Abstract
The form drag of wind on an irregular surface is found to equal the product of U^2^ (U is anemometer windβspeed) and a function of the twoβdimensional spectrum of this surface. For a solid surface this function is, of course, independent of U and the form drag is proportional to U^2^, as observed over land. Over water, Neumann's frequency spectrum and the glitter measurements by Cox and Munk make this function proportional to U, and hence the form drag proportional to U^3^. The computed form drag is not far out of line with measurements by Van Dorn and others.
The dependence of form drag on the βbeamβwidthβ and frequency spectrum of the surface roughness is discussed. The essential roughness statistic comes close to being the meanβsquare slope of the surface, rather than being related to wave height or some other roughness length, as usually assumed. The slope statistics are governed by the highβfrequency part of the wave spectrum, and this feature accounts for the pronounced reduction of form drag by surface βslicks,β and the relatively small effect of a limited fetch. The total drag, c~1~ U^2^ + c~2~ U~3~ (skin friction + form drag), must be larger at very high winds than if it followed a U~2~ law, as usually assumed.