[{"data":1,"prerenderedAt":547},["ShallowReactive",2],{"learn-\u002Flearn\u002Fpressure-systems-highs-and-lows":3,"learn-nav-\u002Flearn\u002Fpressure-systems-highs-and-lows":525},{"id":4,"title":5,"body":6,"date":460,"dateModified":461,"description":462,"draft":463,"extension":464,"faqs":465,"howTo":461,"keyTakeaways":475,"meta":481,"metaDescription":482,"navigation":483,"path":484,"quiz":485,"seo":511,"series":461,"seriesOrder":461,"sources":512,"stem":522,"topic":523,"__hash__":524},"learn\u002Flearn\u002Fpressure-systems-highs-and-lows.md","Pressure systems: highs, lows, ridges and troughs",{"type":7,"value":8,"toc":446},"minimark",[9,13,19,24,78,82,112,127,131,155,174,178,181,211,218,222,237,241,277,281,318,328,332,357,396,400,432,436],[10,11,12],"p",{},"The big patterns on a weather chart, the highs and lows with their families of ridges and troughs, set the scene for everything else. Before the fronts and the showers, it is the pressure systems that decide whether a day is settled or stormy and which way the wind will blow. Reading them is the first step in reading the weather.",[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},"pressure-isobars-and-the-map","Pressure, isobars and the map",[10,25,26,27,31,32,35,36,39,40,42,43,46,47,39,50,42,52,55,56,59,60,63,64,67,68,71,72,77],{},"A surface pressure chart draws lines of ",[28,29,30],"strong",{},"equal pressure"," called ",[28,33,34],{},"isobars",", the contour lines of the atmosphere, and the pattern they make is the pressure systems. Where the isobars close into a centre of ",[28,37,38],{},"low"," pressure you have a ",[28,41,38],{}," or ",[28,44,45],{},"depression","; where they close into a centre of ",[28,48,49],{},"high",[28,51,49],{},[28,53,54],{},"anticyclone",". The ",[28,57,58],{},"spacing"," of the isobars is as important as the shape: closely packed isobars mean a steep ",[28,61,62],{},"pressure gradient"," and ",[28,65,66],{},"strong winds",", while widely spaced isobars mean a slack gradient and ",[28,69,70],{},"light winds",". Our guide to ",[73,74,76],"a",{"href":75},"\u002Flearn\u002Freading-a-surface-analysis-chart","reading a surface analysis chart"," covers the chart itself; this guide is about what the systems on it do.",[20,79,81],{"id":80},"low-pressure-the-unsettled-half","Low pressure: the unsettled half",[10,83,84,85,87,88,91,92,95,96,99,100,103,104,108,109,111],{},"A ",[28,86,38],{}," is an area of low pressure into which surface air ",[28,89,90],{},"converges"," and is forced to ",[28,93,94],{},"rise",". Rising air cools, its moisture condenses, and the result is ",[28,97,98],{},"cloud, precipitation and unsettled, often windy weather",". Depressions are where the active weather lives, and they usually carry ",[28,101,102],{},"fronts",", the warm and cold boundaries covered in our guide to ",[73,105,107],{"href":106},"\u002Flearn\u002Fair-masses-and-fronts","air masses and fronts",", spiralling out from the centre. Because the pressure gradient around a vigorous low is steep, the winds are ",[28,110,28],{},", and the closer the isobars, the stronger they blow.",[10,113,114,115,118,119,122,123,126],{},"The air does not flow straight into the low; the Earth's rotation deflects it, so it ",[28,116,117],{},"circulates"," around the centre. In the ",[28,120,121],{},"northern hemisphere"," the wind blows ",[28,124,125],{},"anticlockwise"," around a low (clockwise in the southern), spiralling slightly inward. That rotation is why a depression appears as a great swirl of cloud on a satellite picture.",[20,128,130],{"id":129},"high-pressure-settled-but-not-always-clear","High pressure: settled, but not always clear",[10,132,84,133,135,136,139,140,143,144,147,148,151,152,154],{},[28,134,49],{}," is the opposite: an area of high pressure where air ",[28,137,138],{},"descends",", or subsides. Sinking air ",[28,141,142],{},"warms and dries",", which suppresses cloud, so a high usually brings ",[28,145,146],{},"settled weather, clear skies and light winds",", the fine, calm days of an anticyclone. The winds circulate the other way, ",[28,149,150],{},"clockwise"," around a high in the ",[28,153,121],{}," (anticlockwise in the southern), spiralling slightly outward.",[10,156,157,158,161,162,165,166,71,169,173],{},"But \"settled\" is not the same as \"clear\", and this is the trap with highs. The descending air can form an ",[28,159,160],{},"inversion"," near the surface, trapping moisture and pollution beneath it, so especially in the cooler months a high can give ",[28,163,164],{},"persistent low cloud, mist, fog and poor visibility",", the