SIGMETs: what they mean and when they ground you

A METAR tells you what the weather is doing right now. A TAF tells you what it's expected to do. A SIGMET tells you something stronger: a meteorologist has decided some weather phenomenon is dangerous enough that all aircraft in a defined region need to know about it. SIGMETs are the meteorologist's flag — they don't appear for routine weather, only when something is genuinely hazardous. If there's a SIGMET on your route, you re-plan or you wait. This is the practical guide.

What "significant" means

SIGMET stands for Significant Meteorological Information. The threshold for issuance is that a phenomenon poses a hazard to "the safety of all aircraft" — not just light aircraft, not just IFR aircraft. The phenomena that qualify (codified by ICAO):

| Code | Meaning | Why it grounds you | |---|---|---| | TS | Thunderstorm | Severe turbulence, hail, icing, lightning, wind shear | | ICE | Icing (severe) | Structural ice accretion can stall an airframe in minutes | | TURB | Severe turbulence | Loss of control, structural damage | | MTW | Mountain wave | Strong vertical air motion exceeding aircraft climb capability | | VA | Volcanic ash | Engine flameout, pitot blockage, structural abrasion | | DUST/SAND | Dust storm | Visibility, engine ingestion |

For UL/LSA pilots, TS, ICE, and TURB are the most common reasons to abort. VA is rare in most regions but a hard no-go anywhere it's issued. MTW is region-specific (Alpine corridors, Pyrenees lee waves). DUST is mostly a concern in southern regions on Saharan dust events.

A real SIGMET decoded

Here's a real SIGMET from the LIBB FIR (Brindisi flight information region):

LIBB SIGMET 03 VALID 141200/141600 LIBB-
LIBB BRINDISI FIR EMBD TS FCST WI N4030 E01700 - N4100 E01900 - N4030 E02100 - N3950 E01900 - N4030 E01700
TOP FL420 MOV NE 25KT INTSF=

Read it as:

• LIBB SIGMET 03 — third SIGMET issued by Brindisi today
• VALID 141200/141600 — valid 12:00 to 16:00 UTC on day 14 (4 hours)
• EMBD TS FCST — embedded thunderstorms forecast (embedded = inside cloud, not visible until you're in them)
• WI N4030 E01700 - N4100 E01900 - N4030 E02100 - N3950 E01900 - N4030 E01700 — coordinates of the 5-vertex polygon defining the affected area. Read each pair as Latitude / Longitude. The polygon closes (the last point equals the first).
• TOP FL420 — thunderstorm tops at flight level 42,000 ft (very tall — significant storm)
• MOV NE 25KT — moving northeast at 25 knots
• INTSF — intensifying

The polygon roughly covers central-southern Italy. Tops at FL420 means this is a big storm. A 25 kt NE movement means the storm will move 100 nm in 4 hours — by 16:00 UTC the affected area will have shifted significantly.

How to read the polygon

The most useful skill: visualize the polygon on a map. The example above defines a 5-vertex polygon with vertices at:

• 40°30'N 17°00'E (off the Apulian coast)
• 41°00'N 19°00'E (over the Adriatic)
• 40°30'N 21°00'E (over Albania/Greece)
• 39°50'N 19°00'E (south Adriatic)

That's roughly a diamond covering the Adriatic and Ionian Seas. If your route is from LIRP (Pisa) to LIBR (Brindisi), the SIGMET says: don't go southeast of Pisa during the validity window. Reroute via the Tyrrhenian coast or wait 4 hours.

Voliqo's planner renders SIGMET polygons as filled overlays on the map. If your plotted route crosses a SIGMET polygon, the planner highlights the intersection — visually obvious.

Geographic vs temporal scope

A SIGMET has both:

• Geographic scope: the polygon (or sometimes a corridor)
• Temporal scope: the validity window (typically 4 hours; up to 6 for some phenomena)

For a 2-hour flight that's planned to start 1 hour after the SIGMET expires, the SIGMET doesn't ground you — it has expired. For a 5-hour flight that overlaps the validity window, you have to plan around it OR delay until it expires.

