Evidence for a critical wind velocity for air‐sea boundary processes

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Abstract
Five thousand simultaneous wind and current observations at Amrum Bank Lightship are analyzed to contribute toward the solution of the controversial question of the existence of a critical wind speed for air‐sea boundary processes. The observations are grouped according to eight wind forces (Beaufort scale) and subjected to harmonic analysis. The residual current is further analyzed to yield a current that can be considered as exclusively wind‐driven. Upon plotting these current velocities W against wind velocities V in cm/sec, the graph shows a definite discontinuity between the wind velocity Groups 1 to 4 and 5 to 8. There is a marked reduction in current speed between wind forces 4 and 5 and a lower ratio of increase of W against V for higher wind velocities.