1. A height scale in turbulent flow over tall roughness elements associated with the average level of action of momentum transfer between the flow and the roughness elements. In neutral stability the logarithmic wind profile becomes a straight line only if the vertical axis is shifted by the zero-plane displacement length. Several formulas exist to relate this height scale as a function of the roughness element geometry (for example, spacing and silhouette area). Tabulated values for various surface types are published in most micrometeorological texts, for example, Oke (1987). See surface roughness. 2. (Also called displacement distance, displacement thickness. ) In a viscous homogeneous fluid, the depth of a boundary layer of shearing flow over a flat plate.