SIGMETs and AIRMETs Explained
Understanding aviation weather advisories for safer flight planning
The Weather Advisory System
SIGMETs and AIRMETs are in-flight weather advisories that alert pilots to hazardous conditions along their route. Understanding these products is essential for safe preflight planning and in-flight decision-making.
Think of them as a tiered warning system: AIRMETs cover moderate conditions that affect light aircraft and VFR pilots, while SIGMETs cover severe conditions that affect all aircraft regardless of size or pilot experience.
Quick Reference
SIGMET
Significant Meteorological Information
- • Hazardous to ALL aircraft
- • Severe turbulence
- • Severe icing
- • Volcanic ash
- • Dust/sandstorms
- • Tropical cyclones
- • Valid 4-6 hours
AIRMET
Airmen's Meteorological Information
- • Hazardous to light/VFR aircraft
- • Moderate turbulence
- • Moderate icing
- • IFR conditions
- • Mountain obscuration
- • Sustained winds >30 kts
- • Valid 6 hours
AIRMET Types
AIRMETs are organized by phenomenon type, identified by letter designators. They're issued every 6 hours and cover the contiguous United States.
AIRMET Sierra (S) - IFR & Mountain Obscuration
Issued when IFR conditions (ceiling below 1,000 feet and/or visibility below 3 miles) or mountain obscuration is expected to affect a significant area.
IFR Criteria
- • Ceiling <1,000 ft AGL
- • Visibility <3 miles
- • Or both combined
Mountain Obscuration
- • Clouds/precipitation hiding mountains
- • Critical for terrain avoidance
- • Common in Pacific NW, Rockies
AIRMET Tango (T) - Turbulence
Issued for moderate turbulence, sustained surface winds of 30 knots or more, and low-level wind shear (non-convective).
Moderate Turbulence
Causes strain against seat belts, unsecured objects move
Surface Winds
Sustained winds ≥30 kts (not gusts)
LLWS
Non-convective wind shear below 2,000 ft AGL
AIRMET Zulu (Z) - Icing & Freezing Level
Issued for moderate icing conditions and always includes freezing level information for the affected area.
Moderate Icing
- • Rate may create problems >1 hour
- • De-icing/anti-icing can handle
- • Diversion should be considered
Freezing Level Info
- • Always included in AIRMET Zulu
- • Given as altitude ranges
- • Essential for altitude planning
SIGMET Types
Non-Convective SIGMETs
Issued for severe weather other than thunderstorms. Valid for 4 hours (6 hours for volcanic ash and tropical cyclones). Named alphabetically (November through Yankee, excluding Sierra and Tango).
Severe Turbulence
Aircraft momentarily out of control. Occupants forced against belts.
Severe Icing
Ice accumulation rate is such that de-ice/anti-ice equipment fails to control. Immediate exit required.
Volcanic Ash
Any concentration. Extremely hazardous - can cause engine failure. AVOID COMPLETELY.
Dust/Sandstorm
Reducing visibility to below 3 miles over a large area.
Convective SIGMETs
Issued specifically for severe thunderstorm activity. Valid for 2 hours. Issued hourly (55 minutes past the hour) with special bulletins as needed. Covers Eastern (E), Central (C), and Western (W) regions.
Criteria (any one of the following):
- • Severe thunderstorms with surface winds ≥50 knots
- • Severe thunderstorms with hail ≥3/4 inch diameter
- • Tornadoes occurring or expected
- • Embedded thunderstorms (hidden in other clouds)
- • Line of thunderstorms (squall line)
- • Area coverage ≥40% with VIP level 4+ intensity
Convective SIGMET = Avoid
Convective SIGMETs describe conditions that are hazardous to all aircraft, regardless of size. When a Convective SIGMET covers your route, plan to deviate around it or delay your flight. The 2-hour validity period means conditions change rapidly— monitor updates frequently.
Decoding Examples
AIRMET Example
DFWS WA 161445
AIRMET TANGO UPDT 2 FOR TURB VALID UNTIL 162100
.
AIRMET TURB...TX OK AR LA MS
FROM MLC TO LIT TO MEI TO IAH TO SAT TO MLC
MOD TURB BTN FL250 AND FL410. CONDS CONTG BYD 2100Z.
Translation:
- • DFWS: Issued by Fort Worth Aviation Weather Center
- • 161445: Issued 16th at 1445Z
- • UPDT 2: Second update to this AIRMET
- • VALID UNTIL 162100: Valid until 16th at 2100Z
- • Area: Texas, Oklahoma, Arkansas, Louisiana, Mississippi
- • FROM...TO: Bounded by VORs MLC-LIT-MEI-IAH-SAT-MLC
- • Conditions: Moderate turbulence between FL250-FL410, continuing past 2100Z
Convective SIGMET Example
CONVECTIVE SIGMET 45C
VALID UNTIL 1855Z
KS OK
FROM 30NW ICT-40S ICT-50SE OKC-30NW OKC-30NW ICT
AREA SEV TS MOV FROM 26025KT. TOPS ABV FL450.
HAIL TO 2 IN...WIND GUSTS TO 65 KTS POSS.
Translation:
- • 45C: 45th convective SIGMET of the day, Central region
- • VALID UNTIL 1855Z: 2-hour validity
- • Area: Kansas and Oklahoma
- • SEV TS: Severe thunderstorms
- • MOV FROM 26025KT: Moving from 260° at 25 knots
- • TOPS ABV FL450: Storm tops above 45,000 ft
- • Hazards: Hail to 2 inches, wind gusts to 65 knots possible
Where to Find SIGMETs and AIRMETs
Online Sources
- • aviationweather.gov - Official AWC products
- • 1800wxbrief.com - Standard/abbreviated briefings
- • ForeFlight/Garmin Pilot - Graphical overlay
- • DUATS - Text-based briefing
In Flight
- • Flight Service - Radio briefing updates
- • HIWAS - Broadcast on VORs
- • ADS-B In - FIS-B weather products
- • SiriusXM Aviation - Satellite weather
Using Advisories in Flight Planning
AIRMET Present
Consider whether you and your aircraft can handle the described conditions. Light aircraft pilots should be particularly cautious. Have escape routes and alternates planned. Monitor conditions for changes.
Non-Convective SIGMET Present
Severe conditions exist or are expected. Plan to avoid the area if possible. If penetration is necessary (rare), ensure you have the equipment, training, and fuel for the encounter. File a PIREP after transiting.
Convective SIGMET Present
Severe thunderstorm activity along your route. Do not attempt to penetrate. Plan deviation around the area (minimum 20nm from cell edges). Consider delaying departure until convective activity diminishes. Watch for rapid changes.
Key Takeaways
- SIGMETs = severe, hazardous to ALL aircraft. AIRMETs = moderate, primarily light aircraft/VFR
- AIRMET Sierra = IFR/mountain obscuration, Tango = turbulence, Zulu = icing
- Convective SIGMETs cover severe thunderstorms—avoid these areas completely
- Check validity times—Convective SIGMETs are only valid 2 hours
- Get updates in flight via Flight Service, HIWAS, or ADS-B FIS-B