[{"data":1,"prerenderedAt":501},["ShallowReactive",2],{"learn-\u002Flearn\u002Fsignificant-weather-sigwx-charts":3,"learn-nav-\u002Flearn\u002Fsignificant-weather-sigwx-charts":469},{"id":4,"title":5,"body":6,"date":395,"dateModified":396,"description":397,"draft":398,"extension":399,"faqs":400,"howTo":410,"keyTakeaways":419,"meta":425,"metaDescription":426,"navigation":427,"path":428,"quiz":429,"seo":455,"series":396,"seriesOrder":396,"sources":456,"stem":466,"topic":467,"__hash__":468},"learn\u002Flearn\u002Fsignificant-weather-sigwx-charts.md","Significant weather (SIGWX) charts",{"type":7,"value":8,"toc":380},"minimark",[9,13,19,24,44,63,67,93,121,129,133,149,153,163,167,200,204,232,236,255,259,291,298,302,320,337,341,373,377],[10,11,12],"p",{},"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.",[14,15,16],"blockquote",{},[10,17,18],{},"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.",[20,21,23],"h2",{"id":22},"what-a-sigwx-chart-is","What a SIGWX chart is",[10,25,26,27,31,32,39,40,43],{},"A SIGWX chart is a ",[28,29,30],"strong",{},"forecast",", not an observation. It is produced under the ",[33,34,38],"a",{"href":35,"rel":36},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fannex-3-meteorological-service-for-international-air-navigation-1",[37],"nofollow","World Area Forecast System"," set out in ICAO Annex 3, and it shows the significant en-route weather expected at a ",[28,41,42],{},"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.",[10,45,46,47,50,51,54,55,58,59,62],{},"SIGWX charts come in ",[28,48,49],{},"level bands",". A ",[28,52,53],{},"high-level"," chart covers the upper flight levels used by jets, a ",[28,56,57],{},"medium-level"," chart the levels in between, and a ",[28,60,61],{},"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.",[20,64,66],{"id":65},"thunderstorms-and-cumulonimbus","Thunderstorms and cumulonimbus",[10,68,69,70,73,74,80,81,84,85,88,89,92],{},"The most important hazard the chart marks is ",[28,71,72],{},"cumulonimbus (CB)",", the cloud of the thunderstorm, because a CB bundles together severe turbulence, icing, hail, lightning and ",[33,75,79],{"href":76,"className":77},"\u002Flearn\u002Fglossary#gt-wind-shear",[78],"glossary-link","wind shear",". The chart shows the ",[28,82,83],{},"areas"," of expected CB, their ",[28,86,87],{},"bases and tops"," as flight levels, and a ",[28,90,91],{},"coverage word"," that tells you how much there is:",[94,95,96,103,109,115],"ul",{},[97,98,99,102],"li",{},[28,100,101],{},"ISOL (isolated):"," individual, separated CBs, covering less than 1\u002F8 of the area;",[97,104,105,108],{},[28,106,107],{},"OCNL (occasional):"," well-separated CBs, covering 1\u002F8 to 4\u002F8 of the area;",[97,110,111,114],{},[28,112,113],{},"FRQ (frequent):"," CBs with little or no separation, covering more than 4\u002F8 of the area, the worst case;",[97,116,117,120],{},[28,118,119],{},"EMBD (embedded):"," CBs hidden within other cloud layers, so you may not see them coming.",[10,122,123,124,128],{},"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 ",[33,125,127],{"href":126},"\u002Flearn\u002Fthunderstorms-and-convective-weather","thunderstorms and convective weather"," explains why these cells are worth a wide berth.",[20,130,132],{"id":131},"turbulence-and-clear-air-turbulence","Turbulence and clear-air turbulence",[10,134,135,136,139,140,143,144,148],{},"Areas of expected ",[28,137,138],{},"moderate or severe turbulence"," are outlined and marked with the turbulence symbol, and a separate concern, ",[28,141,142],{},"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 ",[33,145,147],{"href":146},"\u002Flearn\u002Fjet-stream-and-clear-air-turbulence","the jet stream and clear-air turbulence"," covers in more depth.",[20,150,152],{"id":151},"icing","Icing",[10,154,155,157,158,162],{},[28,156,152],{}," 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 ",[33,159,161],{"href":160},"\u002Flearn\u002Faircraft-icing-explained","aircraft icing"," explains what the intensities mean for the aircraft.",[20,164,166],{"id":165},"jet-streams-and-the-tropopause","Jet streams and the