The height above the displacement plane at which the mean wind becomes zero when extrapolating the logarithmic wind- speed profile downward through the surface layer.

It is a theoretical height that must be determined from the wind-speed profile, although there has been some success at relating this height to the arrangement, spacing, and physical height of individual roughness elements such as trees or houses. The average wind speed [[File:ams2001glos-Aex02.gif