# Skin-friction coefficient

From AMS Glossary

## skin-friction coefficient

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*Also called*friction coefficient, drag coefficient.) A dimensionless drag coefficient expressing the proportionality between the frictional force per unit area, or the shearing stress τ_{0}exerted by the wind at the earth's surface, and the square of the surface wind speed*M*:*C*_{D}is the skin-friction coefficient and ρ the air density.The skin-friction coefficient is independent of wind speed except where the wind modifies the underlying surface, for example, a sheet of water or field of tall grass. It appears to be independent of thermal stability and, over a fully rough surface, of the wind speed at the reference level, but it does depend on the height of the reference level. Values derived for the atmosphere have almost the same magnitudes as those used in aerodynamics, ranging from about 0.005 over smooth water to 0.015 over grassland. Some writers define the skin-friction coefficient as twice the value given here.

*See also*drag;*compare*form drag, wave drag.Sutton, O. G. 1953. Micrometeorology. 255–264.