Every preflight weather briefing should include a check for AIRMETs — Airmen's Meteorological Information advisories that flag weather hazards serious enough to affect safety but not yet at the severity level of a SIGMET. Whereas SIGMETs warn of extreme, potentially catastrophic phenomena, AIRMETs target the everyday hazards that routinely catch VFR pilots and operators of aircraft sensitive to turbulence or icing: low ceilings, mountain obscuration, moderate turbulence, strong surface winds, and moderate icing. Understanding the three AIRMET categories — Sierra, Tango, and Zulu — and knowing how to read the modern Graphical AIRMET (G-AIRMET) is one of the most practically valuable skills a pilot can develop.
The regulatory and procedural backbone for AIRMETs is laid out in Chapter 26 of the FAA Aviation Weather Handbook (FAA-H-8083-28B). This article expands on that foundation, translating the official criteria into language that makes operational sense during a real-world briefing.
The Three AIRMET Categories
Each AIRMET carries a code letter — Sierra, Tango, or Zulu — that immediately tells a pilot what type of hazard to expect. These are not arbitrary labels; each category corresponds to a distinct family of weather phenomena with its own issuance criteria.
AIRMET Sierra (IFR and Mountain Obscuration)
AIRMET Sierra is issued whenever IFR conditions are occurring or expected over a widespread area. The threshold is a ceiling below 1,000 feet and/or surface visibility below 3 statute miles. The responsible forecasters also specify the cause of restricted visibility — precipitation, smoke, haze, mist, fog, or blowing snow. Sierra also covers widespread mountain obscuration, a separate but related hazard in which clouds, precipitation, smoke, haze, mist, or fog hide mountain terrain from pilots who may be navigating visually. Mountain obscuration is particularly dangerous because VFR pilots may not realize terrain is concealed until it is too late to reverse course.
AIRMET Tango (Turbulence and Wind)
AIRMET Tango covers three related mechanical and dynamic hazards. First, it is issued for moderate turbulence — the top and bottom altitudes of the affected layer are always specified so pilots know exactly which portion of their route is affected. Second, Tango covers sustained surface winds of 30 knots or greater, which affect aircraft handling during taxi, takeoff, and landing. Third, it covers non-convective low-level wind shear (LLWS) potential below 2,000 feet AGL. Non-convective LLWS — caused by temperature inversions, frontal boundaries, and terrain effects rather than by thunderstorm outflow — can produce sudden, dramatic airspeed changes in the critical phases of departure and approach. Tango does not cover convective turbulence, which is handled by SIGMETs and convective outlook products.
AIRMET Zulu (Icing and Freezing Levels)
AIRMET Zulu addresses structural icing. It is issued for moderate icing, and like Tango for turbulence, Zulu always specifies the top and bottom altitudes of the icing layer. Importantly, Zulu also provides freezing level heights across the forecast area. When the bottom altitude of the moderate icing layer coincides with the freezing level, that relationship is explicitly noted. In areas with multiple freezing levels — common in the western mountains or during complex frontal passages — those layers are enumerated. Lowest freezing levels are given at intervals of 4,000 feet MSL (or surface as appropriate), giving pilots a picture of where liquid water might be present and where it could freeze on contact with a cold airframe.
Who Issues AIRMETs and Where They Apply
AIRMETs are issued by three U.S. Meteorological Watch Offices (MWOs). The Aviation Weather Center (AWC) in Kansas City covers the contiguous United States (CONUS) and adjacent coastal waters. The Alaska Aviation Weather Unit (AAWU) covers Alaska and its adjacent coastal waters, issuing AIRMETs for three distinct Alaska regions. WFO Honolulu covers Hawaii and adjacent waters. Notably, no AIRMETs are issued for U.S. Oceanic FIRs — the Gulf of Mexico, Caribbean, and Western Atlantic and Pacific Oceans lie outside AIRMET coverage territory. For those areas, pilots must rely on SIGMETs and other oceanic weather products.
Issuance Schedule and Valid Periods
CONUS AIRMETs are issued four times daily at 0245, 0845, 1445, and 2045 UTC, each valid for 6 hours. Alaska AIRMETs are issued three times daily and valid for 8 hours. Hawaii AIRMETs match the CONUS four-times-daily schedule but on its own issuance times (0400, 1000, 1600, 2200 UTC) and are valid for 6 hours. Unscheduled AIRMETs can be issued at any time when hazardous conditions arise that were not captured in the scheduled bulletin. These amendments carry an update number so pilots and dispatchers can readily identify them.
The Graphical AIRMET (G-AIRMET) Over the CONUS
For the CONUS, the traditional text AIRMET has been complemented — and in many respects superseded for practical preflight use — by the Graphical AIRMET, or G-AIRMET. The G-AIRMET is available on aviationweather.gov, 1800wxbrief.com, and through Flight Information Service-Broadcast (FIS-B) equipment in the cockpit. It presents the same hazard information as the text AIRMET but in a higher-resolution, map-based format that makes it far easier to visualize where hazards exist relative to a planned route.
