Every aircraft dispatcher shares legal responsibility for the safe conduct of each flight. That responsibility becomes most tangible when hazardous weather products — SIGMETs, Convective SIGMETs, and AIRMETs — appear on the preflight weather picture. These products are not merely advisories to note and file away; they carry operational and, in some cases, regulatory weight that can prevent a legal release, require an amended flight plan mid-route, or trigger a fuel-stop diversion. Understanding precisely what each product covers, how it is generated, how long it is valid, and what it demands of a dispatcher is central to both the written examination and day-to-day dispatch practice.
The authoritative framework for these products is the FAA Aviation Weather Handbook, FAA-H-8083-28B, which consolidates the meteorological definitions, issuance criteria, and communication formats that dispatchers must internalize. The discussion that follows draws directly from that source, expanding each concept into the practical dispatch context.
SIGMETs: Significant Meteorological Information
A SIGMET — Significant Meteorological Information — is issued by the Aviation Weather Center (AWC) to warn pilots and dispatchers of non-convective weather phenomena that are potentially hazardous to all aircraft. The key phrase is all aircraft: SIGMETs are not limited by aircraft category or certification level. The phenomena that trigger a SIGMET issuance include severe or extreme turbulence not associated with thunderstorms, severe icing not associated with thunderstorms, widespread dust storms or sandstorms lowering visibility below three miles, and volcanic ash. Each of these conditions is considered severe enough to threaten structural integrity, controllability, or powerplant health across the entire fleet.
SIGMETs are valid for up to four hours in the contiguous United States (CONUS) and up to six hours for conditions associated with hurricanes or volcanic ash, because those phenomena are slower-moving and harder to bound precisely in time. Each SIGMET is assigned an alphabetic identifier — using the series November through Yankee, excluding Sierra and Tango, which are reserved for AIRMETs. This naming convention allows controllers, dispatchers, and pilots to cross-reference the same advisory without confusion. SIGMETs are reissued as conditions persist, so a dispatcher monitoring a volcanic ash cloud must track successive issuances and verify that the geographic boundaries have not expanded to affect planned routes.
Convective SIGMETs: The Thunderstorm Benchmark
Convective SIGMETs (WSTs) are a separate product entirely, and their practical impact on dispatch operations is arguably greater than any other single weather advisory. They are issued by the AWC's Aviation Weather Center for areas of significant convective activity — specifically, severe thunderstorms with surface winds of 50 knots or greater, hail at the surface of three-quarters of an inch diameter or greater, or tornadoes. They are also issued for embedded thunderstorms, lines of thunderstorms at least 60 nautical miles long with cells affecting at least 40 percent of the line's length, and areas of thunderstorms covering at least 3,000 square miles with a significant amount of the area being affected by echoes of VIP (Video Integrator and Processor) level 4 or higher.
Convective SIGMETs are issued on a scheduled basis at 55 minutes past each hour, with special Convective SIGMETs issued at any time when conditions warrant. Each issuance is valid for up to two hours, making them a rapidly refreshing product that a dispatcher must check continuously throughout a long-haul operation. They cover the CONUS only; similar hazards in other regions fall under standard SIGMET criteria.
The dispatch significance of Convective SIGMETs cannot be overstated. Under 14 CFR Part 121, no dispatcher may release a flight into known severe icing or known severe turbulence. Since Convective SIGMETs frequently capture both phenomena embedded within convective complexes, their active issuance directly limits the routing options a dispatcher can legally approve. When a Convective SIGMET covers a significant portion of an intended route, dispatchers must evaluate alternate corridors, request altitude deviations (which may themselves be limited by MEA or RVSM constraints), or delay the release until the convective system has dissipated and the advisory has expired.
AIRMETs: Airmen's Meteorological Information
AIRMETs — Airmen's Meteorological Information — occupy the tier below SIGMETs on the hazard scale, but dispatchers must resist treating them as background noise. AIRMETs describe conditions that are operationally significant, particularly for lighter aircraft, but may also affect transport-category operations when they extend over large areas or stack with other weather factors. There are three AIRMET types, each with a standardized identifier:
- AIRMET Sierra (AIRMET S): IFR conditions — ceilings below 1,000 feet and/or visibility below three miles affecting at least 3,000 square miles — and mountain obscuration.
- AIRMET Tango (AIRMET T): Moderate turbulence, sustained surface winds of 30 knots or greater at the surface, and low-level wind shear.