dreary spell sometimes called ",[28,167,168],{},"anticyclonic gloom",[73,170,172],{"href":171},"\u002Flearn\u002Ftemperature-inversions-and-stable-air","temperature inversions and stable air"," explains the mechanism. A high promises calm winds; it does not always promise good visibility.",[20,175,177],{"id":176},"ridges-and-troughs","Ridges and troughs",[10,179,180],{},"Between the closed centres, the isobars bulge into elongated extensions, and these have their own names and weather:",[182,183,184,199],"ul",{},[185,186,187,188,191,192,194,195,198],"li",{},"a ",[28,189,190],{},"ridge"," is an elongated extension of ",[28,193,49],{}," pressure, a finger of high reaching out from an anticyclone, and it brings a ",[28,196,197],{},"spell of fair, settled weather"," as it passes, a brief improvement;",[185,200,187,201,191,204,206,207,210],{},[28,202,203],{},"trough",[28,205,38],{}," pressure, and it brings a ",[28,208,209],{},"band of unsettled weather",", cloud, showers, or a front, as it goes through, a brief deterioration.",[10,212,213,214,217],{},"Reading a ridge or trough on the chart tells you about the ",[28,215,216],{},"timing"," of a change: a ridge building in promises a fine interlude, a trough approaching warns of a passing band of weather even without a fully formed depression.",[20,219,221],{"id":220},"cols-and-the-slack-between-systems","Cols and the slack between systems",[10,223,224,225,228,229,232,233,236],{},"Between two highs and two lows there is sometimes a region of ",[28,226,227],{},"slack pressure"," called a ",[28,230,231],{},"col",", where the isobars are widely spaced and the gradient is weak. Winds are ",[28,234,235],{},"light and variable"," in a col, and the weather is whatever the local conditions make of it: fog and low cloud can form readily in the calm, or it can simply be a quiet, featureless patch between the systems. A col is the pause between the active weather, and its lightness of wind is its defining feature.",[20,238,240],{"id":239},"wind-the-gradient-and-buys-ballots-law","Wind, the gradient and Buys Ballot's law",[10,242,243,244,247,248,251,252,255,256,262,263,266,267,269,270,255,273,276],{},"The link between the pressure pattern and the wind is direct: the wind blows roughly ",[28,245,246],{},"along the isobars",", not across them, deflected by the Earth's rotation, and its ",[28,249,250],{},"strength"," follows the ",[28,253,254],{},"gradient",", the ",[73,257,261],{"href":258,"className":259},"\u002Flearn\u002Fglossary#gt-isobar",[260],"glossary-link","isobar"," spacing. A handy rule ties it together. ",[28,264,265],{},"Buys Ballot's law"," says that in the ",[28,268,121],{},", if you stand with your ",[28,271,272],{},"back to the wind",[28,274,275],{},"low pressure is on your left"," (and the high on your right); in the southern hemisphere it is reversed. So the wind itself tells you where the systems lie, and the chart's isobars tell you how hard it will blow.",[20,278,280],{"id":279},"systems-move-and-develop","Systems move and develop",[10,282,283,284,287,288,290,291,294,295,298,299,290,301,304,305,308,309,312,313,317],{},"A pressure chart is a ",[28,285,286],{},"snapshot",", and the systems on it are alive: they move, and they grow and decay. A ",[28,289,38],{}," can ",[28,292,293],{},"deepen"," (its central pressure falling, its winds and weather intensifying) or ",[28,296,297],{},"fill"," (weakening and dying), while a ",[28,300,49],{},[28,302,303],{},"build"," (strengthening and expanding) or ",[28,306,307],{},"decline",". They are steered by the flow in the ",[28,310,311],{},"upper atmosphere",", the broad pattern aloft and the ",[73,314,316],{"href":315},"\u002Flearn\u002Fjet-stream-and-clear-air-turbulence","jet stream",", which acts as a kind of conveyor for the surface systems beneath it.",[10,319,320,321,323,324,327],{},"For a pilot, this means the ",[28,322,216],{}," matters as much as the pattern. A low that is deepening and racing toward you is a very different proposition from one that is filling and slipping away, even if the snapshot looks similar, and a ridge building in promises a longer fine spell than one already breaking down. Reading a sequence of charts, not just one, tells you which way the systems are ",[28,325,326],{},"trending",", so you can judge whether the weather on your route will be improving or deteriorating by the time you get there. The forecast charts add the missing dimension to the snapshot: where the systems will be, and how strong, at the hour you fly.",[20,329,331],{"id":330},"a-worked-example","A worked