The G-AIRMET provides forecasts at discrete time snapshots no more than 3 hours apart, looking up to 12 hours into the future. Standard snapshot times are at 00, 03, 06, 09, and 12 hours from issuance. During the first 6 hours, additional intermediate snapshots may be inserted at the 01, 02, 04, or 05-hour marks when the situation warrants higher temporal resolution. The 00-hour snapshot represents initial conditions at the time of issuance; subsequent frames show the forecast state of each hazard at that exact valid time.
A Critical Interpolation Rule
One of the most operationally important — and commonly misunderstood — aspects of the G-AIRMET is how to interpret the time between snapshots. The snapshots are not continuous animations; they are discrete moments. If a 00-hour frame shows no hazard and a 03-hour frame shows moderate icing over your route, you must assume that icing begins immediately after the 00-hour time and persists until at least the 03-hour mark. Likewise, if the 06-hour frame shows no hazard again, the icing should be assumed to exist from 0001 through 0559 hours unless the product specifies a narrower initiation or ending time. In practical terms: the absence of a hazard at one snapshot does not guarantee safe conditions in the minutes just before the next snapshot appears.
The G-AIRMET covers these seven specific hazards over the CONUS: IFR conditions (ceiling below 1,000 ft and/or visibility below 3 miles), widespread mountain obscuration, moderate icing, freezing levels, moderate turbulence, non-convective LLWS potential below 2,000 feet AGL, and sustained surface winds exceeding 30 knots. Each hazard is displayed as a shaded polygon on the map, color-coded by type, allowing a pilot to immediately see whether their route of flight penetrates an affected area. Because the G-AIRMET provides higher forecast resolution than the legacy text bulletin, the two products may not be pixel-perfect matches, but the text bulletins valid from 00 through 06 hours are formally aligned with the graphical depiction for that same period.
Why AIRMETs Matter: Operational and Safety Significance
AIRMETs are explicitly designed for VFR pilots and operators of aircraft sensitive to weather — light singles, twins, turboprops, and jets that may lack the performance or certification to safely penetrate icing or severe turbulence. For a VFR pilot, an active AIRMET Sierra along the route could indicate that the intended destination is IFR before arrival. For a pilot in a non-ice-certified aircraft, AIRMET Zulu is a hard stop — flight into known icing conditions in an unequipped aircraft is illegal under 14 CFR Part 91 and potentially fatal. AIRMET Tango for moderate turbulence, while not necessarily prohibitive, should trigger a review of aircraft performance, passenger briefings, and possible altitude adjustments.
En route, AIRMETs are accessible over Flight Service frequencies and through FIS-B datalink. In Alaska and Hawaii, they are broadcast over air traffic frequencies. This ensures pilots can receive updated AIRMET information even if conditions change after departure — an important safety net on long cross-country flights.
Key Numbers and Rules
- IFR threshold: Ceiling below 1,000 ft AGL and/or visibility below 3 statute miles — triggers AIRMET Sierra.
- Surface wind threshold: Sustained winds of 30 knots or greater — triggers AIRMET Tango.
- LLWS coverage: Non-convective wind shear potential below 2,000 ft AGL — part of AIRMET Tango.
- Icing: Moderate icing with layer tops and bottoms specified — AIRMET Zulu; always includes freezing level information.
- CONUS issuance: 0245, 0845, 1445, 2045 UTC — every 6 hours, valid 6 hours.
- Alaska issuance: Three times daily, valid 8 hours.
- Hawaii issuance: Four times daily, valid 6 hours.
- G-AIRMET snapshots: Every 3 hours out to 12 hours; intermediate frames possible in the first 6 hours.
- Altitudes: All heights referenced to MSL unless noted; FL prefix used at or above 18,000 ft.
- No AIRMET coverage: U.S. Oceanic FIRs (Gulf of Mexico, Caribbean, Pacific/Atlantic oceans).
Common Test Traps
- Confusing categories: Students mix up Tango and Zulu. Remember: Tango = Turbulence (and winds); Zulu = icing (Zero degrees = freezing). Sierra = IFR/mountain obscuration (think Sierra Nevada — mountains hidden in clouds).
- Assuming G-AIRMET frames are continuous: A clean 00-hour frame does not mean the route is clear until 03:00. Hazards must be assumed to begin immediately after the clear snapshot unless otherwise stated.
- Forgetting that Zulu always includes freezing levels: Even when no moderate icing is present in a given area, AIRMET Zulu provides freezing level heights — this is a distinct and important piece of the product.
- Thinking AIRMETs cover oceanic FIRs: They do not. Gulf of Mexico, Caribbean, and oceanic Pacific/Atlantic areas rely on SIGMETs for inflight advisory coverage.
- Confusing valid periods: CONUS and Hawaii AIRMETs are valid for 6 hours; Alaska AIRMETs are valid for 8 hours. The exam may present these as the same duration.