[{"data":1,"prerenderedAt":339},["ShallowReactive",2],{"learn-\u002Flearn\u002Fthe-international-standard-atmosphere":3,"learn-nav-\u002Flearn\u002Fthe-international-standard-atmosphere":310},{"id":4,"title":5,"body":6,"date":240,"description":241,"draft":242,"extension":243,"faqs":244,"howTo":254,"keyTakeaways":261,"meta":266,"navigation":267,"path":268,"quiz":269,"seo":293,"series":294,"seriesOrder":294,"sources":295,"stem":307,"topic":308,"__hash__":309},"learn\u002Flearn\u002Fthe-international-standard-atmosphere.md","The International Standard Atmosphere (ISA)",{"type":7,"value":8,"toc":230},"minimark",[9,13,19,24,35,49,53,74,96,99,103,122,125,129,144,147,176,195,199,219,223],[10,11,12],"p",{},"The International Standard Atmosphere is the shared yardstick behind almost every altitude, speed and performance figure a pilot uses, and knowing what it actually says makes the day's corrections far easier to reason about.",[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},"a-model-not-the-weather","A model, not the weather",[10,25,26,27,34],{},"The ",[28,29,33],"a",{"href":30,"rel":31},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fmanual-of-the-icao-standard-atmosphere-extended-to-80-kilometres-262500-feet-doc-7488",[32],"nofollow","International Standard Atmosphere (ISA)"," is a defined, idealised model of how pressure, temperature and density change with height. It is not a forecast and not an average of any real day. It exists so that altimeters, airspeed indicators, performance charts and engineering data all use one agreed reference, instead of every manufacturer and authority choosing their own.",[10,36,37,38,42,43,48],{},"ICAO publishes it as ",[28,39,41],{"href":30,"rel":40},[32],"Doc 7488",", and the same model underpins the performance reference conditions in ",[28,44,47],{"href":45,"rel":46},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fannex-8-airworthiness-of-aircraft",[32],"ICAO Annex 8",". Because it is a model, the real atmosphere almost never matches it exactly, and the difference between the two is the whole reason corrections exist.",[20,50,52],{"id":51},"the-sea-level-values","The sea-level values",[10,54,26,55,61,62,67,68,73],{},[28,56,60],{"href":57,"className":58},"\u002Flearn\u002Fglossary#gt-isa",[59],"glossary-link","ISA"," fixes mean sea level at three reference values, described in the ",[28,63,66],{"href":64,"rel":65},"https:\u002F\u002Fwww.faa.gov\u002Fregulations_policies\u002Fhandbooks_manuals\u002Faviation\u002Fphak",[32],"FAA Pilot's Handbook of Aeronautical Knowledge"," and the ",[28,69,72],{"href":70,"rel":71},"https:\u002F\u002Fwww.faa.gov\u002Fregulationspolicies\u002Fhandbooksmanuals\u002Faviation\u002Ffaa-h-8083-28b-aviation-weather-handbook",[32],"Aviation Weather Handbook",":",[75,76,77,85,91],"ul",{},[78,79,80,84],"li",{},[81,82,83],"strong",{},"Temperature: 15 degrees Celsius"," (59 degrees Fahrenheit).",[78,86,87,90],{},[81,88,89],{},"Pressure: 1013.25 hectopascals"," (29.92 inches of mercury).",[78,92,93],{},[81,94,95],{},"Density: about 1.225 kilograms per cubic metre.",[10,97,98],{},"These are the numbers behind a standard altimeter setting of 1013 hPa or 29.92 inHg, and behind the \"standard day\" assumption printed on most performance charts.",[20,100,102],{"id":101},"the-lapse-rate-and-the-tropopause","The lapse rate and the tropopause",[10,104,105,106,109,110,113,114,117,118,121],{},"From sea level upward, the standard temperature falls at a steady ",[81,107,108],{},"lapse rate of about 1.98 degrees Celsius per 1000 feet",", which almost everyone rounds to ",[81,111,112],{},"2 degrees per 1000 feet"," for mental maths. That holds up to the ",[81,115,116],{},"standard tropopause at about 36 090 feet"," (11 km), where the standard temperature reaches about ",[81,119,120],{},"minus 56.5 degrees Celsius",".",[10,123,124],{},"Above the tropopause, in the standard lower stratosphere, the model holds the temperature constant at that minus 56.5 degrees rather than continuing to fall. So the simple \"2 degrees per 1000 feet\" rule only applies in the troposphere, below roughly 36 000 feet.",[20,126,128],{"id":127},"isa-deviation-the-number-that-actually-matters","ISA deviation: the number that actually matters",[10,130,131,132,135,136,139,140,143],{},"On a real day the air is rarely standard, and the useful quantity is the ",[81,133,134],{},"ISA deviation",": the actual temperature minus the standard temperature at the same level. It is written as ",[81,137,138],{},"ISA plus"," or ",[81,141,142],{},"ISA minus"," a