Part of DIY Wind Turbine

Understanding how hills, valleys, forests, and buildings reshape wind flow so you can exploit acceleration zones and avoid turbulence traps.

Terrain Effects

Why Terrain Effects Matter

Wind does not flow in straight, even lines across the landscape. It bends around hills, accelerates over ridges, funnels through valleys, and churns into chaos behind obstacles. The same 5 m/s regional wind can become 8 m/s on a hilltop, 2 m/s in a sheltered hollow, or a useless swirl of turbulence behind a building — all within a few hundred meters of each other. Understanding terrain effects lets you find the micro-sites where wind is naturally concentrated and smooth, and avoid the dead zones where a turbine will underperform or be destroyed.

This knowledge is the difference between a turbine that powers your community and one that shakes itself apart in turbulent air while producing a trickle of electricity. Every terrain feature within 500 meters of your turbine site affects its performance.

Hilltop Acceleration

When wind encounters a hill or ridge, it has nowhere to go but up and over. The air stream compresses as it flows over the crest, accelerating significantly. This is called the speed-up effect, and it is the single most valuable terrain feature for wind energy.

How It Works

Imagine wind as water flowing over a smooth rock in a stream. The water speeds up as it passes over the top and slows down on the other side. Wind does exactly the same thing over hills.

Hill Shape	Speed-Up Factor	Notes
Smooth, rounded ridge	1.5-2.0x base speed	Best case — clean acceleration, minimal turbulence
Sharp, peaked ridge	1.3-1.6x with turbulence	Speed increase present but accompanied by turbulence at the very top
Broad, flat-topped hill	1.2-1.4x	Moderate acceleration, very clean flow
Steep cliff face	1.4-1.8x at lip, severe turbulence behind	Dangerous — extreme turbulence on lee side

The Ideal Ridge

A gently rounded ridge perpendicular to the prevailing wind is the best natural site you will find. Wind accelerates smoothly over the crest with minimal turbulence. If you have access to such a feature, build there — even if it means a longer wire run. The 50-100% speed increase translates to 3-8x more energy than the surrounding flatland.

Placement on a Hill

Position matters enormously even on a good hill. Place your turbine:

• On the crest or slightly upwind of the crest — this is where acceleration peaks and turbulence is lowest
• Never on the downwind slope — the lee side is a turbulence zone; flow separates from the surface and creates chaotic eddies
• Along the ridge, not at the end — ridge ends create tip vortices (swirling wind around the edges) that produce turbulence

The upwind slope itself experiences moderate acceleration and clean flow — it is acceptable if the crest is not accessible, though not as good as the top.

Valley Channeling

Valleys and gaps in ridgelines act as natural wind tunnels. When a broad air mass is forced through a narrow passage, it accelerates — just as water speeds up when a river narrows.

Identifying Channeling Effects

• Mountain passes and saddles — Wind funnels through gaps between peaks, often reaching speeds 50-100% higher than the surrounding area
• River valleys aligned with prevailing wind — Long, straight valleys oriented in the wind direction channel and accelerate flow
• Gaps between buildings or tree lines — Even small gaps create localized acceleration

Valley Channeling Is a Double-Edged Sword

Channeled wind is fast but often turbulent and gusty. The acceleration comes with increased variability — sudden speed changes that stress turbine components. Valley bottoms also trap cold air at night, creating temperature inversions that suppress wind completely during calm periods. Measure carefully before committing to a valley site.

Valley Floor vs. Valley Sides

The valley floor is usually the worst location — it collects turbulence from both valley walls, traps calm cold air, and experiences the most extreme gusting. The valley sides, particularly the upwind-facing slope, are better — they catch wind deflected upward by the terrain. The ridge crests above the valley are best of all.

Turbulence Zones Behind Obstacles

Every solid obstacle — building, tree, wall, hill — creates a turbulence zone on its downwind side. This is the single most important terrain effect to understand, because it is the most common reason turbines fail to perform.

