Rotor, venturi and compression

How hills, trees and buildings bend and break the wind — rotor in the lee, speed-up through gaps and over crests, wind shadows — and why you never fly behind the crest.

In this lesson you will

Picture a ridge on a breezy afternoon. In front of it, wings hang in smooth rising air. Behind the crest, in the same wind, the air tumbles down the far side in invisible eddies. The difference is a few hundred metres and the shape of the ground.

Wind bends around hills, squeezes through gaps and breaks up behind obstacles. You cannot see it, so you learn to read it from the terrain: where the air is smooth, fast or dangerous.

Mechanical turbulence

When the wind meets trees, buildings, hills or mountains, it breaks into eddies. Pilots call this mechanical turbulence. It is stronger in stronger wind and behind bigger or rougher obstacles, and the wind carries the eddies downstream¹.

How far they travel depends on the wind and the air. Unstable, thermic air makes larger eddies that break up quickly; in stable air the eddies die away slowly and travel further¹.

The lee side and rotor

The lee side is the downwind side of a hill or obstacle. There the air follows the ground downward and becomes increasingly turbulent, which tends to push anything flying there down towards the slope. The stronger the wind, the stronger the sink and the turbulence. Near a steep ridge in strong wind the lee downdraft can reach 10 m/s2,000 ft/min or more — several times a paraglider’s sink rate — and a flat-topped ridge offers little refuge, because the sink and turbulence carry on over the flat top².

On a steep lee slope the flow can break away from the ground altogether and roll into a rotor: a turning eddy in which the air near the ground flows back towards the hill, against the wind. In a compilation of field and wind-tunnel studies, this never happened where the steepest slope was gentler than about 15°, always happened from about 18°, and came and went in between³.

Figure: Follow the dots: smooth and rising in front of the ridge, fastest where the lines squeeze over the crest, then breaking away into a rotor behind it. The lee often looks calm from above — that is what makes it dangerous.

Follow the dots: smooth and rising in front of the ridge, fastest where the lines squeeze over the crest, then breaking away into a rotor behind it. The lee often looks calm from above — that is what makes it dangerous.

Strong winds higher up make all of this worse. Mountain-flying guidance treats a wind of about 20 kt (37 km/h23 mph) at ridge level as strong, with moderate or greater turbulence, strong updrafts and downdrafts and very strong rotor, and says to suspect it anywhere downwind of rugged terrain⁴. That is far beyond any student limit.

How far does it reach?

Sources give different distances for the turbulent zone behind an obstacle, partly because they measure different things⁵:

Source	What it describes	Distance downwind
USPA parachuting manual	Turbulence felt by a canopy	10–20 obstacle heights
Tree-windbreak research	Any measurable shelter and disturbance	Out to about 30 heights
Rule of thumb attributed to Dennis Pagen	Turbulent wash, growing with the wind	Obstacle height × wind speed in mph: about 10 heights at 16 km/h10 mph, 20 at 32 km/h20 mph

For a tree line 15 m50 ft tall, that is roughly 150–450 m500–1,500 ft⁵. The practical rule does not depend on which figure is right: in any real wind, do not land or set up your final close downwind of trees or buildings — and allow more in strong wind or stable air⁵¹.

Figure: A tree line at the upwind edge of a landing field (LZ). Eddies roll downwind across the field; sources put the end of the turbulence anywhere from about 10 to 30 tree heights, and further in strong wind or stable air, so touch down as far from the trees as the field allows.

A tree line at the upwind edge of a landing field (LZ). Eddies roll downwind across the field; sources put the end of the turbulence anywhere from about 10 to 30 tree heights, and further in strong wind or stable air, so touch down as far from the trees as the field allows.

Behind trees the wind near the ground also drops sharply, which makes the wind gradient on final much steeper there.

Venturi: gaps speed the wind up

When the wind is squeezed through a narrowing, it speeds up: the venturi effect. Gaps in a tree line act as funnels, and on their downwind side the wind is often stronger than in the open field. Between mountains, passes and gaps can channel strong gap winds and produce eddies and strong shear zones⁶. The biggest mountain gaps have seen winds of 18–90 km/h11–56 mph with gusts to about 145 km/h90 mph — a measure of how strong the effect can get, not a typical flying-site value⁶.

