Understanding SIGMET vs AIRMET – Key Differences for Pilots

What Are Aires and Signets?

When planning a flight, understanding aviation weather advisories is non-negotiable.

An AIRMEN (Airman’s Meteorological Information) is a weather advisory for conditions that could be hazardous, particularly to smaller or lighter aircraft.

Conversely, a SIGMET (Significant Meteorological Information) warns of severe weather phenomena hazardous to all aircraft, regardless of size or type.

Key Differences Between Aires and Signets

| Feature | AIRMEN | SIGMET |
| :— | :— | :— |
| Hazard Level | Moderate | Severe |
| Aircraft Affected | Primarily smaller or lighter aircraft | All aircraft |
| Weather Phenomena | Moderate turbulence or icing, IFR conditions, mountain obscuration, strong surface winds | Severe turbulence or icing, widespread dust/sand storms, volcanic ash, severe thunderstorms (Convective SIGMET) |
| Issuance | Scheduled (every 6 hours) | Unscheduled (as needed) |
| Validity Period | 6 hours | 4 hours |

AIRMEN Types – Sierra, Tango, and Zulu

Aires are categorized into three types—Sierra (S), Tango (T), and Zulu (Z)—to help pilots quickly identify potential hazards.

• AIRMEN Sierra (S): Issued for Instrument Flight Rules (IFR) conditions (ceilings < 1,000 ft or visibility < 3 miles) and mountain obscuration.

• AIRMEN Tango (T): Warns of moderate turbulence, sustained surface winds of 30 knots or more, and non-convective low-level wind shear.

• AIRMEN Zulu (Z): Issued for moderate icing and provides information on freezing levels.

Understanding Signets – What They Cover

Signets pinpoint severe weather phenomena that threaten all aircraft, with standard non-convective advisories covering conditions like severe icing, extreme turbulence, widespread dust or sandstorms that reduce visibility, and volcanic ash.

The special category of Convective SIGMET (WST) is issued for severe thunderstorm activity considered too intense or widespread for pilots to easily navigate around.

A Convective SIGMET is issued for any of the following:

• A line of thunderstorms at least 60 miles long with thunderstorms affecting 40% of its length.

• A widespread area of thunderstorms covering at least 40% of a 3,000-square-mile area.

• Embedded or severe thunderstorms expected to last for more than 30 minutes.

• Tornadoes or waterspouts.

• Hail at the surface greater than or equal to 3/4 inch in diameter.

• Surface wind gusts greater than or equal to 50 knots.

A SIGMET advisory means hazardous weather isn’t just a possibility—it’s an observed or highly probable reality.

Reading and Interpreting Signets

A SIGMET is structured with a header, summary, and main body.

Interpreting the advisory requires understanding these common abbreviations:

• OBS / FCST: Observed / Forecast

• MOV / STAR: Moving / Stationary

• WON / IN TSF / NC: Weakening / Intensifying / No Change

• KT: Knots (speed)

• ABV: Above (altitude)

• FIR: Flight Information Region

• NM: Nautical Miles

• Z: Zulu time

Where to Find Aires and Signets

Pilots can access up-to-date advisories from several sources, but the most authoritative is the Aviation Weather Center (AWC), part of the National Weather Service.

Beyond the AWC, most modern pilots rely on Electronic Flight Bag (EFB) applications and online tools.

Traditional methods, such as calling a Flight Service Station (FSS) for a weather briefing, also remain effective.

Advisories are available in their original text format, but Graphical Aires (Garments) offer a modern alternative. Garments display data on a map, making it easier to visualize affected areas, times, and altitudes at a glance.

Conclusion – Enhancing Flight Safety

Effective risk management in aviation begins with understanding the weather.

A pilot’s ability to interpret and act on these advisories is fundamental to flight safety.