Wind Grows With Height
At ground level, friction from terrain — buildings, trees, hills — slows the wind dramatically. Rise above these obstructions and wind speed increases, following a predictable profile determined by the roughness of the underlying surface. This atmospheric boundary layer extends from the ground to the gradient height (typically 300-600 m), above which the wind blows at its full geostrophic speed unaffected by surface friction. Understanding this profile is essential for designing every structure that faces the wind.
The Power Law Profile
The simplest and most widely used model is the power law: V(z) = V_ref × (z/z_ref)^α. The exponent α captures terrain roughness in a single parameter. For open water, α ≈ 0.10 — wind increases gently, already strong near the surface. For dense urban centers, α ≈ 0.35 — wind is heavily suppressed near the ground but increases steeply with height. Building codes worldwide (ASCE 7, Eurocode 1, AS/NZS 1170) use terrain categories that correspond to specific α values to determine design wind speeds at any height.
The Logarithmic Profile
Derived from turbulent boundary layer theory, the log-law V(z) = (u*/κ) × ln(z/z₀) has a stronger physical basis. Here u* is the friction velocity, κ ≈ 0.41 is von Kármán's constant, and z₀ is the surface roughness length. The log profile is more accurate near the surface and provides the friction velocity — a key parameter for calculating turbulence intensity and wind loads. This simulation shows both profiles side by side so you can compare their predictions at your target height.
Design Implications
The wind profile directly determines structural design loads. A building in open terrain faces higher wind at low elevations than the same building downtown — but the downtown building may face higher loads at the top because the urban boundary layer concentrates wind speed increase at upper levels. Wind turbine designers use the profile to estimate energy production — a 10% increase in hub height can increase annual energy by 5-7%. The profile also matters for air quality: stronger mixing in open terrain disperses pollutants faster than in calm urban canyons.