The Turbulence Envelope

Zone	Distance from Obstacle	Wind Quality	Suitability
Immediate wake (recirculation)	0-2x obstacle height downwind	Chaotic, swirling, may reverse direction	Impossible — do not place any turbine here
Near wake	2-5x height downwind	Highly turbulent, speed reduced 40-60%	Very poor — rapid fatigue damage to turbine
Far wake	5-15x height downwind	Moderately turbulent, speed reduced 20-40%	Poor — reduced output and shortened lifespan
Recovery zone	15-20x height downwind	Mildly turbulent, speed within 10% of free stream	Acceptable if no better site available
Free stream	Beyond 20x height	Undisturbed flow	Good — obstacle no longer affects turbine

Turbulence Is Invisible

You cannot see turbulence. A site behind a large building may feel windy on a gusty day, but the wind is swirling and reversing direction constantly. Your turbine will spin up, brake, reverse load, spin up again — dozens of times per minute. This cyclic stress causes metal fatigue, bearing failure, and blade cracks within weeks or months. Always assume turbulence exists behind any solid obstacle and site accordingly.

Upwind Effects

Obstacles also affect wind upstream, but much less. Wind begins to slow and deflect about 2-5x the obstacle height upwind. This is far less severe than the downwind turbulence but still measurable. Do not place a turbine immediately in front of a tall obstacle either.

Coastal and Lake Effects

Large bodies of water are the smoothest possible upwind surface. Wind flowing over water experiences almost no friction, arriving at shore fast, smooth, and laminar. This makes shorelines excellent turbine sites — but only when the wind blows from the water.

Onshore and Offshore Breezes

During the day, land heats faster than water, causing air to rise over land. Cooler air flows in from the water — the onshore breeze. At night, the pattern reverses. These thermal breezes are typically 2-5 m/s and very consistent.

Condition	Wind Direction	Speed	Quality	Best For
Daytime onshore breeze	Water to land	2-5 m/s	Very smooth, laminar	Consistent low-power generation
Nighttime offshore breeze	Land to water	1-3 m/s	Smooth but weak	Minimal generation
Prevailing wind from water	Over water to land	Regional speed + friction bonus	Excellent — fast and clean	Maximum generation
Prevailing wind from land	Over land to water	Regional speed - friction penalty	Turbulent from passing over land terrain	Moderate, depends on upwind obstacles

Site on the Upwind Shore

If prevailing winds cross a lake or bay before reaching you, your shoreline is an exceptional turbine site. The water surface has eliminated all upwind turbulence. Place the turbine as close to the water’s edge as practical, with the rotor at least 10 meters above ground to clear any shore bluffs.

Forest Edge Effects

Forests are among the worst upwind obstacles. A dense forest canopy acts like a rough wall — wind must climb over it, creating a massive turbulence zone that extends 20-30 times the tree height downwind.

The Forest Boundary Layer

At the upwind edge of a forest, wind slows abruptly and deflects upward. Above the canopy, wind accelerates slightly (compression effect). Below the canopy, wind nearly stops. At the downwind edge, wind must recover from this disruption.

Rules for forest proximity:

• Downwind of a forest — Place turbine at least 20x tree height away from the forest edge, with the rotor at least 2x tree height above ground. For 15m trees, this means 300m downwind and 30m above ground — impractical without an extremely tall tower.
• Upwind of a forest — Much better. Wind approaching the forest is still undisturbed. Place the turbine at least 5x tree height upwind of the forest edge.
• In a clearing within a forest — Almost always too turbulent. The clearing must be at least 10x the surrounding tree height in diameter (150m for 15m trees) and the tower must lift the rotor above the canopy.
• On a hilltop above forest — If the hill rises well above the surrounding canopy, the hill’s speed-up effect can overcome the forest turbulence. The rotor must be above the tree line.

Urban Environments and Rooftop Mounting

Buildings create some of the most severe and unpredictable turbulence of any terrain feature. Their sharp edges, flat surfaces, and clustered arrangement produce chaotic, swirling wind that changes dramatically with direction.

Why Rooftop Turbines Usually Fail

The idea of mounting a turbine on a rooftop seems logical — it is high up and often windy. In practice, rooftop turbines consistently underperform by 50-80% compared to predictions because:

1. Recirculation zone — Wind separating at the building edge creates a turbulent recirculation bubble that extends upward 1.3x the building height above the roof surface. A turbine on a 10m building needs its rotor at least 13m above roof level to clear this zone — defeating the purpose.

