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Beaufort Scale
Beaufort Scale Sailor's description Wind Speed Effects of wind on sea Probable wave height
(metres)
    mph knots   average max
0 Calm (up to): 0 - 1 0 - 1 Sea is like a mirror and smoke rises vertically. - -
1 Light air 1 - 3 1 - 3 Ripples with the appearance of scales are formed, but without foam crests. 0.1 0.1
2 Light breeze 4 - 7 4 - 6 Small wavelets, still short, but more pronounced. Crests have a glassy appearance and do not break. 0.2 0.3
3 Gentle breeze 8 - 12 7 - 10 Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses. 0.6 1.0
4 Moderate breeze 13 - 18 11 - 16 Small waves, becoming larger; fairly frequent white horses. 1.0 1.5
5 Fresh breeze 19 - 24 17 - 21 Moderate waves, taking a more pronounced long form. Many white horses are formed. Chance of some spray. 2.0 2.5
6 Strong breeze 25 - 31 22 - 27 Large waves (breakers) begin to form; the white foam crests are more extensive everywhere. Probably some spray. 3.0 4.0
7 Near gale 32 - 38 28 - 33 Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind. 4.0 5.5
8 Gale 39 - 46 34 - 40 Moderately high waves. Edges of crests begin to break into spindrift. 5.5 7.5
9 Severe gale 47 - 54 41 - 47 High waves. Crests of waves begin to topple, tumble and roll over. Dense streaks of foam along the direction of the wind. Spray may affect visibility. 7.0 10.0
10 Storm 55 - 63 48 - 55 Very high waves with long over-hanging crests. Streaks of foam form great patches. 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. 9.0 12.5
11 Violent storm 64 - 72 56 - 63 Exceptionally high waves (small and medium-size ships might be for a time lost to view behind the waves). The sea is completely covered with long white patches of foam. The edges of the wave crests are blown into froth. Visibility affected. 11.5 16.0
12 Hurricane (over) 73 + 64 + The air is filled with foam and spray. Sea completely white. Visibility very seriously affected. 14.0 -
Rough Wind Force Scale
The probable height of waves indicates conditions that might generally be encountered on high seas, some distance from the coast, and must never be used the other way round, i.e. to estimate or report the state of the sea. Remember that on internal seas, near the coast, and with land winds, waves are less high and not as steep.
History
The scale is named after the Irish admiral Francis Beaufort (1774-1857) who worked for the British Hydrographic Office. In 1806, he proposed a scale for classifying wind strength into 13 degrees, which was then adopted by the British admiralty in 1838, and later by all the other countries.