Wind Speed Height Adjustment Calculator
Extrapolate a wind speed measured at one height to any other hub height using the wind power law. Turbine engineers use this when only met-mast data is available but hub heights differ from the measurement point.
About this calculator
Wind speed increases with height above ground because surface friction slows air near the ground—a phenomenon called wind shear. The most common model for this is the power law: v₂ = v₁ × (h₂ / h₁)^α, where v₁ is the known wind speed at height h₁, v₂ is the estimated speed at target height h₂, and α (alpha) is the power law exponent. The exponent α characterises surface roughness: α ≈ 0.10 for open water, α ≈ 0.14–0.16 for flat open terrain (the most common default), and α ≈ 0.25–0.40 for forested or urban areas. Because wind speed and therefore power both depend on this extrapolation, choosing a correct α for the terrain type is critical. The alternative log-law model is also used, but the power law is more common in engineering practice for heights below 200 m.
How to use
A met mast at 10 m records a wind speed of 5 m/s. You want to estimate speed at a hub height of 80 m over flat terrain (α = 0.143). Step 1 — Divide heights: 80 / 10 = 8. Step 2 — Raise to exponent: 8^0.143 ≈ 1.337. Step 3 — Multiply: 5 × 1.337 ≈ 6.68 m/s. Enter knownSpeed = 5, knownHeight = 10, targetHeight = 80, and powerLawExponent = 0.143 into the calculator to confirm approximately 6.7 m/s at hub height.
Frequently asked questions
What power law exponent should I use for different terrain types?
The exponent α varies significantly with surface roughness. For open water or smooth coastal areas, use α ≈ 0.10–0.11. For flat open land with sparse vegetation—the most common default—use α ≈ 0.14–0.16. Farmland with occasional trees warrants α ≈ 0.20, and suburban or forested terrain can reach α ≈ 0.25–0.35. Urban environments with tall buildings may push α above 0.40. Using the wrong exponent can lead to significant errors in energy yield estimates, so site-specific measurements are preferred whenever available.
Why does wind speed increase with height above the ground?
The Earth's surface exerts friction on air flowing over it, which slows wind near the ground and creates a gradient called the atmospheric boundary layer. The rougher the surface—trees, buildings, uneven terrain—the thicker and more pronounced this layer, and the stronger the shear effect. At higher altitudes the air is less affected by surface drag and flows more freely. This is why wind turbines are built on tall towers: hub heights of 80–120 m can access significantly faster and more consistent winds than measurements taken at standard 10 m meteorological station height.
How accurate is the power law compared to other wind shear models?
The power law is a simple empirical model that works well for neutral atmospheric stability conditions and heights below 200 m over homogeneous terrain. It can over- or under-predict in complex terrain, during stable atmospheric conditions at night, or in highly turbulent environments. The logarithmic wind profile (log-law) is more physically grounded and tends to be more accurate at lower heights and over rougher surfaces. For high-stakes projects, multi-year measurements at actual hub height combined with computational fluid dynamics (CFD) modelling provide the most reliable wind resource data.