Force 4

− Wind speed 11 − 16 kn or 5.5-7.9 m/sec

mean, 13kn or 7 m/sec

(Small waves, becoming longer; fairly frequent white horses)

Force 0

− Wind speed less than 1 kn or 0-0.2 m/sec

(Sea like a mirror)

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 12

− Wind speed greater than 63kn or 32.7 m/sec

(The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected)  

Force 2

− Wind speed 4 − 6 kn or 1.6-3.3 m/sec

mean, 5 kn or 3 m/sec

(Small wavelets, still short but more pronounced − crests have a glassy appearance and do not break) 

Force 3

− Wind speed 7 − 10 kn or 3.4-5.4 m/sec

mean, 9kn or 5 m/sec

(Large wavelets. Crests begin to break. Foam glassy appearance. Perhaps scattered horses) 

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 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.) 

Force 6

− Wind speed 22−27 kn or 10.8-13.8 m/sec

mean 24 kn or 12 m/sec

(Large waves begin to form; the white foam crests are more extensive everywhere (Probably some spray))  

Force 1

− Wind speed 1 − 3 kn or 0.3-1.5 m/sec

mean, 2 kn or 1 m/sec

(Ripples with the appearance of scales are formed, but without foam crests)

 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 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) 

Beaufort wind scale

Force 5

− Wind speed 17 − 21 kn or 8.0-10.7 m/sec

mean, 19 kn or 9 m/sec

(Moderate waves, taking a more pronounced long form; many white horses are formed (Chance of some spray))