The validity is fixed at issuance — but new SIGMETs can be issued for the same area as the situation evolves. For thunderstorms, expect a re-issuance every 4–6 hours as long as the convective activity persists.

What VFR pilots should do

The conservative VFR rule: avoid the polygon by at least 20 nm. For TS specifically, the FAA recommends 20 nm of clearance from any cumulonimbus; SIGMET polygons are usually drawn with margin already, but adding your own 20 nm is good discipline.

For ICE, route below or above the affected layer (SIGMETs include altitude bands — BTN FL080/FL150 means between 8,000 and 15,000 ft). For UL/LSA pilots staying below 5,000 ft AGL, an icing SIGMET above FL080 isn't your problem.

For severe TURB, stay out of the polygon entirely. Turbulence intensity can damage a 600 kg airframe; route around it.

What IFR pilots should do

IFR pilots have more options because:

• They can climb above many phenomena (TS tops aside)
• They can request reroutes around SIGMET polygons
• They can plan alternates that aren't affected

The IFR pilot's tool is route negotiation with ATC. If a SIGMET polygon blocks your filed route, request a deviation — most controllers will accommodate within practical limits. Plan fuel reserves that account for a longer reroute.

For thunderstorm SIGMETs especially: even IFR-rated, IFR-equipped aircraft should not fly through embedded TS. Lightning, turbulence, hail damage, and massive vertical drafts are all real. Reroute or land short.

SIGMETs vs AIRMETs

A related document is the AIRMET — same idea, lower threshold. AIRMETs cover phenomena hazardous to LIGHT aircraft but not necessarily to all aircraft (e.g., moderate icing, moderate turbulence, mountain obscuration, low ceilings).

For UL/LSA pilots, AIRMETs are at least as important as SIGMETs because the AIRMET threshold matches your aircraft's capability. A "moderate" turbulence AIRMET that an airliner pilot ignores will toss your airplane around hard.

Voliqo currently surfaces SIGMETs in the planner. AIRMET coverage is in the backlog — until then, supplement Voliqo with your national MET service for AIRMET data.

Reading the altitude band

SIGMETs include altitude information:

• BTN FL080/FL150 — between FL080 and FL150
• TOP FL420 — top of phenomenon (storm cell, ash plume) at FL420
• SFC/FL150 — surface to FL150

For UL pilots cruising at FL050 (5,000 ft), a SIGMET valid BTN FL080/FL150 doesn't affect you. But a SIGMET SFC/FL150 does — surface to FL150 means the entire troposphere up to FL150 is hazardous.

Read altitude bands carefully. They're easy to miss in dense METAR-style formatting.

How to use SIGMETs in route planning

Voliqo's planner overlays SIGMET polygons on the map alongside METAR/TAF station markers. The workflow:

1. Plan your route (departure, intermediate stops, arrival)
2. Check for SIGMET overlays — they appear as colored polygons
3. If your route crosses a polygon, decide: reroute, delay, or cancel
4. Cross-reference the polygon's altitude band with your planned cruise altitude

For a Tecnam P92 Echo MkII cruising at 3,000 ft AGL, an altitude band starting at FL080 doesn't affect the route geometry. For a cruising Tecnam P2010 TDI climbing to FL120 to maximize endurance, the same SIGMET might require staying at FL050 instead.

Bottom line

SIGMETs aren't routine weather — they're meteorologist-issued flags that something is dangerous. Don't skip them. The discipline is:

1. Pull SIGMETs as part of every pre-flight briefing
2. Visualize the polygon on a map (mental or planner-rendered)
3. Cross-reference the validity window and altitude band against your flight plan
4. If your route + altitude + timing crosses a SIGMET polygon, either reroute, delay, or stay home

For VFR/UL pilots, the rule is conservative: 20 nm clearance from any TS or severe TURB polygon. For IFR pilots, plan reroutes with extra fuel reserve. SIGMETs are the cheapest piece of pre-flight intelligence available — they just need a few minutes of attention.