number of degrees.",[10,145,146],{},"A worked example, using the rounded lapse rate:",[75,148,149,155,166],{},[78,150,151,152,121],{},"At 10 000 feet, the standard temperature is 15 minus (10 times 2), or ",[81,153,154],{},"minus 5 degrees Celsius",[78,156,157,158,161,162,165],{},"If the actual outside air temperature is ",[81,159,160],{},"minus 15",", you are ",[81,163,164],{},"ISA minus 10"," (10 degrees colder than standard).",[78,167,168,169,161,172,175],{},"If it is ",[81,170,171],{},"plus 5",[81,173,174],{},"ISA plus 10"," (10 degrees warmer than standard).",[10,177,178,179,183,184,188,189,194],{},"That single figure feeds straight into the corrections pilots care about. Warm air (ISA positive) is less dense, which raises ",[28,180,182],{"href":181},"\u002Flearn\u002Fdensity-altitude-and-aircraft-performance","density altitude"," and stretches takeoff and climb performance. Cold air (ISA negative) makes the altimeter over-read true altitude, which is why ",[28,185,187],{"href":186},"\u002Flearn\u002Faltimetry-qnh-qfe-qne","cold-temperature altitude corrections"," exist. ISA deviation also appears in ",[28,190,193],{"href":191,"className":192},"\u002Flearn\u002Fglossary#gt-true-airspeed",[59],"true airspeed"," and Mach calculations, because the speed of sound depends on temperature.",[20,196,198],{"id":197},"common-pitfalls","Common pitfalls",[75,200,201,207,213],{},[78,202,203,206],{},[81,204,205],{},"The 2-degrees rule stops at the tropopause."," Above about 36 090 feet the standard temperature is held constant, so do not keep subtracting.",[78,208,209,212],{},[81,210,211],{},"ISA is not an average day."," It is a fixed reference. Treating it as \"normal weather\" leads to under-correcting on genuinely hot, cold or high days.",[78,214,215,218],{},[81,216,217],{},"Mind the sign."," ISA deviation is actual minus standard. Warmer than standard is ISA plus; colder is ISA minus.",[20,220,222],{"id":221},"in-pilot-efb","In Pilot EFB",[10,224,225,226,229],{},"Pilot EFB includes the unit and altitude conversions and the ",[28,227,228],{"href":181},"density-altitude"," and cold-temperature corrections that all lean on the standard atmosphere, with the working shown so you can follow each step rather than trust a single output. The figures are for your own planning and reference; Pilot EFB is offline-first and is not a certified Electronic Flight Bag, so verify against your aircraft's approved data and current official sources before you act on them.",{"title":231,"searchDepth":232,"depth":232,"links":233},"",2,[234,235,236,237,238,239],{"id":22,"depth":232,"text":23},{"id":51,"depth":232,"text":52},{"id":101,"depth":232,"text":102},{"id":127,"depth":232,"text":128},{"id":197,"depth":232,"text":198},{"id":221,"depth":232,"text":222},"2026-06-20","What the ICAO International Standard Atmosphere is, its sea-level values and lapse rate, and how ISA deviation underpins altimetry, performance and density altitude.",false,"md",[245,248,251],{"q":246,"a":247},"What are the ISA sea-level values?","The International Standard Atmosphere defines mean sea level as 15 degrees Celsius, a pressure of 1013.25 hectopascals (29.92 inches of mercury), and a density of about 1.225 kilograms per cubic metre. These are reference values, not a forecast of the air on any given day.",{"q":249,"a":250},"What is the ISA temperature lapse rate?","In the standard troposphere the temperature falls by about 1.98 degrees Celsius per 1000 feet, almost always rounded to 2 degrees per 1000 feet for mental maths. This continues up to the standard tropopause at about 36 090 feet (11 km), above which the standard temperature stays constant at about minus 56.5 degrees Celsius.",{"q":252,"a":253},"What does 'ISA plus 10' mean?","It means the actual temperature is 10 degrees Celsius warmer than the standard atmosphere predicts for that altitude. ISA deviation is the difference between the real temperature and the ISA value at the same level, and it drives corrections for true altitude, true airspeed and aircraft performance.",{"name":255,"steps":256},"Work out the ISA temperature and your ISA deviation at an altitude",[257,258,259,260],"Start from the ISA sea-level temperature of 15 degrees Celsius.","Subtract 2 degrees for every 1000 feet of altitude (the rounded standard lapse rate), valid up to about 36 090 feet.","That figure is the standard temperature for the altitude, for example 15 minus 20 equals minus 5 degrees at 10 000 feet.","Compare it with the actual outside air temperature: the difference is your ISA