A significant weather chart, usually written SIGWX, is a single picture of the hazards a flight might meet en route: where the thunderstorms, the turbulence, the icing and the jet streams are expected to be at a given time. It is one of the most information-dense documents in a briefing, and once you know the symbols it is also one of the fastest to read.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
What a SIGWX chart is
A SIGWX chart is a forecast, not an observation. It is produced under the World Area Forecast System set out in ICAO Annex 3, and it shows the significant en-route weather expected at a fixed valid time, the snapshot the forecast is made for. That single valid time is the first thing to read, because the chart is only describing that moment; the weather before and after it will differ.
SIGWX charts come in level bands. A high-level chart covers the upper flight levels used by jets, a medium-level chart the levels in between, and a low-level chart the lower airspace most relevant to general aviation, often including surface fronts. Reading the right band for your flight is the second thing to get right: a high-level chart's jet streams and tropopause are of little use to a light aircraft below them.
Thunderstorms and cumulonimbus
The most important hazard the chart marks is cumulonimbus (CB), the cloud of the thunderstorm, because a CB bundles together severe turbulence, icing, hail, lightning and wind shear. The chart shows the areas of expected CB, their bases and tops as flight levels, and a coverage word that tells you how much there is:
- ISOL (isolated): individual, separated CBs, covering less than 1/8 of the area;
- OCNL (occasional): well-separated CBs, covering 1/8 to 4/8 of the area;
- FRQ (frequent): CBs with little or no separation, covering more than 4/8 of the area, the worst case;
- EMBD (embedded): CBs hidden within other cloud layers, so you may not see them coming.
EMBD deserves particular respect, because an embedded CB is concealed in surrounding cloud and is exactly the one you can blunder into on instruments. Our guide to thunderstorms and convective weather explains why these cells are worth a wide berth.
Turbulence and clear-air turbulence
Areas of expected moderate or severe turbulence are outlined and marked with the turbulence symbol, and a separate concern, clear-air turbulence (CAT), is shown around the jet streams where there is no cloud to warn you of it. The chart gives the intensity and the level band affected. Because CAT comes with no visual cue, the SIGWX chart is one of the few warnings you get of it in advance, which our guide to the jet stream and clear-air turbulence covers in more depth.
Icing
Icing areas are outlined with the icing symbol and an intensity, marking where moderate or severe airframe icing is forecast. Icing on a SIGWX chart is the planning-level companion to the in-flight reports and warnings, and it lets you see in advance which levels and regions to avoid or to plan a climb or descent through quickly. Our guide to aircraft icing explains what the intensities mean for the aircraft.
Jet streams and the tropopause
On medium and high-level charts, jet streams are drawn as bold arrows along their core, labelled with the core flight level and the maximum wind speed, with tick marks indicating speed bands along the arrow. The tropopause height is shown at points across the chart as boxed values, marking the level where the troposphere gives way to the stratosphere. Together these tell you where the strongest winds and the sharpest temperature structure are, which matter for fuel, for turbulence and for the tropopause itself.
Fronts, and how this differs from a surface analysis
Low-level SIGWX charts also show fronts, warm, cold and occluded, and their movement, which is where the SIGWX chart can be confused with a surface analysis chart. The distinction is worth holding clearly. A surface analysis chart depicts the analysed surface pressure pattern, the isobars and the fronts, often as a current or near-current picture of the surface. A SIGWX chart is a forecast of significant weather aloft for a level band at a fixed time, with the hazards, CB, turbulence, icing and jet streams, that the surface chart does not show. They are complementary: the surface chart tells you the pressure pattern and the fronts driving the weather, and the SIGWX chart tells you the airborne hazards that weather will produce.
Reading the symbols
Because the chart is built almost entirely from symbols, the legend is, again, the key. The cloud, turbulence and icing symbols, the jet-stream arrows, the boxed tropopause values and the area outlines are all defined there, along with the coverage abbreviations. The standards behind them are international, set in ICAO Annex 3 and the WMO technical regulations and described for pilots in the FAA Aviation Weather Handbook, so a SIGWX chart from one provider reads much like another once you know the symbols.
The other hazards
Thunderstorms, turbulence, icing and jet streams are the staples, but a SIGWX chart also flags a set of less frequent hazards that matter when they appear, each with its own symbol defined in the legend. Volcanic eruptions are marked at their location, because volcanic ash is destructive to engines and is often invisible until you are in it. Tropical cyclones are shown where they threaten a route. Widespread sandstorms and duststorms, mountain waves, and areas of widespread reduced surface visibility on low-level charts all carry their own marks, as does the symbol for a radiation incident. These are not on every chart, but when one is there it is telling you about a hazard the routine reports may not, and the SIGWX chart is often where you first see it on a planning sheet.
On a low-level chart, the freezing level and areas of widespread low cloud or hill fog may also be shown, rounding out the picture for flight in the lower airspace. The lesson is to read the whole chart and its legend rather than scan only for the familiar CB and jet-stream symbols, because the rarer marks are the ones easiest to overlook and sometimes the most serious.
A worked example
You pull the low-level SIGWX chart for your flight and check the valid time: it is the snapshot nearest your en-route phase. Across your track you see an area marked EMBD CB, with bases at the surface and tops well above your cruising level, so you note that embedded thunderstorms are forecast and you cannot count on seeing them. Around that area is an outlined moderate icing region in the levels you planned to cruise at, so you reconsider your level.
A cold front is drawn across the route with its direction of movement, telling you the band of weather will be sweeping through around your timing, and you cross-check the surface picture against the surface analysis chart. Higher up, were this a medium-level chart, you would also read the jet stream with its core level and speed and the boxed tropopause heights. From the symbols you have built the en-route hazard picture: embedded storms, icing at your level, and a front moving through, all before you started the engine.
Common pitfalls
- Ignoring the valid time. The chart describes one fixed moment; weather either side of it differs.
- Reading the wrong level band. A high-level chart's jet streams mean little to a light aircraft far below them.
- Underrating embedded CB. EMBD thunderstorms are concealed in other cloud and are easy to fly into on instruments.
- Confusing it with a surface analysis. SIGWX forecasts hazards aloft; the surface chart analyses the pressure pattern and fronts.
- Skipping the legend. The chart is almost all symbols; their meanings are defined there.
In Pilot EFB
Pilot EFB is a study and planning companion that keeps the forecast charts and the rest of a briefing in one offline-first place, so a briefing you have already pulled stays readable away from a signal. It helps you study the symbols and build the en-route hazard picture, but it does not forecast the weather, fly around the hazards, or replace the official charts, and pulling a fresh forecast needs a connection. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and brief from your official source of record.