Beaufort Scale

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Beaufort Wind Force Mean Wind Speed in knots Limits of wind speed in knots Descriptive Term Sea Criterion Probable height of waves in metres* Probable max. height of waves in metres*
Measured at a height of 10 m above sea level.
0 0 <1 Calm Sea like a mirror.    
1 2 1 - 3 Light air Ripples with the appearance of scales are formed, but without foam crests. 0.1 0.1
2 5 4 - 6 Light breeze Small waves, still short but more pronounced, crests have a glassy appearance and do not break. 0.2 0.3
3 9 7 - 10 Gentle breeze Large wavelets, Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses. 0.6 1
4 13 11 - 16 Moderate breeze Small waves, becoming longer, fairly frequent white horses. 1 1.5
5 19 17 - 21 Fresh breeze Moderate waves, taking a more pronounced lingo form: many white horses are formed. (Chance of some spray.) 2 2.5
6 24 22 - 27 Strong breeze Large waves begin to form; the white foam crests are more extensive everywhere. (Probably some spray.) 3 4
7 30 28 - 33 Near gale Sea heaps up and white foam from breaking waves begin to be blown in streaks along the direction of the wind. 4 5.5
8 37 34 - 40 Gale Moderately high waves of greater length; the edges of crests begin to break into spindrift. The foam is blown in well marked streaks along the direction of the wind. 5.5 7.5
9 44 41 - 47 Strong Gale High waves. Dense streaks of foam along the direction of the wind. Crests begin to topple, tumble and roll over. Spray may affect visibility. 7 10
10 52 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. The whole surface of the sea takes on a white appearance. Tumbling of the sea becomes heavy and shock-like. Visibility affected. 9 12.5
11 60 56 - 63 Violent Storm Exceptionally high waves. (Small and medium sized ships may 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 waves’ crests are blown into froth. Visibility affected. 11.5 16
12   64 and over Hurricane The air is filled with foam and spray. Sea completely white with driving spray; visibility is seriously affected. 14 or over  
(*) These columns are added as a guide to show roughly what may be expected in the open sea, remote from land. In enclosed waters, or when near land off-shore wind, wave heights will be smaller, and the water steeper.

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