deviation, written as ISA plus or ISA minus a number of degrees.",[262,263,264,265],"The ISA is a fixed reference atmosphere: 15 degrees Celsius, 1013.25 hectopascals and about 1.225 kilograms per cubic metre at mean sea level.","Standard temperature falls about 2 degrees Celsius per 1000 feet up to roughly 36 090 feet, then holds near minus 56.5 degrees in the standard stratosphere.","ISA deviation, the gap between actual and standard temperature, is what altimetry, true airspeed and performance corrections are built on.","It is a model, not a measurement: the real atmosphere is rarely standard, which is exactly why the corrections matter.",{},true,"\u002Flearn\u002Fthe-international-standard-atmosphere",[270,279,287],{"q":271,"options":272,"answer":277,"explanation":278},"What is the ISA temperature at mean sea level?",[273,274,275,276],"0 degrees Celsius","15 degrees Celsius","25 degrees Celsius","It depends on the day",1,"The International Standard Atmosphere fixes mean sea level at 15 degrees Celsius, 1013.25 hectopascals and about 1.225 kilograms per cubic metre.",{"q":280,"options":281,"answer":277,"explanation":286},"Roughly how much does standard temperature fall per 1000 feet in the troposphere?",[282,283,284,285],"About 0.5 degrees Celsius","About 2 degrees Celsius","About 5 degrees Celsius","It rises with height","The standard lapse rate is about 1.98 degrees Celsius per 1000 feet, rounded to 2 for mental maths, up to the standard tropopause near 36 090 feet.",{"q":288,"options":289,"answer":277,"explanation":292},"If the standard temperature at your level is minus 5 and the actual temperature is plus 5, what is your ISA deviation?",[164,174,290,291],"ISA plus 5","Standard, no deviation","ISA deviation is actual minus standard. Plus 5 against a standard of minus 5 is 10 degrees warmer than standard, so ISA plus 10.",{"title":5,"description":241},null,[296,298,300,302,305],{"label":297,"url":30},"ICAO Doc 7488: Manual of the ICAO Standard Atmosphere",{"label":299,"url":64},"FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C)",{"label":301,"url":70},"FAA Aviation Weather Handbook (FAA-H-8083-28B)",{"label":303,"url":304},"Met Office: aviation weather services","https:\u002F\u002Fwww.metoffice.gov.uk\u002Fservices\u002Ftransport\u002Faviation\u002Fregulated\u002Fnational-aviation\u002Findex",{"label":306,"url":45},"ICAO Annex 8: Airworthiness of Aircraft (performance reference conditions)","learn\u002Fthe-international-standard-atmosphere","Weather","Gj6ZgkAhwoAjqzt8F3c38UyN-27A30mVR4aQ-tvIGRU",{"related":311,"newer":330,"older":335,"series":294},[312,317,325],{"path":313,"title":314,"description":315,"date":240,"topic":308,"draft":242,"minutes":316,"series":294,"seriesOrder":294},"\u002Flearn\u002Freading-a-surface-analysis-chart","How to read a surface analysis chart","Decode a surface analysis chart: isobars and the pressure gradient, highs and lows, warm, cold and occluded fronts, and what the big picture tells you before you read the METAR.",3,{"path":318,"title":319,"description":320,"date":321,"topic":308,"draft":242,"minutes":322,"series":323,"seriesOrder":324},"\u002Flearn\u002Fhow-to-read-a-sigmet-and-airmet","How to read a SIGMET and an AIRMET","What SIGMETs and AIRMETs warn of, the phenomena codes, how long each stays valid, and a worked SIGMET decoded field by field, with the ICAO baseline and the US differences.","2026-06-18",5,"decode-the-weather",4,{"path":326,"title":327,"description":328,"date":329,"topic":308,"draft":242,"minutes":322,"series":323,"seriesOrder":277},"\u002Flearn\u002Fhow-to-read-a-metar","How to read a METAR","A plain-language guide to decoding a METAR field by field, with a worked example and the EASA\u002FUK and FAA differences that trip pilots up.","2026-06-17",{"path":331,"title":332,"description":333,"date":240,"topic":334,"draft":242,"minutes":316,"series":294,"seriesOrder":294},"\u002Flearn\u002Fsquawk-codes-and-transponders","Squawk codes and transponders explained","What a transponder squawk code is, the difference between Mode A, C and S, and the emergency codes 7500, 7600 and 7700 every pilot must know.","Operations",{"path":336,"title":337,"description":338,"date":240,"topic":334,"draft":242,"minutes":316,"series":294,"seriesOrder":294},"\u002Flearn\u002Fwake-turbulence-categories-and-separation","Wake turbulence categories and separation","Why aircraft trail wingtip vortices, the ICAO Light, Medium, Heavy and Super categories by maximum take-off mass, and how wake separation keeps a following aircraft clear.",1781989192075]