[{"data":1,"prerenderedAt":398},["ShallowReactive",2],{"learn-\u002Flearn\u002Ftaf-change-groups":3,"learn-nav-\u002Flearn\u002Ftaf-change-groups":357},{"id":4,"title":5,"body":6,"date":284,"description":285,"draft":286,"extension":287,"faqs":288,"howTo":298,"keyTakeaways":307,"meta":312,"navigation":313,"path":314,"quiz":315,"seo":342,"series":343,"seriesOrder":344,"sources":345,"stem":354,"topic":355,"__hash__":356},"learn\u002Flearn\u002Ftaf-change-groups.md","TAF change groups: TEMPO, BECMG, PROB and FM",{"type":7,"value":8,"toc":274},"minimark",[9,25,31,36,52,118,122,147,151,158,162,165,169,174,177,226,229,233,259,263],[10,11,12,13,18,19,24],"p",{},"A ",[14,15,17],"a",{"href":16},"\u002Flearn\u002Fhow-to-read-a-taf","TAF"," would be far less useful if it could only describe one state of the weather. What makes it a forecast rather than a snapshot is the set of change groups bolted onto it, the small words that say when the weather will shift, whether the shift lasts, and how sure the forecaster is. Get these four right and a long ",[14,20,17],{"href":21,"className":22},"\u002Flearn\u002Fglossary#gt-taf",[23],"glossary-link"," reads cleanly.",[26,27,28],"blockquote",{},[10,29,30],{},"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.",[32,33,35],"h2",{"id":34},"the-four-change-groups","The four change groups",[10,37,38,39,45,46,51],{},"The TAF code form is defined in ",[14,40,44],{"href":41,"rel":42},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fannex-3-meteorological-service-for-international-air-navigation-1",[43],"nofollow","ICAO Annex 3"," and the ",[14,47,50],{"href":48,"rel":49},"https:\u002F\u002Flibrary.wmo.int\u002Frecords\u002Fitem\u002F35713-manual-on-codes-international-codes-volume-i-1",[43],"WMO Manual on Codes",", and there are four change indicators to know:",[53,54,55,72,82,100],"ul",{},[56,57,58,62,63,66,67,71],"li",{},[59,60,61],"strong",{},"FM"," (from): a ",[59,64,65],{},"rapid, lasting change"," at a specific time, given to the minute, as in ",[68,69,70],"code",{},"FM121500"," for 1500 UTC on the 12th. From that time, the new conditions replace the old ones entirely until the next change group.",[56,73,74,77,78,81],{},[59,75,76],{},"BECMG"," (becoming): a ",[59,79,80],{},"permanent change"," that arrives gradually during the stated period and then holds. The change period is normally short, not exceeding two hours, and in any case no more than four.",[56,83,84,87,88,91,92,95,96,99],{},[59,85,86],{},"TEMPO"," (temporary): ",[59,89,90],{},"temporary fluctuations"," that come and go, each instance lasting ",[59,93,94],{},"less than one hour"," and in aggregate covering ",[59,97,98],{},"less than half"," the period stated.",[56,101,102,105,106,109,110,113,114,117],{},[59,103,104],{},"PROB"," (probability): a ",[59,107,108],{},"30 or 40 per cent probability"," of the conditions that follow, written ",[68,111,112],{},"PROB30"," or ",[68,115,116],{},"PROB40",", and often combined with TEMPO.",[32,119,121],{"id":120},"the-probability-rule","The probability rule",[10,123,124,126,127,113,129,131,132,45,137,142,143,146],{},[68,125,104],{}," only ever appears as ",[68,128,112],{},[68,130,116],{},". As the ",[14,133,136],{"href":134,"rel":135},"https:\u002F\u002Fwww.faa.gov\u002Fair_traffic\u002Fpublications\u002Fatpubs\u002Faim_html\u002Fchap7_section_1.html",[43],"FAA AIM",[14,138,141],{"href":139,"rel":140},"https:\u002F\u002Fwww.metoffice.gov.uk\u002Fservices\u002Ftransport\u002Faviation\u002Fregulated\u002Ftraining-resources-for-aviation\u002Fmetars-and-tafs",[43],"UK Met Office"," note, a likelihood below 30 per cent is simply left out, and a likelihood of 50 per cent or more is stated as part of the main forecast rather than hedged as a probability. So ",[68,144,145],{},"PROB40 TEMPO 0820 4000 SHRA"," means a 40 per cent