What they are
An AIRMET (AIRmen's METeorological Information) and a SIGMET (SIGnificant METeorological Information) are both in-flight weather advisories. The Aviation Weather Center issues them to warn pilots about conditions that could affect flight safety — turbulence, icing, low ceilings, mountain obscuration, thunderstorms, volcanic ash, dust storms.
The fastest way to keep them straight: AIRMET = "hazardous to small/light aircraft." SIGMET = "hazardous to all aircraft." Same idea, two severity levels.
Why pilots care
These are the FAA's official "we're telling you, in writing, that this hazard exists." If you take off into known icing without checking AIRMET Zulu, and something goes wrong, you'll have a hard time arguing you didn't have the information. Both products are part of a complete weather briefing under FAR 91.103 — preflight action.
Practically: AIRMETs and SIGMETs are how you find out about hazards that don't show up cleanly on a METAR or TAF. A surface report at your departure airport tells you it's clear and a million. It doesn't tell you there's moderate rime icing forecast along your route at 8,000 ft.
Checkride context: AIRMETs and SIGMETs fall under K2g of the weather task — that's PA.I.C.K2g for Private, IR.I.B.K2g for Instrument, and CA.I.C.K2g for Commercial. The K2 sub-elements are essentially identical across all three ratings. If your DPE picks K2, they'll assess at least three of the seven products listed (K2a–K2g), so know this one alongside METARs, TAFs, the GFA, surface analysis charts, winds aloft, and the convective outlook.
The three AIRMETs
AIRMETs come in three flavors. Each one covers a specific type of hazard. The names are phonetic alphabet letters, which is mostly memorable once you know the trick:
IFR & Mountain Obscuration
"Sight" — anything that limits how far you can see.
- Ceilings below 1,000 ft AGL
- Visibility below 3 SM
- Mountains obscured by clouds, precip, or haze
Turbulence & Strong Surface Winds
"Turbulence" — the easy one to remember.
- Moderate turbulence
- Sustained surface winds 30 kt or greater
- Non-convective low-level wind shear
Icing & Freezing Levels
"Zero degrees" — think freezing.
- Moderate icing
- Forecast freezing levels
Heads up: AIRMETs only describe moderate conditions. If turbulence or icing is forecast as severe, that's a SIGMET. Same hazard, more severe — different product.
AIRMET vs SIGMET — the key distinction
This is the one most students get wrong on the oral. Read the table top-to-bottom, not column-by-column:
AIRMET
- Severity
- Moderate
- Affects
- Light aircraft, often pilots without certain ratings or equipment
- Issued every
- 6 hours, with amendments as needed
- Valid for
- 6 hours
- Examples
- Moderate icing, ceiling 800 ft, moderate turbulence
SIGMET
- Severity
- Severe — hazardous to all aircraft
- Affects
- Everyone, including airliners
- Issued
- As needed, when conditions warrant
- Valid for
- Up to 4 hours (longer for volcanic ash and tropical cyclones)
- Examples
- Severe icing, severe turbulence, volcanic ash, dust storms reducing vis below 3 SM, tropical cyclones
One way I explain it to my tutoring students: an AIRMET is the equivalent of a winter weather advisory on a regular forecast. A SIGMET is the equivalent of a winter storm warning. Same hazard family, but the warning version means it's serious enough that everyone should pay attention.
Convective SIGMETs — the third category
Convective SIGMETs are the FAA's separate product specifically for thunderstorms and the things thunderstorms produce. They get their own category because thunderstorms are uniquely dangerous and uniquely common, and pilots need a dedicated product for them.
AC 00-45H (the FAA's Aviation Weather Services advisory circular) divides the criteria into two categories. Knowing the distinction is useful — and testable.
