Beaufort Wind Scale

How to determine wind speed by sea state

Knots Descriptive Sea Conditions Wind Force (Beaufort) Probable Wave Height ft
0-1 Calm Sea smooth and mirror- like. 0 -
1-3 Light Air Scale-like ripples without foam crests. 1 3 inches
4-6 Light Breeze Small, short wavelets; crests have glassy appearance and do not break. 2 6 inches
7-10 Gentle Breeze Large wavelets; some crests begin to break; foam of glassy appearance. Occasional white foam crests. 3 2
11-16 Moderate Breeze Small waves, becoming longer; fairly frequently white foam crests. 4 4
17-21 Fresh Breeze Moderate waves, taking a more pronounced long form; many white foam crests; there may be some spray. 5 6
22-27 Strong Breeze Large waves begin to form; white foam crests are more extensive everywhere; there may be some spray. 6 10
28-33 Near Gale Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind; spindrift begins. 7 14
34-40 Gale Moderately high waves of greater length; edges of crests break into spindrift; foam is blown in well marked streaks along the direction of the wind. 8 18
41-47 Strong Gale High waves; dense streaks of foam along the direction of the wind; crests of waves begin to topple, tumble and roll over; spray may reduce visibility. 9 23
48-55 Storm 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 is white in appearance. The tumbling of the sea becomes heavy and shock-like.  Visibility is reduced. 10 29
56-63 Violent Storm Exceptionally high waves that may obscure small and medium sized ships.   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.   Visibility is reduced. 11 37
64-71 Hurricane The air is filled with foam and spray.  Sea completely white with driving spray; visibility very much reduced. 12 45