Wind shear and hub height: why 10 m data misleads

Key Takeaways

• The wind profile power law, u = u_r(z/z_r)^α, is the standard formula for extrapolating wind speed from a reference height to hub height, but its accuracy depends entirely on choosing the right exponent (α) for the specific terrain.
• A commonly used default of α ≈ 0.143 (1/7) applies to neutral atmospheric stability over open land; open water sites are better modelled with a lower exponent around 0.11, and using the land value offshore (or vice versa) introduces a real error.
• The fixed 1/7 exponent doesn't account for surface roughness, zero-plane displacement from obstacles, or atmospheric stability, which is why it can yield "quite erroneous estimates" in forested, urban, or otherwise obstructed terrain.
• A 10-metre weather station reading extrapolated to a 50-100 metre hub height compounds any exponent error over a large height ratio, which is exactly the gap most preliminary wind assessments are trying to bridge.

Most publicly available wind data comes from weather stations measuring at 10 metres, a height chosen for meteorological standardisation, not for turbine planning. A modern turbine's hub sits at 50-100 metres or more. Bridging that gap requires extrapolation, and the accuracy of that extrapolation depends entirely on an assumption, the wind shear exponent, that varies by terrain in ways a single default value doesn't capture.

The formula, and the exponent that decides everything

The standard extrapolation is the wind profile power law: u = u_r(z/z_r)^α, where u is the estimated wind speed at the target height z, u_r is the known wind speed at reference height z_r, and α is an empirically-derived exponent describing how wind speed changes with height in the local atmospheric and terrain conditions (Wind Profile Power Law, retrieved 2026-09-10). Every part of the formula is precise except α, which is the part doing the actual work, and it's exactly the part that's often assumed rather than measured.

A commonly cited default is α ≈ 0.143, or 1/7, representing neutral atmospheric stability over open land. Open water sites are better represented by a lower exponent, around 0.11, since water's smoother surface produces less wind shear than land (Wind Profile Power Law, retrieved 2026-09-10). Applying the land-appropriate 0.143 exponent to an offshore or near-coastal site, or vice versa, produces a genuinely different extrapolated wind speed at hub height, not a rounding difference.

Why the fixed exponent breaks down in real terrain

The 1/7 default doesn't account for surface roughness, the zero-plane displacement caused by trees, buildings, or other obstacles near the measurement point, or the atmospheric stability conditions prevailing at the time of measurement (Wind Profile Power Law, retrieved 2026-09-10). In forested or urban terrain specifically, using a constant 1/7 exponent "may yield quite erroneous estimates," which is why the logarithmic wind profile, a more terrain-sensitive alternative, is preferred in those conditions. A candidate turbine site near buildings, trees, or other obstructions is exactly the case where a generic exponent assumption is least trustworthy.

Why the height gap magnifies whatever error exists

The power-law formula raises the height ratio (z/z_r) to the exponent α, which means any error in α compounds as the height ratio grows. Extrapolating from a 10-metre reading to a 100-metre hub height involves a height ratio of 10, considerably larger than the height differences (commonly under 50 metres) that standard assessments were designed around (Wind Profile Power Law, retrieved 2026-09-10). A modest error in the assumed exponent, applied across that large a ratio, can shift the estimated hub-height wind speed, and therefore the projected capacity factor, by a meaningful margin. Run a candidate site's actual reference-height reading and terrain type through the wind viability calculator to see how sensitive the extrapolated hub-height estimate is to the exponent chosen, rather than treating a single default value as settled.

What this means for a preliminary wind assessment

A 10-metre data point extrapolated with a generic 1/7 exponent is a screening estimate, useful for ruling a site clearly in or clearly out, not a number to finance a project against. Where the extrapolated estimate lands in genuinely marginal territory, and much of the practical decision-making happens there, the exponent assumption itself becomes the thing worth verifying before committing further, ideally through measurement closer to actual hub height (a met mast, or remote sensing such as LiDAR) rather than a taller extrapolation from a lower, more conveniently available reading. For a site where a single large turbine investment doesn't clear that bar, it's worth checking whether a roadside VAWT wind solution fits the local wind profile at smaller scale instead.

Frequently asked questions

Can I just use the standard 1/7 wind shear exponent for any site?

Only as a rough approximation for open land under neutral atmospheric conditions. Over water, forested terrain, or urban/obstructed sites, the 1/7 default introduces meaningful error, and terrain-specific data or a different model (such as the logarithmic wind profile) gives a more reliable estimate.

How much does the height difference between measurement and hub height matter?

Considerably. The power-law formula raises the height ratio to the exponent power, so a larger gap between the reference height (commonly 10m) and hub height (50-100m+) magnifies any error in the assumed exponent. Smaller height gaps are inherently more forgiving of exponent assumptions.

Is 10-metre wind data useless for early-stage site screening?

No, it's a legitimate first-pass screening tool for ruling a site clearly viable or clearly unviable. It becomes unreliable as the sole basis for a financing decision, particularly for sites landing in marginal territory, where measurement closer to actual hub height is worth the additional cost before committing capital.

The bottom line

Extrapolating 10-metre wind data to hub height isn't wrong in principle, it's exactly the standard first-pass method, but it's only as good as the wind shear exponent chosen, and a generic default doesn't hold across every terrain type. Treat an extrapolated estimate as a screening figure, and invest in closer-to-hub-height measurement before it becomes the number a project's economics are actually built on.

Figures were verified on 10 September 2026 against published wind resource assessment methodology. Wind shear exponents vary by specific site terrain and atmospheric conditions; commission a site-specific measurement campaign rather than relying on a generic exponent for investment-grade assessments.