[{"data":1,"prerenderedAt":282},["ShallowReactive",2],{"learn-\u002Flearn\u002Fmountain-waves-and-rotor":3,"learn-nav-\u002Flearn\u002Fmountain-waves-and-rotor":254},{"id":4,"title":5,"body":6,"date":194,"description":195,"draft":196,"extension":197,"faqs":198,"howTo":208,"keyTakeaways":209,"meta":214,"navigation":215,"path":216,"quiz":217,"seo":243,"series":208,"seriesOrder":208,"sources":244,"stem":251,"topic":252,"__hash__":253},"learn\u002Flearn\u002Fmountain-waves-and-rotor.md","Mountain waves and rotor",{"type":7,"value":8,"toc":184},"minimark",[9,13,19,24,36,53,78,81,85,102,106,109,131,138,142,145,148,152,172,176],[10,11,12],"p",{},"A range of hills in a strong wind does not just block the air; it sets the whole atmosphere downwind ringing like a struck bell. The result, mountain waves and the rotor beneath them, can throw a light aircraft around hard, and it is one of the few hazards where the cloud, when there is any, tells you exactly what is going on.",[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},"how-a-wave-forms","How a wave forms",[10,25,26,27,31,32,35],{},"When wind meets a ridge, the air is forced up and over. If the air is ",[28,29,30],"strong",{},"stable",", it does not keep rising once it clears the top; instead it sinks back down the lee side, overshoots its level, and bounces back up again, setting off a train of ",[28,33,34],{},"standing waves"," that march downwind from the ridge. They are called standing waves because the wave pattern stays fixed relative to the ground while the air pours through it.",[10,37,38,39,46,47,52],{},"The ",[40,41,45],"a",{"href":42,"rel":43},"https:\u002F\u002Fwww.faa.gov\u002Fregulationspolicies\u002Fhandbooksmanuals\u002Faviation\u002Ffaa-h-8083-28b-aviation-weather-handbook",[44],"nofollow","FAA Aviation Weather Handbook"," and ",[40,48,51],{"href":49,"rel":50},"https:\u002F\u002Fskybrary.aero\u002Farticles\u002Fmountain-waves",[44],"SKYbrary"," set out the three ingredients that have to come together:",[54,55,56,64,71],"ul",{},[57,58,59,60,63],"li",{},"a wind blowing ",[28,61,62],{},"roughly perpendicular to the ridge",", within about 30 degrees of square on;",[57,65,66,67,70],{},"a wind speed at ridge level that is ",[28,68,69],{},"reasonably strong and increases with height",";",[57,72,73,74,77],{},"a ",[28,75,76],{},"stable layer"," of air that resists the vertical motion.",[10,79,80],{},"When all three are present, the waves can extend tens of miles downwind and reach far above the ridge top, sometimes into the lower stratosphere.",[20,82,84],{"id":83},"rotor-the-part-that-hurts","Rotor: the part that hurts",[10,86,87,88,91,92,97,98,101],{},"Under the crest of each wave, close to the ground, the airflow rolls over on itself into a churning, horizontal-axis circulation called ",[28,89,90],{},"rotor",". As the ",[40,93,96],{"href":94,"rel":95},"https:\u002F\u002Fwww.faa.gov\u002Fregulations_policies\u002Fhandbooks_manuals\u002Faviation\u002Fphak",[44],"FAA Pilot's Handbook"," describes, rotor is where the wave system produces its worst turbulence, frequently ",[28,99,100],{},"severe or extreme",", together with strong and rapidly shifting up and down draughts. It is the most dangerous element precisely because it sits at low level, on the lee side of high ground, where an aircraft has the least height and the least performance in reserve.",[20,103,105],{"id":104},"reading-the-clouds","Reading the clouds",[10,107,108],{},"When the air is moist enough, the wave system advertises itself:",[54,110,111,118,124],{},[57,112,113,114,117],{},"smooth, lens-shaped ",[28,115,116],{},"lenticular clouds"," (altocumulus lenticularis) sit at the wave crests, stacked like a pile of plates;",[57,119,73,120,123],{},[28,121,122],{},"cap cloud"," can drape over the ridge itself;",[57,125,126,127,130],{},"a ragged, turbulent-looking ",[28,128,129],{},"rotor cloud"," forms in the rotor below the crests.",[10,132,133,134,137],{},"These clouds look frozen in place because they form continually on the upwind side and dissipate on the downwind side as air streams through them. The trap, as the handbook