What is the difference between a SIGMET and an AIRMET?

A SIGMET warns of weather phenomena considered hazardous to all aircraft regardless of type, such as severe icing, severe or extreme turbulence, and convective activity meeting specific criteria. An AIRMET covers conditions that are less extreme — such as moderate icing or moderate turbulence — but still potentially hazardous especially to lighter aircraft. The key dividing line is severity: severe or extreme hazards generate SIGMETs, while moderate hazards typically generate AIRMETs, as described in the Aviation Weather handbook (FAA-H-8083-28) and the AIM.

How long is a convective SIGMET valid and when is it issued?

A convective SIGMET (WST) is valid for 2 hours from the time of issuance. The Aviation Weather Center issues scheduled convective SIGMETs at 55 minutes past each hour, and unscheduled special convective SIGMETs are issued at any time when new significant convective hazards develop between those hourly updates. Pilots and dispatchers should never assume the most recent hourly product is current if a significant convective development has occurred since issuance.

Does an active SIGMET legally prohibit a flight under Part 91?

There is no specific regulation in 14 CFR Part 91 that automatically prohibits flight solely because an active SIGMET has been issued for a route. However, under 14 CFR §91.3, the pilot in command is ultimately responsible for the safe operation of the flight, and knowingly penetrating a hazard area described in an active SIGMET without adequate justification or equipment could constitute careless or reckless operation under 14 CFR §91.13. In Part 121 operations, the dispatcher and PIC share responsibility for the flight release, and an active SIGMET along the route is a direct factor in release legality and safety.

SIGMET and AIRMET Decoding for Dispatch and In-Flight Decision Making

Every preflight weather analysis and every in-flight decision loop for ATP-level operations centers on the same question: what are the current and forecast hazards along my route, and how severe are they? SIGMETs and AIRMETs are the FAA's primary in-flight advisory products designed to answer exactly that question. Knowing how to decode them quickly, cross-reference them with other data, and act on them operationally is an ATP Airman Certification Standards core competency — and a genuine life-safety skill.

The Two-Tier Advisory System

The FAA structures in-flight weather advisories in two tiers based on severity. A SIGMET (Significant Meteorological Information) represents the higher tier and warns of conditions considered hazardous to all aircraft regardless of type, equipment, or pilot certification. An AIRMET (Airmen's Meteorological Information) represents the lower tier, covering conditions that may be less extreme but remain potentially hazardous — especially to lighter aircraft, single-engine operations, and pilots with limited instrument experience. Understanding which tier a hazard falls into determines the urgency of your response and informs dispatch release decisions under Part 121 and Part 135.

SIGMETs: Non-Convective and Convective

Non-Convective SIGMETs (WS)

Non-convective SIGMETs address weather phenomena unrelated to active thunderstorm cells. The Aviation Weather Center (AWC) issues WS products for: severe or extreme turbulence not associated with thunderstorms, severe icing not associated with thunderstorms, and widespread dust storms or sandstorms that lower visibility below 3 statute miles. Additional criteria include volcanic ash clouds and tropical cyclones. A standard non-convective SIGMET is valid for up to 4 hours. When the hazard involves volcanic ash or a tropical cyclone, validity extends to 6 hours because those phenomena evolve more slowly and require longer planning horizons.

Convective SIGMETs (WST)

Convective SIGMETs are the highest-urgency weather advisories in the domestic U.S. system. The AWC issues them automatically — without any pilot request — whenever one or more of the following criteria are met: a line of thunderstorms at least 60 nautical miles long with cells well-developed along at least 60% of the line; an area of thunderstorms covering at least 3,000 square miles with thunderstorms affecting at least 40% of that area; or a severe or embedded thunderstorm (regardless of area coverage). Convective SIGMETs are issued at 55 minutes past each hour on a scheduled basis, plus special (unscheduled) issues whenever an urgent new development occurs between hourly updates. Each convective SIGMET is valid for 2 hours from issuance, or until superseded by a new product.

A critical operational note: convective SIGMETs implicitly cover severe turbulence, severe icing, and low-level wind shear within the convective area as a matter of meteorological physics, even though those hazards are not always restated in the text. Planning a route through an active WST area while assuming only one hazard exists is a dangerous oversimplification.

AIRMETs: Sierra, Tango, and Zulu

AIRMETs are issued by the AWC every 6 hours on a scheduled basis, with special (unscheduled) AIRMETs issued between cycles when conditions deteriorate unexpectedly. Each AIRMET is valid for 6 hours. Sequential letter suffixes (e.g., TANGO 3) allow you to track whether you are reading the most current version of a product.

