Analysis of logarithmic wind profiles measured to a height of 10 m on the south shore of Long Island indicates three distinct classes of aerodynamic roughness of the sea surface. On the basis of the roughness Reynolds number, defined a u* i 0/&ngr;, where u* is the surface friction velocity, z0 the roughness length, and &ngr; the kinematic viscosity, the sea surface is aerodynamically smooth for u*z0/&ngr;?0.15. There is a moderately rough transition regime for 0.1 <u*z0/&ngr;<4, the surface becoming fully rough for the roughness Reynolds numbers exceeding 4. The surface drag coefficient CD6 obtained from the mean wind speed at a height of 6 m is found to vary from 0.75¿10-3 for an aerodynamically smooth sea surface to 1.9¿10-3 for a fully rough surface. Moderately rough flow conditions yielded a CD6 of 1.15¿10-3. Wind profile measurements from two other offshore or coastal sites showed good agreement. Mean wind speeds ranged from 3.5 to 10 m s-1. The drag coefficient did not vary with mean wind speed but was weakly dependent on the atmospheric stability. Charnock's constant α in iz0=αu*2/g, where g is the gravitational acceleration, was found to have a mean value of 0.016 for moderately rough conditions and 0.072 for fully rough conditions.