tropopause",[10,168,169,170,173,174,182,183,186,187,190,191,194,195,199],{},"On medium and high-level charts, ",[28,171,172],{},"jet streams"," are drawn as bold arrows along their core, labelled with the ",[28,175,176,177],{},"core ",[33,178,181],{"href":179,"className":180},"\u002Flearn\u002Fglossary#gt-flight-level",[78],"flight level"," and the ",[28,184,185],{},"maximum wind speed",", with tick marks indicating speed bands along the arrow. The ",[28,188,189],{},"tropopause height"," is shown at points across the chart as ",[28,192,193],{},"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 ",[33,196,198],{"href":197},"\u002Flearn\u002Fthe-tropopause","tropopause"," itself.",[20,201,203],{"id":202},"fronts-and-how-this-differs-from-a-surface-analysis","Fronts, and how this differs from a surface analysis",[10,205,206,207,210,211,215,216,219,220,223,224,227,228,231],{},"Low-level SIGWX charts also show ",[28,208,209],{},"fronts",", warm, cold and occluded, and their movement, which is where the SIGWX chart can be confused with a ",[33,212,214],{"href":213},"\u002Flearn\u002Freading-a-surface-analysis-chart","surface analysis chart",". The distinction is worth holding clearly. A ",[28,217,218],{},"surface analysis"," chart depicts the ",[28,221,222],{},"analysed surface pressure pattern",", the isobars and the fronts, often as a current or near-current picture of the surface. A ",[28,225,226],{},"SIGWX"," chart is a ",[28,229,230],{},"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.",[20,233,235],{"id":234},"reading-the-symbols","Reading the symbols",[10,237,238,239,242,243,248,249,254],{},"Because the chart is built almost entirely from symbols, the ",[28,240,241],{},"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 ",[33,244,247],{"href":245,"rel":246},"https:\u002F\u002Flibrary.wmo.int\u002Frecords\u002Fitem\u002F35699-technical-regulations-volume-ii",[37],"WMO technical regulations"," and described for pilots in the ",[33,250,253],{"href":251,"rel":252},"https:\u002F\u002Fwww.faa.gov\u002Fregulationspolicies\u002Fhandbooksmanuals\u002Faviation\u002Ffaa-h-8083-28-aviation-weather-handbook",[37],"FAA Aviation Weather Handbook",", so a SIGWX chart from one provider reads much like another once you know the symbols.",[20,256,258],{"id":257},"the-other-hazards","The other hazards",[10,260,261,262,265,266,270,271,274,275,278,279,282,283,286,287,290],{},"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. ",[28,263,264],{},"Volcanic eruptions"," are marked at their location, because ",[33,267,269],{"href":268},"\u002Flearn\u002Fvolcanic-ash-and-sigmet","volcanic ash"," is destructive to engines and is often invisible until you are in it. ",[28,272,273],{},"Tropical cyclones"," are shown where they threaten a route. Widespread ",[28,276,277],{},"sandstorms and duststorms",", ",[28,280,281],{},"mountain waves",", and areas of widespread reduced ",[28,284,285],{},"surface visibility"," on low-level charts all carry their own marks, as does the symbol for a ",[28,288,289],{},"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.",[10,292,293,294,297],{},"On a low-level chart, the ",[28,295,296],{},"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.",[20,299,301],{"id":300},"a-worked-example","A worked example",[10,303,304,305,307,308,311,312,315,316,319],{},"You pull the ",[28,306,61],{}," SIGWX chart for your flight and check the ",[28,309,310],{},"valid time",": it is the snapshot nearest your en-route phase. Across your track you see an area marked ",[28,313,314],{},"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 ",[28,317,318],{},"moderate icing"," region in the levels you planned to cruise at, so you reconsider your level.",[10,321,322,323,326,327,329,330,333,334,336],{},"A ",[28,324,325],{},"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 ",[33,328,214],{"href":213},". Higher up, were this a medium-level chart, you would also read the ",[28,331,332],{},"jet stream"," with its core level and speed and the boxed ",[28,335,198],{}," 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.",[20,338,340],{"id":339},"common-pitfalls","Common pitfalls",[94,342,343,349,355,361,367],{},[97,344,345,348],{},[28,346,347],{},"Ignoring