain-language description. For example:
DFWT WA 121345 AIRMET TANGO UPDT 3 FOR TURB...VALID UNTIL 121800
This indicates an updated AIRMET Tango issued by the Dallas/Fort Worth AWC office on the 12th at 1345Z, valid until 1800Z. The body identifies the geographic area (by VOR boundaries), affected altitudes, and intensity.
Operational Use
Under 14 CFR 91.103, the pilot in command must become familiar with all available information concerning the flight, which includes weather reports and forecasts. AIRMETs and SIGMETs should be reviewed during preflight planning and monitored in flight via Flight Service (1-800-WX-BRIEF), HIWAS where available, ATC, EFBs, and ADS-B FIS-B datalink.
A student or VFR-only pilot should regard an active AIRMET Sierra along the route as a strong indicator that VFR flight may not be possible and a divert/cancel decision is appropriate. An AIRMET Zulu indicates known icing — VFR aircraft without anti-/de-ice equipment must avoid the affected altitudes when visible moisture is present and temperatures are at or below freezing. A SIGMET or Convective SIGMET requires immediate consideration regardless of aircraft capability; deviation, delay, or cancellation is the prudent response.