When wind passes over land the unevenness of the ground and interference to wind flow from trees or undergrowth will cause a significant amount of turbulence. Turbulent air creates an additional strain on a wind turbine blade, accelerating the onset of fatigue damage. In order to limit this damage as much as possible wind turbine designers will normally place the turbine on a tower, which is tall enough to raise the blades above this turbulent layer of air.
The wind offshore is generally less disturbed because the surface of the sea is smoother, resulting in a thinner turbulent layer and less overall turbulence. Waves in rough seas will increase turbulence and wave height itself needs to be taken into account offshore. Turbine blades should be lifted high enough to avoid the highest waves likely at a particular site. Generally, however, tower heights offshore can be lower than onshore. In both cases site measurements will be needed to ascertain what the optimum turbine height should be.
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