the valid time."," The chart describes one fixed moment; weather either side of it differs.",[97,350,351,354],{},[28,352,353],{},"Reading the wrong level band."," A high-level chart's jet streams mean little to a light aircraft far below them.",[97,356,357,360],{},[28,358,359],{},"Underrating embedded CB."," EMBD thunderstorms are concealed in other cloud and are easy to fly into on instruments.",[97,362,363,366],{},[28,364,365],{},"Confusing it with a surface analysis."," SIGWX forecasts hazards aloft; the surface chart analyses the pressure pattern and fronts.",[97,368,369,372],{},[28,370,371],{},"Skipping the legend."," The chart is almost all symbols; their meanings are defined there.",[20,374,376],{"id":375},"in-pilot-efb","In Pilot EFB",[10,378,379],{},"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.",{"title":381,"searchDepth":382,"depth":382,"links":383},"",2,[384,385,386,387,388,389,390,391,392,393,394],{"id":22,"depth":382,"text":23},{"id":65,"depth":382,"text":66},{"id":131,"depth":382,"text":132},{"id":151,"depth":382,"text":152},{"id":165,"depth":382,"text":166},{"id":202,"depth":382,"text":203},{"id":234,"depth":382,"text":235},{"id":257,"depth":382,"text":258},{"id":300,"depth":382,"text":301},{"id":339,"depth":382,"text":340},{"id":375,"depth":382,"text":376},"2026-04-27",null,"What a significant weather chart forecasts, the CB, turbulence, icing and jet-stream symbols it uses, and how it differs from a surface analysis chart.",false,"md",[401,404,407],{"q":402,"a":403},"Is a SIGWX chart a forecast or an observation?","A forecast. It shows the significant en-route weather expected at a fixed valid time, which is the first thing to read, because the chart only describes that moment. SIGWX charts come in level bands, high, medium and low level, so you also read the band relevant to your flight rather than, say, a high-level chart's jet streams far above a light aircraft.",{"q":405,"a":406},"What do ISOL, OCNL, FRQ and EMBD mean?","They describe the coverage or nature of cumulonimbus cloud. ISOL is isolated, individual separated cells; OCNL is occasional, well-separated cells; FRQ is frequent, cells with little or no separation; and EMBD is embedded, cumulonimbus hidden within other cloud. EMBD deserves particular respect because an embedded thunderstorm is concealed and easy to fly into on instruments.",{"q":408,"a":409},"How is a SIGWX chart different from a surface analysis chart?","A SIGWX chart forecasts significant weather aloft, the thunderstorms, turbulence, icing and jet streams, for a level band at a fixed time. A surface analysis chart depicts the analysed surface pressure pattern, the isobars and the fronts. They are complementary: the surface chart shows the pressure pattern and fronts, and the SIGWX chart shows the airborne hazards that weather will produce.",{"name":411,"steps":412},"How to read a significant weather (SIGWX) chart",[413,414,415,416,417,418],"Note the valid time and the level band the chart covers, such as low, medium or high level.","Read the cumulonimbus and thunderstorm areas, their coverage word and their bases and tops.","Identify the turbulence and clear-air-turbulence areas and their intensity.","Identify the icing areas and their intensity.","Find the jet streams, with their core level and speed, and the boxed tropopause heights.","On a low-level chart, read the fronts and their movement, and cross-check every symbol against the legend.",[420,421,422,423,424],"A SIGWX chart is a forecast of significant en-route weather valid at a fixed time and level band.","Cumulonimbus coverage is given as ISOL, OCNL, FRQ or EMBD, and embedded CB is concealed within other cloud.","It shows turbulence, clear-air turbulence, icing, jet streams with their level and speed, and boxed tropopause heights.","It differs from a surface analysis chart, which depicts the analysed surface pressure pattern and fronts.","The chart is almost all symbols, so read the legend, and read the right level band for your flight.",{},"What a SIGWX chart forecasts, the CB, turbulence, icing and jet-stream symbols it uses, and how it differs