The previous lesson built winds from forces; this one watches them at work at three scales. At the largest, the whole planet runs a standing circulation of three cells per hemisphere. At the smallest, an afternoon of sunshine on a slope or a coastline is enough to start a wind of its own. And in between sits the mountain wave — a local wind pattern with the reach and the violence to matter at every flight level.
Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.
Regarding the vertical extent of mountain-wave effects:
The surface pressure pattern in January, in the northern hemisphere, features:
Regarding mountain-wave effects on aircraft performance and structure:
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Heat drives the whole machine. The equator receives the most sun, its air rises, and the rising branch of that heat engine is the ITCZ (intertropical convergence zone), the belt of equatorial low pressure where the trade winds of both hemispheres meet. The risen air travels poleward aloft, cools, and sinks near 30° latitude into the subtropical highs; part of the sunk air returns equatorward as the trade winds (north-easterly in the northern hemisphere: Coriolis at work on a southbound surface flow), closing the Hadley cell. Poleward of the highs blow the mid-latitude westerlies of the Ferrel cell, until they meet the cold polar easterlies flowing out of the polar high — the meeting line is the polar front near 60°, where air rises again and the polar cell closes the set.
The pattern is real but it breathes with the seasons, because the whole system follows the sun (north in July, south in January) and because continents heat and cool faster than oceans. In January the northern winter builds intense cold thermal highs over the continents (the Siberian high above all) while deep lows sit over the northern oceans (the Icelandic and Aleutian lows); in July the same continents flip to heat lows (the Asian monsoon low the largest) while the oceanic subtropical highs (the Azores high in the Atlantic) strengthen and spread. Aloft, the picture is simpler in both months: broad westerlies through the mid-latitude troposphere, strongest in winter, and easterlies over the deep tropics. The climatology module redraws all of this on real maps; here the shape of the machine is what matters.
Shrink the same physics to a single slope or shoreline and you get the local winds — every one of them a small thermal circulation. On slopes:
Anabatic wind — by day the sun heats the slope, the slope heats the air touching it, and the warmed, lighter air flows up the slope. Gentle, usually a few knots.
Katabatic wind — by night the slope radiates its heat away, chills the air on it, and the dense, cold air drains downhill under its own weight. Over long ice slopes the drainage can reach gale force.
A whole valley runs the same cycle at a larger size:
Valley wind — by day the heated valley air flows up the valley's axis.
Mountain wind — by night the cooled air drains back down the same axis.
And the terrain shapes whatever wind exists already: where a valley narrows, the flow squeezed through it accelerates (the Venturi effect), so gaps and passes carry winds well above the free-air value; and where converging valleys bring two flows together, the crowded air converges and accelerates the same way, exactly as the accelerated flow over a ridge did in the altimetry lesson.
On the coast the circulation is horizontal. By day the land heats faster than the sea, air rises over the land, and the sea breeze flows in from the water to replace it; its leading edge, the sea-breeze front, is a miniature front that can trigger a line of cumulus and a sharp wind change a few tens of kilometres inland. By night the land cools below the sea and the gentler land breeze flows offshore. One more evening habit is worth knowing: as the surface layer decouples after sunset, local low-level jet streams can develop in the evening — narrow ribbons of fast air a few hundred metres up, above the calmed surface layer, waiting across the approach path.
Force a stable airstream over a ridge and it does not simply descend on the far side — it overshoots, is pulled back by its stability, and oscillates downstream in a train of standing waves (mountain waves, also called lee waves): the wave pattern stands still over the ground while the air pours through it. Formation has a recognisable recipe:
Wind — roughly 20 kt (knots) or more at ridge level, blowing within about 30° of perpendicular to the ridge, and holding or increasing with height.
Stability — a stable layer near crest level, with less stable air below and above: the stable layer is the spring that makes the overshooting air swing back.
The structure is a paradox of smooth and violent. The waves themselves are smooth, laminar flow with strong sustained updraughts and downdraughts: wavelengths of a few kilometres to tens of kilometres, amplitudes that can span thousands of feet. But underneath the wave crests, near the ground, the flow rolls up into rotors: closed, violently turbulent eddies, the most dangerous air in the system. The clouds are the giveaway, when the moisture allows them:
Lenticular (lens-shaped) clouds — standing at the wave crests.
Rotor cloud — a ragged cloud marking the rotor.
Cap cloud — hugging the ridge top.
All three stand still while the wind blows through them — and in dry air the whole wave system can exist with no cloud at all.
Two warnings finish the picture. First, mountain-wave effects can exceed the performance or the structural capability of an aircraft: a sustained downdraught can outpace the climb capability of a piston aircraft against a ridge, and rotor turbulence can overstress any type. Second, the waves do not stay low: wave effects can propagate from low to high level — over Greenland and other great ridges they are felt at jet cruise altitudes, far above the terrain that made them.
The cell names, wind belts and pressure features are recall, as are the mountain-wave conditions and cloud names; the understanding marks sit in why each circulation runs and what the wave system does to an aircraft. The overview, following the learning objectives:
Describe the three-cell circulation, and the surface pressure and wind pattern with its January/July shift, plus the upper-level westerlies and easterlies
Describe and explain anabatic and katabatic winds, mountain and valley winds, and the Venturi effect in valleys
Describe land and sea breezes, the sea-breeze front, and the evening low-level jets
Explain the origin, formation conditions, structure and properties of mountain waves
Identify mountain waves by their clouds, and state that they can exceed aircraft performance or structural capability and propagate to high level