Figure: Seen from above, with the wind from the top. The streamlines crowd through the gap, where the wind is strongest, and leave it as a gusty jet; behind each hill is sheltered air with sink and rotor. The inset shows the same effect through a gap in a tree line.

Seen from above, with the wind from the top. The streamlines crowd through the gap, where the wind is strongest, and leave it as a gusty jet; behind each hill is sheltered air with sink and rotor. The inset shows the same effect through a gap in a tree line.

For you this means two things. A launch in a saddle or gap can be much windier and gustier than the valley suggests. And a gap in a tree line at the edge of a landing field can send a jet of stronger, rougher wind across your final⁶.

Compression over the crest

Wind also speeds up as it crosses a hill: the air pushed up the slope squeezes against the air above, and the streamlines crowd together over the top. Pilots call this compression.

How much? A widely used engineering guideline puts the greatest speed-up just above the top at about 2 × (hill height ÷ L) for a long ridge, and 1.6 × (height ÷ L) for a round hill, where L is the horizontal distance from the top down to half the height. For a ridge 100 m330 ft high with L of 250 m820 ft, that is roughly 80% more wind just above the crest than at the same height upstream. The guideline only holds for slopes gentle enough not to separate, and the speed-up changes with wind direction and stability⁷.

Instructors estimate that on windy days the wind just above and behind the top of a ridge is roughly 8–11 km/h5–7 mph stronger than in front of the slope, so your forward speed over the ground drops by about that much there — possibly more than your speed bar adds⁸.

Figure: Wind arrows at the same heights above the ground: short over the flat ground in front, much longer just above the crest. The sock on launch flies higher than the one lower down the slope — and behind the crest lies sink and rotor.

Wind arrows at the same heights above the ground: short over the flat ground in front, much longer just above the crest. The sock on launch flies higher than the one lower down the slope — and behind the crest lies sink and rotor.

So expect launch on top to be windier than the bottom of the hill. And expect to struggle to make progress into wind just where the hill drops away behind you — one reason beginners do not top-land.

Wind shadow

Terrain upwind can put a site in a wind shadow: a ridge upwind can block the low-level wind of an otherwise good slope⁹. Obstacles shelter some distance upwind of themselves too — a tree windbreak slows the wind for two to five heights in front of it as well as far behind⁹.

A wind shadow can fool you on launch. The air at your feet is light, but you may be in the lee of something upwind, with rotor and much stronger wind above you. That is why you compare the launch indicators with the overall wind signs¹⁰.

Never fly behind the crest

Never fly on the crest or behind it on the downwind side: stay in front of the summit whatever the wind, and make every turn away from the ridge, because a downwind turn, even above the top, can carry you over the crest into heavy sink¹¹. Just above the crest the wind can be much stronger than in front — roughly 80% more in the guideline example above⁷ — so your margin for making progress into wind can shrink to nothing there.

Treat a growing crab angle or a falling groundspeed as the wind building¹²: when the headwind beats your airspeed, you go backwards and get blown back¹³. Ridge lift covers flying the ridge, and Blown back and cloud suck what to do.

Crosswinds make a hillside rougher: one school warns that every spine and gully can make rotor when the wind blows across the slope¹⁴. That is one reason student wind limits include the direction.

Reading the terrain before you fly

Find the wind’s direction and strength, on launch and higher up.
Cue Clouds, smoke, trees, water and wings already flying.
Why Launch indicators can sit in a shadow or in rotor¹⁰.
Trace the wind’s path across the ground upwind of you.
Cue Hills, tree lines, buildings and gaps between you and the wind.
Why Every obstacle upwind leaves turbulence downwind¹.
Treat the lee of every obstacle as a no-go zone, planning for up to about 30 obstacle heights in a real wind.
Cue Mentally shade the area behind each tree line, building and crest.
Why Sources differ on how far turbulence reaches; plan for the larger figure⁵.
Look for gaps and saddles.
Cue Narrowings where the wind has to squeeze through.
Why Gaps speed the wind up and make it gusty⁶.
Expect more wind at the crest than at the bottom.
Cue Launch sock flying higher than the landing-field sock.
Why Compression over the crest⁷.
Plan to fly in front of the crest and to land well away from obstacles upwind.
Cue An escape out front, and a touchdown point clear of every wake.
Why Behind the crest and behind obstacles is where the sink and rotor are¹¹⁵.