AIRMET Sierra (IFR / Mountain Obscuration): Issued when instrument meteorological conditions (IMC) are forecast over an area of at least 3,000 square miles — specifically ceilings below 1,000 feet AGL and/or surface visibility below 3 statute miles. Sierra also covers mountain obscuration where terrain becomes obscured by clouds, precipitation, smoke, or haze even if ceiling and visibility criteria are not individually met.
AIRMET Tango (Turbulence): Covers moderate turbulence (not severe or extreme, which rises to SIGMET level), sustained surface winds of 30 knots or more, and low-level wind shear at or below 2,000 feet AGL. Tango is particularly relevant to cargo operators flying lighter turboprops and to any operation concerned with structural fatigue accumulation on repeated moderate-turbulence flights.
AIRMET Zulu (Icing): Addresses moderate icing (not severe, which is SIGMET territory) and provides freezing level information. Zulu products often include multiple freezing level altitudes for different geographic sub-areas, which is essential for calculating icing exposure during climbs and descents.

Decoding the Text Product

Whether you receive an AIRMET or SIGMET as a text string from a dispatcher, a flight service briefing, or a cockpit datalink product, the structure is consistent. Every product contains: a product identifier and sequence number (e.g., CONVECTIVE SIGMET 14C); the valid period in UTC (Zulu) expressed as a four-digit time group; the geographic boundary defined either by named VOR fixes and compass-bearing/distance references or by latitude/longitude coordinates; the affected altitude range in feet MSL or flight levels (e.g., FL180–FL350); and a plain-language description of the hazard with movement direction and speed when determinable. Many products also include an intensity trend: INTSF (intensifying), WKNG (weakening), or STNR (stationary).

When working with the geographic boundary, practice mentally plotting the fix chain on an en-route chart or moving map. A text boundary that looks abstract becomes immediately actionable when you can visualize whether your route of flight penetrates the defined area or merely parallels it outside the boundary.

Cross-Referencing for Dispatch and In-Flight Use

No single product tells the complete story. A convective SIGMET defines the currently observed hazard area, but it is a nowcast product — it reflects conditions at issuance. The Graphical Forecasts for Aviation (GFA) and Area Forecasts provide the temporal dimension, showing how the hazard is expected to evolve over the next several hours. PIREPs (Pilot Reports) add the dimension of direct observation — a PIREP of moderate icing in airspace covered only by an AIRMET Zulu does not upgrade it to SIGMET level automatically, but it confirms the hazard is being encountered and warrants immediate operational attention. Winds aloft forecasts (FB winds) help you assess turbulence potential at altitude and plan altitude changes to remain above or below documented icing layers.

In Part 121 operations, the dispatcher and PIC share legal responsibility for flight release under 14 CFR Part 121. An active convective SIGMET on the filed route at the estimated time of arrival at that segment is not a mere advisory to acknowledge — it is a direct input into whether the release is legal and safe. Dispatchers must document their weather analysis and coordinate rerouting or departure delays when SIGMETs are active along the route. In Part 91 and Part 135 operations, 14 CFR §91.3 places the ultimate go/no-go authority with the PIC, making real-time in-flight awareness of new or updated advisories non-negotiable.

Key Numbers and Rules

Convective SIGMET criteria: line of thunderstorms ≥ 60 NM long; area ≥ 3,000 sq mi with ≥ 40% coverage; or any severe/embedded thunderstorm.
Convective SIGMET valid time: 2 hours; issued at :55 past the hour plus specials.
Non-convective SIGMET valid time: 4 hours (6 hours for volcanic ash or tropical cyclone).
AIRMET valid time: 6 hours; issued every 6 hours with special updates as needed.
AIRMET Sierra IFR criteria: ceiling < 1,000 ft AGL and/or visibility < 3 SM over ≥ 3,000 sq mi.
AIRMET Tango surface wind criterion: sustained winds ≥ 30 knots.
Severe icing or severe/extreme turbulence → SIGMET; moderate icing or moderate turbulence → AIRMET.

Common Test Traps

Convective SIGMETs are issued automatically — they are unilateral AWC products broadcast without any pilot or operator request. Assuming you must ask for them mirrors a dangerous real-world misconception.
Severity thresholds cross between products: Moderate icing = AIRMET Zulu; severe icing = non-convective SIGMET. Confusing these on a knowledge test reflects a gap in operational understanding.
All times are Zulu (UTC). Converting a valid period to local time is a common arithmetic error. Train yourself to work in UTC exclusively when reading weather products.
AIRMET Sierra is NOT just about low ceilings — mountain obscuration qualifies independently. An area with adequate ceiling and visibility that obscures mountainous terrain can still generate a Sierra.
A convective SIGMET covers implied hazards beyond thunderstorms. Severe turbulence, severe icing, and wind shear are physically embedded in the convective environment even when not explicitly listed in the text product.
Special AIRMETs are real AIRMETs. They carry the same operational weight as scheduled products and must be checked between standard 6-hour issuance cycles, especially during rapidly evolving weather situations.

Memory Aid

To keep the three AIRMETs distinct: Sierra = Sight (IFR and mountain obscuration), Tango = Turbulence (and strong surface winds), Zulu = Zero tolerance for ice (moderate icing and freezing levels). For severity tiers: if the hazard threatens all aircraft, it belongs in a SIGMET; if it primarily threatens lighter or less-equipped aircraft, it may qualify only for an AIRMET.