- AIRMET Zulu (AIRMET Z): Moderate icing and the freezing level.
AIRMETs are issued and valid for six hours, including AIRMET Sierra for IFR conditions and mountain obscuration. Scheduled bulletins are issued by the AWC at 0245, 0845, 1445, and 2045 UTC, with updates and amendments issued as needed. For dispatchers operating Part 121 flights into mountainous terrain or regions prone to widespread IFR conditions, AIRMET Sierra demands close scrutiny: if an alternate is required under 14 CFR Part 121.619 and the alternate itself is forecast within an AIRMET Sierra area, the dispatcher must reassess whether the alternate meets the standard-alternate or nonstandard-alternate weather minimums.
Why These Products Matter to Dispatch Operations
The legal framework of Part 121 places the dispatcher and the captain in joint command of the flight's safety. The dispatcher's release — the Dispatch Release or Lockheed Release — must reflect all known hazardous conditions at the time of departure. If a SIGMET or Convective SIGMET is active along the route of flight and the dispatcher signs a release without accounting for it, that action may constitute a regulatory violation and, more importantly, places the crew in danger without an approved deviation plan.
Practically, dispatchers integrate these products through the following workflow during preflight: First, identify all active SIGMETs, Convective SIGMETs, and AIRMETs overlapping the proposed route, including a lateral buffer of at least 20 nautical miles on each side of centerline as a general planning margin (the actual avoidance distance is operationally determined by company Operations Specifications and the captain's judgment). Second, evaluate whether the covered phenomena meet the threshold of known severe icing or known severe turbulence that prohibits release under Part 121. Third, coordinate alternate routing or altitude with the captain, annotating the Dispatch Release accordingly. Fourth, during the flight, continuously monitor for new issuances and amend the release or issue an amended flight plan when conditions deteriorate.
The concept of in-flight amendment authority is equally important. A dispatcher monitoring a developing convective system may need to issue a new clearance, reroute the flight around an expanding Convective SIGMET, or coordinate an unscheduled fuel stop to ensure adequate reserves if a longer deviation route is required. This real-time function distinguishes the aircraft dispatcher from a simple preflight planner and underscores why current weather literacy is a core dispatcher competency.
Key Numbers and Rules
- SIGMET validity: Up to 4 hours (CONUS); up to 6 hours for hurricanes or volcanic ash.
- Convective SIGMET validity: Up to 2 hours; scheduled issuance at H+55 each hour.
- AIRMET validity: 6 hours, including AIRMET Sierra for IFR and mountain obscuration; scheduled issuance at 0245, 0845, 1445, and 2045 UTC.
- Convective SIGMET triggers: Severe thunderstorms (50 kt surface winds, ¾-inch hail, tornadoes), embedded thunderstorms, lines ≥60 NM with ≥40% coverage, areas ≥3,000 sq mi at VIP level 4+.
- AIRMET Sierra IFR threshold: Ceilings below 1,000 ft and/or visibility below 3 SM over at least 3,000 square miles.
- AIRMET identifiers: Sierra (IFR/mtn obscuration), Tango (turbulence/wind shear), Zulu (icing/freezing level).
- SIGMET series: November through Yankee, excluding Sierra and Tango (reserved for AIRMETs).
Common Test Traps
- Confusing validity periods: Students frequently assign 6-hour validity to Convective SIGMETs (which is actually the SIGMET/AIRMET window). Convective SIGMETs are valid for only 2 hours.
- Assuming AIRMETs are irrelevant to Part 121: AIRMETs describe conditions — widespread IFR, moderate icing, moderate turbulence — that directly affect alternate planning, release legality, and passenger safety for large transport-category operations.
- Forgetting scheduled Convective SIGMET timing: The scheduled issuance at H+55 (55 minutes past each hour) is a commonly tested fact. Special Convective SIGMETs may be issued at any time and must be monitored continuously.
- Mixing up SIGMET alphabetic series: Sierra and Tango are NOT used for SIGMETs — they are reserved for AIRMET Sierra and AIRMET Tango. SIGMETs use November through Yankee.
- Overlooking the dispatcher's in-flight amendment obligation: The written exam often presents scenarios where conditions change after departure; dispatchers must recognize their continuing legal duty to amend the release or reroute, not simply log the new SIGMET and move on.