warns, is that in ",[28,135,136],{},"dry air the waves can be present with no cloud at all",", so a strong cross-ridge wind on a clear day still deserves the same caution.",[20,139,141],{"id":140},"a-worked-example","A worked example",[10,143,144],{},"You are routing downwind of a 4000-foot ridge with a wind of 30 knots blowing almost straight across it and increasing with height, under a stable inversion. Stacked lenticular clouds sit at around 8000 feet and a ragged band of cloud hangs low on the lee side.",[10,146,147],{},"Every ingredient for a mountain wave is present. The smart plan is to cross the high ground with plenty of margin, climbing to give yourself height before you reach the ridge, and to expect strong sink on the lee side that can exceed a light aircraft's climb rate. The low ragged cloud marks the rotor, so a low-level transit beneath the wave, tempting because it looks calmer, is exactly where the worst turbulence lives. If you cannot out-climb the downdraught, turning back toward the rising air on the upwind side is safer than pressing on into sinking air over rising ground.",[20,149,151],{"id":150},"common-pitfalls","Common pitfalls",[54,153,154,160,166],{},[57,155,156,159],{},[28,157,158],{},"No cloud is not no wave."," In dry air the waves and rotor are invisible; judge by the wind and the terrain, not only by what you can see.",[57,161,162,165],{},[28,163,164],{},"The downdraught can beat your climb."," Lee-side sink in a strong wave can exceed a light aircraft's best rate of climb, so plan height in hand before you commit.",[57,167,168,171],{},[28,169,170],{},"Rotor is lowest and worst."," Do not duck low to escape turbulence near high ground in a strong cross-ridge wind; that is where rotor is.",[20,173,175],{"id":174},"in-pilot-efb","In Pilot EFB",[10,177,178,179,183],{},"Pilot EFB helps you read the ",[40,180,182],{"href":181},"\u002Flearn\u002Freading-winds-and-temperatures-aloft","winds and temperatures aloft"," and the forecasts and pilot reports that flag wave activity over high ground, so a strong cross-ridge wind is a planned-for factor rather than a surprise on the lee side. A briefing you have already pulled stays readable with no signal; fetching fresh forecasts 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":185,"searchDepth":186,"depth":186,"links":187},"",2,[188,189,190,191,192,193],{"id":22,"depth":186,"text":23},{"id":83,"depth":186,"text":84},{"id":104,"depth":186,"text":105},{"id":140,"depth":186,"text":141},{"id":150,"depth":186,"text":151},{"id":174,"depth":186,"text":175},"2026-06-22","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.",false,"md",[199,202,205],{"q":200,"a":201},"What conditions create a mountain wave?","Three ingredients together: a wind blowing roughly perpendicular to the ridge, within about 30 degrees of square on, a wind speed at ridge level that is reasonably strong and increases with height, and a stable layer of air that resists being lifted. When all three are present the air, having been pushed up over the ridge, oscillates up and down downwind in a series of standing waves.",{"q":203,"a":204},"What is rotor and why is it dangerous?","Rotor is a zone of churning, rolling air that forms beneath the crests of mountain waves, often marked by a ragged rotor cloud that appears to stay in place while the air boils through it. It can produce severe or extreme turbulence and strong, shifting up and down draughts at low level, which is exactly where an aircraft has least height to recover, so it is one of the most hazardous parts of the wave system.",{"q":206,"a":207},"What clouds signal a mountain wave?","Smooth, lens-shaped lenticular clouds (altocumulus lenticularis) mark the wave crests, a cap cloud can sit over the ridge itself, and a ragged rotor cloud forms in the turbulent rotor below. These clouds look stationary because they continually form on the upwind side and dissipate on the downwind side as air flows through them. In dry air the waves can be present with no cloud at all.",null,[210,211,212,213],"Mountain waves form when a reasonably strong wind blows roughly