chance of temporary spells of 4000 metres visibility in rain showers between 0800 and 2000 UTC.",[32,148,150],{"id":149},"the-lasting-versus-temporary-distinction","The lasting-versus-temporary distinction",[10,152,153,154,157],{},"The single most important contrast is ",[59,155,156],{},"BECMG against TEMPO",". BECMG is a one-way change: once the new conditions arrive, they stay. TEMPO is a recurring blip: the prevailing conditions remain in force, and the TEMPO conditions interrupt them briefly before fading back. Reading a TEMPO as a permanent change makes the forecast look far worse than it is; reading a BECMG as temporary makes it look far better.",[32,159,161],{"id":160},"a-worked-example","A worked example",[10,163,164],{},"Here is a TAF with all four groups at work:",[166,167],"report-decoder",{"type":168},"taf",[10,170,171],{},[68,172,173],{},"TAF EGLL 121100Z 1212\u002F1318 24010KT 9999 SCT035 FM121500 27015G25KT 9999 SCT025 BECMG 1218\u002F1220 02008KT TEMPO 1220\u002F1306 6000 -RA BKN012 PROB30 1300\u002F1306 3000 BR",[10,175,176],{},"Read in order:",[53,178,179,185,188,197,206,216],{},[56,180,181,184],{},[68,182,183],{},"1212\u002F1318"," is the validity: from 1200 UTC on the 12th to 1800 UTC on the 13th.",[56,186,187],{},"The base forecast is a wind from 240 at 10 knots, more than 10 km visibility, scattered cloud at 3500 feet.",[56,189,190,192,193,196],{},[68,191,70],{}," is a clean break: ",[59,194,195],{},"from 1500 UTC"," the wind veers and freshens to 270 at 15 gusting 25 knots, cloud lowering to 2500 feet. This now replaces the base.",[56,198,199,202,203,205],{},[68,200,201],{},"BECMG 1218\u002F1220"," is a ",[59,204,80],{}," between 1800 and 2000 UTC: the wind backs to 020 at 8 knots and stays there.",[56,207,208,211,212,215],{},[68,209,210],{},"TEMPO 1220\u002F1306"," is ",[59,213,214],{},"temporary",": between 2000 UTC and 0600 the visibility may drop to 6000 metres in light rain with broken cloud at 1200 feet, in spells of under an hour, then recover.",[56,217,218,221,222,225],{},[68,219,220],{},"PROB30 1300\u002F1306"," adds a ",[59,223,224],{},"30 per cent chance"," that, between 0000 and 0600, the visibility falls further to 3000 metres in mist.",[10,227,228],{},"If you are arriving at 0400 UTC, your planning case is the BECMG wind plus the TEMPO and PROB conditions overlapping it: light wind, but a real chance of 3000 metres in mist with a low cloud base, which is what your alternate and fuel planning has to cover.",[32,230,232],{"id":231},"common-pitfalls","Common pitfalls",[53,234,235,241,247],{},[56,236,237,240],{},[59,238,239],{},"TEMPO is not the main forecast."," It interrupts the prevailing conditions; do not plan as if it holds all period, but do plan to meet it.",[56,242,243,246],{},[59,244,245],{},"PROB is never below 30 or used for a near-certainty."," Seeing only PROB30 and PROB40 is correct, not a gap in the forecast.",[56,248,249,252,253,258],{},[59,250,251],{},"All times are UTC."," As with the ",[14,254,257],{"href":255,"className":256},"\u002Flearn\u002Fglossary#gt-metar",[23],"METAR",", convert deliberately; an arrival time read in local time against UTC change groups is a classic error.",[32,260,262],{"id":261},"in-pilot-efb","In Pilot EFB",[10,264,265,266,269,270,273],{},"Pilot EFB shows the ",[59,267,268],{},"decoded"," TAF alongside the ",[59,271,272],{},"raw"," text and keeps the raw report, so you can check the plain-language change groups against the original code. A briefing you have already pulled stays readable with no signal; 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":275,"searchDepth":276,"depth":276,"links":277},"",2,[278,279,280,281,282,283],{"id":34,"depth":276,"text":35},{"id":120,"depth":276,"text":121},{"id":149,"depth":276,"text":150},{"id":160,"depth":276,"text":161},{"id":231,"depth":276,"text":232},{"id":261,"depth":276,"text":262},"2026-06-22","A