Surface Roughness

From Wind wiki

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The objects on the surface (roughness elements) over which the wind is flowing will have a frictional effect upon the wind speed nearest the surface. The numbers given at various heights are the wind speed at the height relative to the gradient wind, in percent. The gradient wind occurs at the height above the surface where the effects of the surface are no longer felt. It can be considered as the free-stream flow and is in response to the pressure and temperature gradients. Both the height and the spacing of the roughness elements on the surface will influence the frictional effect on the wind. A single parameter, the surface roughness length, z0, is used to signify this effect.