Routine issuance criteria
A routine Convective SIGMET is issued when any of these are occurring or expected:
- A line of thunderstorms at least 60 miles long, with thunderstorms affecting at least 40% of its length
- An area of active thunderstorms covering at least 40% of an area of 3,000 sq mi or greater, exhibiting strong radar reflectivity or a significant satellite or lightning signature
- Embedded or severe thunderstorms expected for more than 30 minutes during the valid period — regardless of area size
Special issuance criteria
A special Convective SIGMET may be issued when any of these are occurring or expected for more than 30 minutes of the valid period:
- Tornado
- Hail ≥ 3/4 inch in diameter at the surface
- Wind gusts ≥ 50 knots at the surface
Routine Convective SIGMETs are issued at H+55 (five minutes before the top of each hour). Special issuances happen whenever the criteria are met — they don't wait for the schedule. All Convective SIGMETs are valid for up to 2 hours and cover three regions: Eastern, Central, and Western U.S. Source: AC 00-45H, Aviation Weather Services.
Important implication: any Convective SIGMET implies severe or greater turbulence, severe icing, and low-level wind shear — even if those hazards aren't separately listed in the report. Don't expect to see a separate SIGMET for severe turbulence inside a thunderstorm; the Convective SIGMET already covers it.
Red flags
Any SIGMET on your route
- A SIGMET means conditions are hazardous to all aircraft — not just light aircraft or unpressurized piston aircraft
- Severe or extreme turbulence, severe icing, volcanic ash, or tropical cyclone SIGMETs are go/no-go items: either route around the hazard area or cancel
- SIGMETs are valid up to 4 hours (6 hours for hurricanes) — check issuance time against your planned departure and arrival (AC 00-45H §7.5)
Convective SIGMET on your route
- A Convective SIGMET implies severe turbulence, severe icing, and LLWS even if those are not separately listed
- Valid only 2 hours — but cells evolve faster than the advisory cycle; always check current radar alongside the SIGMET text
- Your options are: divert around the area, hold on the ground, or cancel. There is no "proceed through" option for a Convective SIGMET
AIRMET Zulu covering your altitude and route
- Moderate icing forecast for non-FIKI aircraft — an active Zulu creates "known icing conditions" legally
- Non-FIKI aircraft may not enter known icing conditions — this is not advisory, it is a legal prohibition (14 CFR 91.527)
- Zulu always includes the freezing level — that altitude tells you where the icing threat begins, not where it's worst
AIRMET Sierra at destination
- IFR conditions (ceiling <1,000 ft / vis <3 SM) or mountain obscuration forecast in your destination area
- Cross-check the TAF: if the TAF already shows IFR and the Sierra confirms it, the picture is consistent — conditions are bad and likely to persist
- If the METAR shows worse conditions than the TAF forecasted, the Sierra is already confirmed and the TAF forecast may be too optimistic
AIRMET Tango near mountainous terrain
- Tango in a mountain wave environment means moderate turbulence with a potential for severe in the rotor zone — the moderate label understates the risk in complex terrain
- Sustained surface winds ≥30 kt over terrain (also part of Tango criteria) signal strong wave activity even without an explicit wave call
- Slow to Va before entering any mountain wave area covered by AIRMET Tango (AC 00-45H §7.5)
G-AIRMET not matching text AIRMET
- The Graphical AIRMET (G-AIRMET) on aviationweather.gov may show a different hazard boundary than the text AIRMET — the G-AIRMET updates more frequently and is generally more current
- When in doubt, use the boundary that shows the greater hazard area — and check issuance times on both
- G-AIRMETs are issued every 3 hours between regular 6-hour cycles, making them the most current hazard advisory product available (AC 00-45H §7.5.4)
Checkride questions you'll actually be asked
Pulled from real ACS oral exam questions and what your DPE is likely to push on. Try to answer each before reading the response.
Q: What's the difference between an AIRMET and a SIGMET?
Severity. AIRMETs cover moderate conditions hazardous to light aircraft. SIGMETs cover severe conditions hazardous to all aircraft. Same hazard families (turbulence, icing) — different intensity thresholds.
Q: What three types of AIRMETs exist and what does each cover?
