Force 7

− Wind speed 28 − 33 kn or 13.9-17.1 m/sec

mean, 30 kn or 15 m/sec

(Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind) 

 Force 10

− Wind speed 48 − 55 kn or 24.5-28.4 m/sec

mean 52kn or 27 m/sec

(Very high waves with long overhanging crests. The resulting foam, in great patches, is blown in dense white streaks along the direction of the wind. On the whole, the surface of the sea takes a white appearence. The tumbling of the sea becomes heavy and shock-like. Visibility affected) 

Force 8

− Windspeed 34 − 40 kn or 17.2-20.7 m/sec

mean 37kn or 19 m/sec

(Moderate high waves of greater length; edges of crests begin to break into the spindrift. The foam is blown in well−marked streaks along the direction of the wind) 

Force 9

− Wind speed 41 − 47 kn or 20.8-24.4 m/sec

mean 44kn or 22 m/sec

(High waves. Dense streaks of foam along direction of the wind. Crests of waves begin to topple, tumble and roll over. Spray might affect visibility) 

Force 11

− Wind speed 56 − 63 kn or 28.5-32.6 m/sec

mean 60kn or 31 m/sec (Exceptionally high waves.)

(Small and medium sized ships might be for a time lost to view behind the waves.) The sea is completely covered with long white patches of foam lying along the direction of the wind. Everywhere the edges of the wave crests are blown into froth. Visiblity affected.)