Most processes at the air-sea boundary are modified by the existing wind pattern. In general the rate of these processes varies gradually with wind speed. An exception seems to occur at a wind speed of about 7 m/sec; at that speed a number of apparently unrelated processes have been observed to undergo such abrupt changes that, in some instances, one is led to suspect the existance of discontinuities. Not only the rate, but the very nature of these processes seems to be altered at this critical wind speed....
Munk, Walter H.. 1947. "A critical wind speed for air-sea boundary processes." Journal of Marine Research 6, (3). https://elischolar.library.yale.edu/journal_of_marine_research/637