deeper look at the change groups that make a TAF a forecast rather than a snapshot, with the exact meaning of FM, BECMG, TEMPO and PROB and a worked example that decodes them in order.",false,"md",[289,292,295],{"q":290,"a":291},"What is the difference between BECMG and TEMPO in a TAF?","BECMG, becoming, signals a permanent change to a new set of conditions that arrives during the stated period and then stays. TEMPO signals temporary fluctuations that come and go: each instance is expected to last less than one hour, and in total they cover less than half the period. So BECMG is a lasting change, TEMPO is a recurring blip.",{"q":293,"a":294},"What does PROB30 or PROB40 mean?","PROB is a probability group. PROB30 means a 30 per cent probability and PROB40 a 40 per cent probability that the conditions that follow will occur. Only 30 and 40 are used: a likelihood below 30 per cent is left out of the forecast, and one of 50 per cent or more is stated as the main forecast rather than as a probability. PROB is often combined with TEMPO, as PROB30 TEMPO.",{"q":296,"a":297},"How is the FM group used in a TAF?","FM, from, marks a rapid and lasting change at a specific time, given to the minute, for example FM121500 meaning from 1500 UTC on the 12th. Everything stated after an FM group replaces the previous conditions entirely from that time, until the next change group. It is used when one air mass gives way to another quickly, such as a front passing through.",{"name":299,"steps":300},"How to read the change groups in a TAF",[301,302,303,304,305,306],"Start with the base forecast after the validity period: this is the prevailing condition until a change group says otherwise.","Read an FM group as a clean break: from the stated time, the new conditions replace the old ones entirely.","Read a BECMG group as a permanent change arriving during its stated period and then holding.","Read a TEMPO group as temporary fluctuations: brief, under an hour each, covering less than half the period, then back to the prevailing conditions.","Read a PROB30 or PROB40 group as a 30 or 40 per cent chance of the conditions that follow, often attached to a TEMPO.","Always check the change times in UTC and carry the worst credible case across your time of arrival.",[308,309,310,311],"FM marks a rapid, lasting change at a stated time; everything after it replaces the previous conditions entirely.","BECMG marks a permanent change that arrives during its period and then holds.","TEMPO marks temporary fluctuations, each under an hour and together less than half the period.","PROB30 and PROB40 give a 30 or 40 per cent probability, often attached to a TEMPO; nothing lower or higher is coded as PROB.",{},true,"\u002Flearn\u002Ftaf-change-groups",[316,325,333],{"q":317,"options":318,"answer":323,"explanation":324},"What does a BECMG group signal in a TAF?",[319,320,321,322],"A temporary fluctuation that comes and goes","A permanent change that arrives during the stated period and then stays","A 40 per cent probability of a change","The end of the forecast",1,"BECMG, becoming, marks a lasting change to new conditions during the stated period; TEMPO is the one that marks temporary, recurring fluctuations.",{"q":326,"options":327,"answer":323,"explanation":332},"A TEMPO group describes conditions that last how long?",[328,329,330,331],"Permanently from the stated time","Less than one hour in each instance and under half the period in total","Exactly two hours","The whole validity period","TEMPO marks temporary fluctuations, each expected to last less than an hour and in aggregate covering less than half the period.",{"q":334,"options":335,"answer":340,"explanation":341},"What does PROB40 TEMPO mean?",[336,337,338,339],"A 40 per