Wind shear
The change in wind speed (and sometimes direction) with height above the surface, driven by surface roughness and atmospheric stability.
Surface friction from terrain, crops, forests and buildings slows the lowest layers of the atmosphere, producing a vertical gradient in wind speed. Wind resource assessments extrapolate measurements from a met mast or lidar to hub height using one of two profiles:
Log law: v(h) = vref ln(h / z0) / ln(href / z0)
Typical values of the shear exponent α and roughness length z0:
- Open sea: α ≈ 0.08–0.12, z0 ≈ 0.0002 m
- Open grassland: α ≈ 0.14–0.16 (the classic "1/7 law"), z0 ≈ 0.03 m
- Farmland with hedges: α ≈ 0.2, z0 ≈ 0.1 m
- Forest and suburbs: α ≈ 0.25–0.4, z0 ≈ 0.5–1 m
Shear varies by time of day. At night a stable boundary layer can produce α above 0.4 and low-level jets, which is why many inland sites produce more at night; on sunny afternoons convective mixing flattens the profile.
For a turbine, shear means the top of a 170 m rotor sees much stronger wind than the bottom, causing cyclic blade loading (a fatigue driver) and departures from the warranted power curve. IEC 61400-12-1 therefore recommends rotor-equivalent wind speed measured at several heights.
Related terms
All terms →- Hub height
The height of a wind turbine's rotor centre above ground or sea level; taller hubs reach stronger, steadier winds and are a main driver of rising capacity factors.
- Weibull distribution
A two-parameter probability distribution used to describe how often each wind speed occurs at a site, defined by a scale factor c and a shape factor k.
- Wind power density
The mean power available in the wind per square metre of area perpendicular to the flow, in W/m², the standard measure of wind resource quality.
- Power curve
The relationship between hub-height wind speed and a turbine's electrical output, used with a wind speed distribution to estimate annual energy production.
- Offshore wind
Wind turbines installed in seas or large lakes, usually on fixed foundations in water up to about 60 m deep, exploiting stronger and steadier winds than onshore sites.