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Beaufort Wind Scale

Descriptive term Beaufort scale number Mean wind speed in knots Sea criterion Probable height of waves in metres Probable maximum height of waves in metres
Calm 0 <1 Sea like a mirror. - -
Light 1 1-3 Ripples with the appearance of scales are formed but without foam crests. 0.1 0.1
Light 2 4-6 Small wavelets, still short but more pronounced; crests have a glassy appearance and do not break. 0.2 0.3
Moderate 3 7-10 Large wavelets; crests begin to break; foam of glassy appearance; perhaps scattered white horses. 0.6 1
Moderate 4 11-16 Small waves, becoming longer; fairly frequent white horses. 1 1.5
Fresh 5 17-21 Moderate waves, taking a more pronounced long form; many white horses are formed (chance of some spray). 2 2.5
Strong 6 22-27 Large waves begin to form; the white foam crests are more extensive everywhere (probably some spray). 3 4
Strong 7 28-33 Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind. 4 5.5
Gale 8 34-40 Moderately high waves of greater length; 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
Gale 9 41-47 High waves; dense streaks of foam along the direction of the wind; crests of waves begin to topple, tumble and roll over; spray may affect visibility. 7 10
Storm 10 48-55 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 on a white appearance; the tumbling of the sea becomes heavy and shocklike; visibility affected. 9 12.5
Storm 11 56-63 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; visibility affected. 11.5 16
Hurricane 12 64 and over The air is filled with foam and spray; sea completely white with driving spray; visibility very seriously affected. 14 or over -