cent chance of temporary fluctuations of the conditions that follow","A guaranteed change at 40 minutes past the hour","Visibility of 40 kilometres","A 4 per cent probability",0,"PROB40 is a 40 per cent probability, and combined with TEMPO it means a 40 per cent chance of the temporary fluctuating conditions that follow.",{"title":5,"description":285},"decode-the-weather",6,[346,348,350,352],{"label":347,"url":41},"ICAO Annex 3: Meteorological Service for International Air Navigation",{"label":349,"url":48},"WMO No. 306: Manual on Codes, Volume I.1",{"label":351,"url":134},"FAA Aeronautical Information Manual, Chapter 7 Section 1 (Meteorology)",{"label":353,"url":139},"UK Met Office: METARs and TAFs","learn\u002Ftaf-change-groups","Weather","Ckfnth8fI7WlkCDnHJdi9-VT4e7Vhr7Xp0slN1MB-R0",{"related":358,"newer":374,"older":379,"series":383},[359,365,370],{"path":360,"title":361,"description":362,"date":284,"topic":355,"draft":286,"minutes":363,"series":364,"seriesOrder":364},"\u002Flearn\u002Fcloud-types-explained","Cloud types and what they tell a pilot","The ten cloud genera grouped into high, middle and low, the difference between heaped and layered cloud, and how to read what each type is telling you about stability, moisture and the weather ahead.",3,null,{"path":366,"title":367,"description":368,"date":284,"topic":355,"draft":286,"minutes":369,"series":364,"seriesOrder":364},"\u002Flearn\u002Fjet-stream-and-clear-air-turbulence","The jet stream and clear-air turbulence","What a jet stream is, where it sits near the tropopause, and why the wind shear around it produces clear-air turbulence that arrives with no cloud to warn you.",4,{"path":371,"title":372,"description":373,"date":284,"topic":355,"draft":286,"minutes":369,"series":364,"seriesOrder":364},"\u002Flearn\u002Fmountain-waves-and-rotor","Mountain waves and rotor","How stable air flowing over high ground sets up standing mountain waves and the violent rotor beneath them, the cloud signs that give them away, and why the turbulence and downdraughts deserve respect.",{"path":375,"title":376,"description":377,"date":284,"topic":378,"draft":286,"minutes":369,"series":364,"seriesOrder":364},"\u002Flearn\u002Fsids-and-stars-explained","SIDs and STARs explained","What standard instrument departures and standard terminal arrival routes are, why they exist, and how to read the climb gradients and crossing restrictions that turn a busy terminal area into orderly, repeatable traffic flows.","Operations",{"path":380,"title":381,"description":382,"date":284,"topic":355,"draft":286,"minutes":369,"series":364,"seriesOrder":364},"\u002Flearn\u002Ftemperature-inversions-and-stable-air","Temperature inversions and stable air","What a temperature inversion is, how it makes the air stable, and why that one fact explains haze, fog, smooth rides, trapped pollution, low cloud and the wind shear that catches pilots out.",{"slug":343,"title":384,"part":344,"total":385,"prev":386,"next":393},"Decode the weather",8,{"path":387,"title":388,"description":389,"date":390,"topic":391,"draft":286,"minutes":369,"series":343,"seriesOrder":392},"\u002Flearn\u002Fhow-to-read-a-pirep","How to read a PIREP","Decode a pilot weather report field by field, understand the UA and UUA types and the slash-coded elements, and see how a PIREP fills the gaps between weather stations.","2026-06-07","Briefing",5,{"path":394,"title":395,"description":396,"date":284,"topic":355,"draft":286,"minutes":363,"series":343,"seriesOrder":397},"\u002Flearn\u002Fturbulence-reporting-and-intensity","Turbulence reporting and intensity scales","What light, moderate, severe and extreme turbulence actually mean, how the intensity is judged by the aircraft's reaction rather than a guess, and how pilots report it so the aircraft behind benefits.",7,1782089963543]