example",[10,333,334,335,338,339,342,343,346,347,349,350,352,353,356],{},"You compare two charts. On the first, a ",[28,336,337],{},"deep low"," sits to your northwest with ",[28,340,341],{},"tightly packed isobars"," sweeping across your route and a ",[28,344,345],{},"cold front"," trailing from it. You read the picture at once: ",[28,348,66],{}," circulating ",[28,351,125],{}," around the low, ",[28,354,355],{},"unsettled weather"," with the front's cloud and rain, and, by Buys Ballot's law, with your back to the strong southwesterly the low is on your left where the chart shows it. It is a day for strong-wind and frontal-weather planning.",[10,358,359,360,362,363,366,367,369,370,372,373,376,377,379,380,383,384,386,387,389,390,395],{},"On the second chart a broad ",[28,361,49],{}," sits over your area with ",[28,364,365],{},"widely spaced isobars",". You read ",[28,368,70],{},", circulating ",[28,371,150],{}," around the high, and ",[28,374,375],{},"settled"," conditions, but because it is a cool, damp season you flag the risk of an ",[28,378,160],{}," trapping ",[28,381,382],{},"fog and low cloud"," beneath the subsiding air, the anticyclonic-gloom trap. A ",[28,385,190],{}," extending from the high promises a fair interlude, while a ",[28,388,203],{}," marked further out warns of a band of weather to come. From two pressure patterns you have read the wind, the weather and the timing of the changes, before looking at a single ",[73,391,394],{"href":392,"className":393},"\u002Flearn\u002Fglossary#gt-metar",[260],"METAR",".",[20,397,399],{"id":398},"common-pitfalls","Common pitfalls",[182,401,402,408,414,420,426],{},[185,403,404,407],{},[28,405,406],{},"Equating high pressure with good flying weather."," A high is settled but can trap fog and low cloud under an inversion.",[185,409,410,413],{},[28,411,412],{},"Ignoring the isobar spacing."," The gradient, not just the system, sets the wind strength; tight isobars mean strong winds.",[185,415,416,419],{},[28,417,418],{},"Getting the circulation backwards."," In the northern hemisphere it is anticlockwise around a low and clockwise around a high, reversed in the south.",[185,421,422,425],{},[28,423,424],{},"Overlooking ridges and troughs."," They mark the brief improvements and deteriorations between the main systems.",[185,427,428,431],{},[28,429,430],{},"Forgetting the col."," Slack pressure means light, variable winds and a tendency to fog and low cloud in the calm.",[20,433,435],{"id":434},"in-pilot-efb","In Pilot EFB",[10,437,438,439,255,442,445],{},"Pilot EFB is a study and planning companion that helps you make sense of the pressure pattern behind a forecast, alongside the decoded ",[73,440,394],{"href":441},"\u002Flearn\u002Fhow-to-read-a-metar",[73,443,444],{"href":75},"surface analysis chart"," and the rest of your briefing in one offline-first place. A briefing you have already pulled stays readable away from a signal, and pulling fresh weather needs a connection. Pilot EFB does not forecast the weather and 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":447,"searchDepth":448,"depth":448,"links":449},"",2,[450,451,452,453,454,455,456,457,458,459],{"id":22,"depth":448,"text":23},{"id":80,"depth":448,"text":81},{"id":129,"depth":448,"text":130},{"id":176,"depth":448,"text":177},{"id":220,"depth":448,"text":221},{"id":239,"depth":448,"text":240},{"id":279,"depth":448,"text":280},{"id":330,"depth":448,"text":331},{"id":398,"depth":448,"text":399},{"id":434,"depth":448,"text":435},"2026-06-09",null,"What highs, lows, ridges and troughs are, the weather each brings, which way the wind circulates around them, and how to read them on a pressure chart.",false,"md",[466,469,472],{"q":467,"a":468},"What weather does a low-pressure system bring?","A low, or depression, is an area of low pressure into which air converges and rises. Rising air cools and condenses, so a low typically brings cloud, precipitation and unsettled, often windy weather, frequently with fronts attached. Closely spaced isobars around a low mean a strong pressure gradient and strong winds. It is the active, changeable half of the pressure pattern.",{"q":470,"a":471},"Why can a high-pressure system still bring poor weather?","A high, or anticyclone, brings descending air, which usually means settled weather, clear skies and light winds. But the sinking air can also trap moisture and form an inversion near the surface, so in the cooler months a high can give persistent low cloud, mist, fog and poor visibility, sometimes called anticyclonic gloom. So a high is usually settled, but settled does not always