across a ridge, increasing with height, through a stable layer.","Rotor is the churning turbulence beneath the wave crests and is the most hazardous part, because it is severe and at low level.","Lenticular clouds mark the crests, a cap cloud sits over the ridge and a rotor cloud marks the rotor; in dry air the wave can be invisible.","Waves can reach far above the ridge and downwind, with strong updraughts and downdraughts, so give high ground a wide and high margin in a strong cross-ridge wind.",{},true,"\u002Flearn\u002Fmountain-waves-and-rotor",[218,227,235],{"q":219,"options":220,"answer":225,"explanation":226},"Which set of conditions is most likely to set up mountain waves?",[221,222,223,224],"Light wind along the ridge and unstable air","Strong wind roughly across the ridge, increasing with height, in stable air","Calm wind and a deep convective layer","Any wind direction as long as the air is unstable",1,"Mountain waves need a reasonably strong wind blowing roughly perpendicular to the ridge that increases with height, together with a stable layer that resists the lifting.",{"q":228,"options":229,"answer":225,"explanation":234},"What is rotor in a mountain-wave system?",[230,231,232,233],"The smooth lens-shaped cloud at the wave crest","A churning zone of turbulence beneath the wave crests, often with a ragged rotor cloud","The steady downslope wind on the lee side","The jet stream above the tropopause","Rotor is the rolling, turbulent air that forms below the wave crests, frequently severe or extreme and dangerous because it is at low level.",{"q":236,"options":237,"answer":225,"explanation":242},"Why do lenticular and rotor clouds appear to stay in one place?",[238,239,240,241],"They are frozen to the mountain top","Air flows through them, forming on the upwind side and dissipating downwind, so the cloud holds its position","They are too cold to move","They are actually moving fast but look still","Wave clouds form continually on the upwind side and dissipate on the downwind side as air flows through, so the cloud appears stationary while the air keeps moving.",{"title":5,"description":195},[245,247,249],{"label":246,"url":42},"FAA Aviation Weather Handbook (FAA-H-8083-28B)",{"label":248,"url":94},"FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C)",{"label":250,"url":49},"SKYbrary: Mountain Waves","learn\u002Fmountain-waves-and-rotor","Weather","RDdiR_rUQQ1ZrtOqhpVdKDscslAf2cTFLcA9CW41bLw",{"related":255,"newer":272,"older":278,"series":208},[256,261,266],{"path":257,"title":258,"description":259,"date":194,"topic":252,"draft":196,"minutes":260,"series":208,"seriesOrder":208},"\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,{"path":262,"title":263,"description":264,"date":194,"topic":252,"draft":196,"minutes":265,"series":208,"seriesOrder":208},"\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":267,"title":268,"description":269,"date":194,"topic":252,"draft":196,"minutes":265,"series":270,"seriesOrder":271},"\u002Flearn\u002Ftaf-change-groups","TAF change groups: TEMPO, BECMG, PROB and FM","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.","decode-the-weather",6,{"path":273,"title":274,"description":275,"date":194,"topic":276,"draft":196,"minutes":260,"series":277,"seriesOrder":271},"\u002Flearn\u002Fminimum-safe-altitudes-msa-mora-mea-moca","Minimum safe altitudes: MSA, MORA, MEA, MOCA and MEF","The family of minimum altitudes that keep you clear of terrain and obstacles, what each one guarantees, and the difference between an altitude that also promises navigation signal and a chart figure that only tells you the highest obstacle.","Operations","plan-a-vfr-cross-country",{"path":279,"title":280,"description":281,"date":194,"topic":276,"draft":196,"minutes":265,"series":208,"seriesOrder":208},"\u002Flearn\u002Freading-an-instrument-approach-chart","Reading an instrument approach chart","How an instrument approach chart is laid out, what the plan view, profile view, minimums box and missed approach each tell you, and how to read one in order so you brief the approach the way you will fly it.",1782089963539]