Sierra (IFR conditions and mountain obscuration), Tango (turbulence and strong surface winds), Zulu (icing and freezing levels). Memory aid: Sight, Turbulence, Zero degrees.
Q: How long is an AIRMET valid for? A SIGMET?
AIRMETs are issued every 6 hours and valid for 6 hours. SIGMETs are issued as needed and valid for up to 4 hours (longer for volcanic ash and tropical cyclones — up to 6 hours for those).
Q: Why is there a separate Convective SIGMET product?
Thunderstorms are common and uniquely dangerous, with multiple hazards bundled together (severe turbulence, severe icing, hail, wind shear, tornadoes). Giving them a dedicated product issued hourly means pilots get current convective info without it being buried in other advisories.
Q: What's the difference between a routine and a special Convective SIGMET?
Per AC 00-45H, routine Convective SIGMETs cover lines of thunderstorms (≥ 60 mi, ≥ 40% affected), areas of thunderstorms (≥ 3,000 sq mi, ≥ 40% covered), or embedded/severe thunderstorms expected for over 30 minutes. They're issued on the hourly schedule at H+55. Special Convective SIGMETs cover specific dangerous events — tornadoes, surface hail ≥ 3/4 inch, surface wind gusts ≥ 50 kt — and can be issued any time the criteria are met, not just at H+55.
Q: You're VFR-only and there's an AIRMET Sierra along your route. Can you legally fly?
"Legally" — yes, AIRMETs are advisory, not prohibitive. But the AIRMET tells you to expect ceilings below 1,000 ft or visibility below 3 SM somewhere in the affected area, which would put you in IFR conditions you're not rated for. The right answer is to identify where the IFR conditions are forecast, decide whether your route avoids them, and have an alternate plan. The AIRMET doesn't make the decision — it gives you information to make one.
Decision scenario
Educational example only. Not for real flight planning. Real go/no-go decisions require official sources, current data, and your own pilot-in-command judgment.
The setup: January morning. You're a Private Pilot, no instrument rating, planning a VFR cross-country in a Cessna 172 from a low-elevation airport over moderate mountain terrain to a destination on the other side. Cruise altitude 9,500 ft. Surface conditions at both airports look fine — clear skies, light winds, 10 SM visibility.
The briefing shows an AIRMET Zulu in effect along your route: "Moderate icing between freezing level and FL180. Freezing level surface to 4,000 ft."
What questions should you be asking?
- Where exactly is the freezing level relative to my route? If it's at the surface to 4,000 ft, my 9,500 ft cruise puts me well into icing territory.
- Is there visible moisture at my altitude? No moisture, no icing — but the AIRMET wouldn't be issued if forecasters didn't expect moisture somewhere along the route.
- What's my aircraft equipped for? A 172 is not certified for flight into known icing. "Known icing" includes a forecast you've read.
- What's my exit strategy if I encounter ice? Descending below freezing level might not be possible over mountains. Climbing above might not be possible in a 172.
- What does the rest of my briefing show? PIREPs of actual icing? Cloud tops? Temperature aloft data?
The AIRMET didn't tell you whether to fly. It told you that ice is forecast in the airspace you're planning to use. Whether you go is your decision as PIC, informed by the AIRMET, the rest of your briefing, your aircraft's capabilities, your personal minimums, and your willingness to turn around if it gets worse than forecast.
Pilot takeaway
- AIRMET = moderate, light-aircraft hazard. Sierra (visibility), Tango (turbulence), Zulu (icing).
- SIGMET = severe, all-aircraft hazard. Same families as AIRMETs, but the severe versions.
- Convective SIGMET = thunderstorm-specific. Issued hourly, valid 2 hours, covers TS-related hazards in one bundle.
- They're advisories, not commands. Your job is to read them, understand what they say about your specific route and altitude, and use them as one input into a PIC decision.
- Read freshly every time. AIRMETs are valid 6 hours, SIGMETs up to 4. By the time you're rolling onto the runway, the briefing you got 90 minutes ago might already be stale.