Common mistake Drifting back over the crest

Symptom: The ground stops moving forward, the crab angle grows, and the wing is closer to the top than you planned.
Cause: The wind built while you were soaring, or you turned downwind near the crest.
Fix: Turn away from the ridge every time, crab to hold your line, and push out in front at the first sign¹¹¹².

Common mistake A final close behind a tree line

Symptom: Below tree-top height the wind seems to vanish and the wing is thrown about.
Cause: Landing in the trees’ shelter and wake.
Fix: Touch down as far from obstacles upwind as the field allows — the sources range from 10 to about 30 obstacle heights, and more in strong wind⁵.

Common mistake Trusting a calm launch

Symptom: Launch is light, but once airborne the air is rough and the wind much stronger.
Cause: Launch was in the wind shadow of terrain upwind.
Fix: Compare launch indicators with the overall signs, and ask why they differ before you fly⁹¹⁰.

Instructor's note Read the ground, not the air

You will rarely see rotor itself, so you learn to predict it: wind direction, then everything upwind of you, then the lee of each feature. Two exceptions trip students up. A gentle lee slope may not form a separated rotor at all³, but its air still sinks and is often rough, so treat every lee as no-go². And a stable, smooth-looking morning is not a guarantee of gentle lee air: in stable air the eddies behind obstacles travel further¹.

Standards differ Terrain turbulence in the syllabus

Common ground: All the bodies teach students to recognise lee-side turbulence and rotor before they fly hills.

USHPA (USA): P2 ground school teaches students to look for rotor by comparing the overall wind signs with the launch indicators¹⁰. P3 adds a spoken analysis of the hill covering wind shadows, wind gradients, lift, sink, laminar air, turbulence and rotors¹⁵.
BHPA (UK): The Club Pilot environment exam for hill pilots covers rotor, the wind gradient, ridge etiquette, over-crowding, top landing and wave¹⁶.
APPI (international): First-level theory covers airflow over ground and obstacles, lee-side turbulence, the wind gradient, venturi and slope lift¹⁷.
FAI/CIVL SafePro (international reference): Puts terrain effects, valleys, venturi, obstructions, corners and rotors, and mechanical, shear and thermal turbulence into its second stage, and turbulence behind obstructions into its third¹⁸.

Summarised from each body’s published material, which changes over time. Check the current official documents and follow your school.

Questions students ask

How far behind a tree line is it safe to land?

The sources disagree — from 10–20 tree heights to about 30, and further in strong wind⁵. Land as far from the trees as the field allows, and never close behind them in any real wind.

Can I see rotor?

Usually not directly. You predict it from the wind and the terrain; clues include launch indicators that disagree with the overall wind signs¹⁰.

Why is launch so much windier than the valley?

Wind gets stronger with height and speeds up over the crest; a launch in a saddle or gap adds the venturi effect⁷⁶.

Key points

Obstacles break the wind into eddies; turbulence grows with the wind and the size of the obstacle, and travels further in stable air.
Behind a hill the air sinks and tumbles; on steep lee slopes it rolls into rotor, with the air near the ground flowing back towards the hill.
Sources differ on how far turbulence reaches behind obstacles — from 10 to about 30 obstacle heights, more in strong wind — so plan for the larger figure and land as far from trees and buildings upwind as you can.
Gaps (venturi) and crests (compression) speed the wind up: expect launch to be windier than the valley.
A wind shadow can make launch calm under strong wind; compare launch indicators with the overall signs.
Never fly behind the crest: turn away from the ridge, crab to hold your line, and move out front when the wind builds.

Ground to Solo is a study aid. Paragliding is learned in person with a qualified instructor at a recognised school; nothing here certifies you to fly.