The sea surface is aerodynamically rough even under light winds

The sea surface is generally considered to be aerodynamically rough at high winds (U>7 m/s), where the roughness length increases with wind velocity; below this velocity, the atmospheric surface layer enters a transition region and then becomes aerodynamically smooth as the wind velocity further decreases. The sea surface is shown, however, to reach its smoothest condition at a wind velocity of about 5 m/s, and then become rough again at lower velocities. In the latter case, the roughness length increases as the wind velocity decreases in accordance with the surface-tension relation governing wind-wave interactions.