from a surface analysis chart.",true,"\u002Flearn\u002Fsignificant-weather-sigwx-charts",[430,439,447],{"q":431,"options":432,"answer":437,"explanation":438},"What kind of information does a SIGWX chart present?",[433,434,435,436],"An observation of the current surface pressure","A forecast of significant en-route weather valid at a fixed time","A list of NOTAMs","The runway condition report",1,"A significant weather chart is a forecast of significant en-route weather, such as thunderstorms, turbulence, icing and jet streams, valid at a fixed time and level band.",{"q":440,"options":441,"answer":437,"explanation":446},"On a SIGWX chart, what do the words ISOL, OCNL, FRQ and EMBD describe?",[442,443,444,445],"Wind speeds","The coverage or nature of cumulonimbus cloud","Runway lengths","Cloud colours","ISOL (isolated), OCNL (occasional), FRQ (frequent) and EMBD (embedded) describe the coverage or nature of cumulonimbus cloud, from individual cells to CBs concealed within other cloud.",{"q":448,"options":449,"answer":437,"explanation":454},"How does a SIGWX chart differ from a surface analysis chart?",[450,451,452,453],"They are the same chart","A SIGWX chart forecasts significant en-route weather aloft, while a surface analysis depicts the analysed surface pressure pattern and fronts","A SIGWX chart shows only runways","A surface analysis is always a forecast","A SIGWX chart forecasts significant en-route weather such as CB, turbulence and icing for a level band, while a surface analysis chart depicts the analysed surface pressure pattern, isobars and fronts.",{"title":5,"description":397},[457,459,461,464],{"label":458,"url":35},"ICAO Annex 3: Meteorological Service for International Air Navigation (World Area Forecast System)",{"label":460,"url":251},"FAA Aviation Weather Handbook (FAA-H-8083-28)",{"label":462,"url":463},"UK Met Office: aviation forecast charts","https:\u002F\u002Fwww.metoffice.gov.uk\u002Fservices\u002Ftransport\u002Faviation\u002Fregulated",{"label":465,"url":245},"WMO No. 49: Technical Regulations, Volume II (meteorological service for air navigation)","learn\u002Fsignificant-weather-sigwx-charts","Briefing","OTMXOTsM6xGkZbelfWXztltgPsMT0uY5OiPoHmWWU9g",{"related":470,"newer":488,"older":495,"series":396},[471,477,483],{"path":472,"title":473,"description":474,"date":475,"topic":467,"draft":398,"minutes":476,"series":396,"seriesOrder":396},"\u002Flearn\u002Freading-an-aerodrome-chart","Reading an aerodrome chart","How an aerodrome chart maps the runways, taxiways, holding positions, hot spots and aprons, and how to use it to plan a taxi and avoid a runway incursion.","2026-06-18",7,{"path":478,"title":479,"description":480,"date":481,"topic":467,"draft":398,"minutes":482,"series":396,"seriesOrder":396},"\u002Flearn\u002Ficao-vs-iata-codes","ICAO vs IATA codes explained","The 4-letter ICAO location indicators used for flight planning and weather versus the 3-letter IATA codes on your boarding pass, plus airline codes.","2026-06-08",3,{"path":484,"title":485,"description":486,"date":487,"topic":467,"draft":398,"minutes":476,"series":396,"seriesOrder":396},"\u002Flearn\u002Ftemporary-airspace-restrictions","Temporary airspace restrictions","What temporary flight restrictions are, why they are imposed, and how the US TFR compares with the UK restricted area and emergency restriction of flying.","2026-06-07",{"path":489,"title":490,"description":491,"date":492,"topic":493,"draft":398,"minutes":494,"series":396,"seriesOrder":396},"\u002Flearn\u002Ffog-mist-and-the-dewpoint-spread","Fog, mist and the dewpoint spread","Why the gap between temperature and dewpoint predicts fog, the difference between mist and fog in a METAR, and the main fog types from radiation to advection.","2026-04-28","Weather",8,{"path":496,"title":497,"description":498,"date":499,"topic":500,"draft":398,"minutes":476,"series":396,"seriesOrder":396},"\u002Flearn\u002Flogging-night-flying-time","Logging night flying time","How night is defined for logging under the FAA and EASA, why the FAA has two nights, what civil twilight means, and how to find the night portion.","2026-04-26","Logbook",1782839405205]