Knots Beaufort m/s km/h Label Effect on sea
1 0 0 - 0.2 1 Calm Sea like a mirror
1-3 1 0.3-1.5 1-5 Light Air Ripples with the appearance of scales are formed, but without foam crests
4-6 2 1.6-3.3 6-11 Light Breeze Small wavelets, still short, but more pronounced. Crests have a glassy appearance and do not break
7-10 3 3.4-5.4 12-19 Gentle Breeze Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses
11-15 4 5.5-7.9 20-28 Moderate Breeze Small waves, becoming larger fairly frequent white horses
16-21 5 8.0-10.7 29-38 Fresh Breeze Moderate waves, taking a more pronounced long form many white horses are formed. Chance of some spray
22-27 6 10.8-13.8 39-49 strong Breeze Large waves begin to form the white foam crests are more extensive everywhere. Probably some spray
28-33 7 13.9-17.1 50-61 Near Gale Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind
34-40 8 17.2-20.7 62-74 Gale 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
41-47 9 20.8-24.4 75-88 Severe 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 affect visibility
48-55 10 24.5-28.4 89-102 Storm Very high waves with long over-hanging 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 shock-like. Visibility affected
56-63 11 28.5-32.6 103-117 Violent Storm Exceptionally high waves (small and medium-size ships might disappear behind the waves). The sea is completely covered with long white patches of foam flying along the direction of the wind. Everywhere the edges of the wave crests are blown into froth. Visibility affected
64-71 12 32.7-36.9 118-133 Hurricane The air is filled with foam and spray. Sea completely white with driving spray visibility very seriously affected