mean clear.",{"q":473,"a":474},"What is the difference between a ridge and a trough?","A ridge is an elongated extension of high pressure, a finger of high reaching out, and it brings a spell of fair, settled weather as it passes. A trough is an elongated extension of low pressure, and it brings a band of unsettled weather, often cloud, showers or a front, as it goes through. A ridge is a brief improvement; a trough is a brief deterioration.",[476,477,478,479,480],"Isobar spacing shows the pressure gradient: closely spaced isobars mean strong winds.","In a low, air converges and rises, bringing cloud, precipitation and unsettled weather; in a high, air descends, usually settled but able to trap fog and low cloud.","In the northern hemisphere wind circulates anticlockwise around a low and clockwise around a high, reversed in the south.","A ridge is an extension of high pressure bringing fair weather; a trough is an extension of low pressure bringing unsettled weather.","By Buys Ballot's law in the northern hemisphere, with your back to the wind the low is on your left.",{},"What highs, lows, ridges and troughs are, the weather each brings, the wind circulation around them, and how to read them on a chart.",true,"\u002Flearn\u002Fpressure-systems-highs-and-lows",[486,495,503],{"q":487,"options":488,"answer":493,"explanation":494},"In a low-pressure system, what is the air doing, and what weather does that bring?",[489,490,491,492],"Sinking, bringing clear settled skies","Converging and rising, bringing cloud, precipitation and unsettled weather","Standing still, bringing no weather","Flowing straight out, bringing strong heat",1,"In a low, air converges and rises; rising air cools and condenses, so a low brings cloud, precipitation and unsettled, often windy weather, frequently with fronts.",{"q":496,"options":497,"answer":493,"explanation":502},"What does the spacing of the isobars tell you?",[498,499,500,501],"The temperature","The strength of the wind, with closely spaced isobars meaning a strong gradient and strong winds","The cloud type","The time of day","The isobar spacing shows the pressure gradient. Closely spaced isobars mean a steep gradient and strong winds; widely spaced isobars mean light winds.",{"q":504,"options":505,"answer":493,"explanation":510},"Using Buys Ballot's law in the northern hemisphere, if you stand with your back to the wind, where is the low pressure?",[506,507,508,509],"On your right","On your left","Directly ahead","Directly behind","In the northern hemisphere, with your back to the wind, low pressure is on your left. This is Buys Ballot's law, and it is reversed in the southern hemisphere.",{"title":5,"description":462},[513,516,519],{"label":514,"url":515},"FAA Aviation Weather Handbook (FAA-H-8083-28)","https:\u002F\u002Fwww.faa.gov\u002Fregulationspolicies\u002Fhandbooksmanuals\u002Faviation\u002Ffaa-h-8083-28-aviation-weather-handbook",{"label":517,"url":518},"UK Met Office: high and low pressure","https:\u002F\u002Fwww.metoffice.gov.uk\u002Fweather\u002Flearn-about\u002Fweather\u002Fhow-weather-works\u002Fhigh-and-low-pressure",{"label":520,"url":521},"ICAO Doc 8896: Manual of Aeronautical Meteorological Practice","https:\u002F\u002Fstore.icao.int\u002Fen\u002Fmanual-of-aeronautical-meteorological-practice-doc-8896","learn\u002Fpressure-systems-highs-and-lows","Weather","oqGIcuMfLi1fTSP1ldwFrbb-CEvR3wjSDBKDiMNWSpg",{"related":526,"newer":532,"older":541,"series":461},[527,532,536],{"path":171,"title":528,"description":529,"date":530,"topic":523,"draft":463,"minutes":531,"series":461,"seriesOrder":461},"Temperature inversions and stable air","What a temperature inversion is, how it makes the air stable, and why that explains haze, fog, smooth rides, trapped pollution, low cloud and wind shear.","2026-06-12",4,{"path":533,"title":534,"description":535,"date":460,"topic":523,"draft":463,"minutes":531,"series":461,"seriesOrder":461},"\u002Flearn\u002Fmountain-waves-and-rotor","Mountain waves and rotor","How stable air over high ground sets up mountain waves and the violent rotor beneath, the cloud signs that reveal them, and why they deserve respect.",{"path":75,"title":537,"description":538,"date":539,"topic":523,"draft":463,"minutes":540,"series":461,"seriesOrder":461},"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 the big picture before the METAR.","2026-06-06",3,{"path":542,"title":543,"description":544,"date":545,"topic":546,"draft":463,"minutes":540,"series":461,"seriesOrder":461},"\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","Briefing",1782839402969]