2. Direction chaos — Buildings deflect wind in all directions. The wind at rooftop level shifts constantly, meaning the turbine spends most of its time yawing (turning to face the wind) instead of generating.

3. Structural vibration — Turbine vibration transmits through the mounting into the building structure, causing noise, cracking, and potential structural damage.

4. Gustiness — Urban wind is extremely gusty with rapid speed changes. This stresses the turbine and produces less energy than the same average speed in smooth flow.

Avoid Rooftop Mounting

In almost all cases, a ground-mounted turbine on a proper tower at the edge of the built area will vastly outperform a rooftop installation. The only exception is a very tall building that towers above all surrounding structures by at least 2x — and even then, the turbine must be mounted well above the roofline on a dedicated mast.

Urban Siting Strategy

If you must install a turbine in an urban area:

• Place it at the upwind edge of the built area, where the prevailing wind arrives fresh from open ground
• Use a tower that lifts the rotor at least 1.5x the height of the tallest nearby building
• Avoid narrow streets and enclosed courtyards — these are turbulence traps
• Look for parks, playing fields, or cleared lots on the upwind side of town

Identifying the Best Micro-Site

Walk your entire available area with these priorities, in order:

1. Hilltops and ridgelines perpendicular to prevailing wind — best natural acceleration, cleanest flow
2. Open, flat ground with 300m+ clear upwind fetch — no acceleration but no turbulence either
3. Upwind shoreline of a lake or large river — smooth upwind surface, consistent thermal breezes
4. Upwind edge of a settlement or forest — catches wind before it hits obstacles
5. Valley aligned with prevailing wind — channeling effect but check for gustiness

Avoid, in order of severity:

1. Lee side of hills or ridges (worst — severe turbulence)
2. Rooftops and urban canyons
3. Downwind of dense forest
4. Valley bottoms perpendicular to wind
5. Downwind of large buildings within 20x their height

Terrain Features and Their Effects

Terrain Feature	Effect on Wind Speed	Effect on Turbulence	Net Impact on Turbine
Rounded ridge crest	+50-100%	Minimal increase	Excellent — best natural site
Broad hilltop	+20-40%	Low	Very good
Valley aligned with wind	+30-50%	Moderate to high	Good if turbulence tolerable
Flat open ground	Baseline	Low	Good — reliable and predictable
Upwind shoreline	+10-20% (smoother)	Very low	Good — cleanest flow
Mountain pass/saddle	+50-100%	High (gusty)	Mixed — high energy but hard on equipment
Forest clearing	-30-50%	High	Poor — avoid unless clearing is very large
Lee side of hill	-20-50%	Severe	Terrible — never site here
Behind building (within 20x H)	-30-60%	Severe	Terrible — turbine will fail prematurely
Urban rooftop	Variable	Extreme	Very poor — rooftop myth

Common Mistakes

Mistake	Cause	Fix
Building on lee side of ridge	”The hilltop is too exposed / hard to access”	Accept the difficulty; the crest is worth it. Lee side turbulence is devastating.
Placing turbine in a “windy” urban gap	Feels windy between buildings due to channeling	Measure for turbulence, not just speed. Gusty, swirling wind damages turbines.
Ignoring forest downwind effects	Trees seem far away	Calculate: 15m trees create turbulence 300m downwind. Map the actual shadow.
Rooftop mounting	Seems high and convenient	Mount on a proper ground tower at the settlement edge instead.
Not checking multiple micro-sites	Assuming wind is uniform across property	Walk the entire area, test multiple points. Differences of 50m can mean 2x the wind.
Overlooking seasonal terrain effects	Deciduous trees lose leaves in winter, changing wind flow	A site shielded in summer by full canopy may open up in winter. Measure in both seasons.

Key Takeaways

• Terrain can double or halve wind speed within a few hundred meters — micro-siting is critical
• Hilltops and ridgelines perpendicular to prevailing wind are the best natural sites, offering 50-100% speed increases
• Turbulence behind obstacles extends 20x the obstacle height downwind — always map these zones before siting
• Valley channeling boosts speed but introduces gustiness — measure carefully before committing
• Coastal sites with prevailing wind from water are exceptional due to smooth, laminar flow
• Rooftop turbines almost always underperform — a ground tower at the settlement edge is better
• Walk your entire available area and compare multiple candidate micro-sites before building