Friction Layer Depth by Terrain – Why Forests Kill Wind and Deserts Boost It
We know that obstacles block the wind. But the effect goes far beyond the shadow of the object. It changes the entire "Friction Layer" of the atmosphere.
Meteorologists use a value called "Roughness Length" ($z_0$) to calculate this. It measures how "grippy" the land is. The higher the $z_0$, the deeper the friction layer, and the slower the wind speed.
The Forest Effect ($z_0$ = 1.0 m)
Trees are the enemy of wingfoil wind. A forest has a roughness length of about 1.0 meter. This is huge. It creates a friction layer that can extend 500 meters into the sky.
If the wind blows over a forest before hitting your lake, the air is turbulent and slow. The "clean" gradient wind cannot mix down to the water. You get gusty, hole-filled wind. Even if the trees are 1 km away, their friction shadow ruins the wind quality.
The Desert/Water Effect ($z_0$ = 0.0002 m)
Open water and flat deserts are slippery. Their roughness length is tiny (0.0002 m). The friction layer is shallow—perhaps only 50 meters deep.
This allows the high-speed gradient wind to mix almost all the way down to the surface.
- Result: 20 knots at 500 meters becomes 18 knots at the surface.
- Efficiency: You get 90% of the available power.
Why "Offshore" Wind is Gusty
When the wind blows from land to sea (Offshore), it carries the "memory" of the land's friction. It takes roughly 2 to 5 km of fetch over the water for the wind profile to recover and become smooth again.
If you ride 100 meters off the beach in offshore winds, you are riding in the land's friction layer. The wind gusts are violent because the air is tumbling over the houses and trees behind you.
Summary
Check the upwind terrain on Google Maps.
- Trees/City: High friction. Subtract 5 knots from the forecast. Expect turbulence.
- Sand/Water: Low friction. Trust the forecast. Expect steady 15–25 knots power.