Beaufort scale
 

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In the beginning of the 19th century, sailors were describing wind forces instead of wind speeds, yet they lacks a uniform scale to estimate these forces.
His scale as well as the more precise wind speeds in Knots or sometimes in meters per second are now widely used.
Beaufort scale
 Specification and speeds Equivalent speed at 10 m
above sea level
 
Mean Limits Description in forecasts State
of
sea
Probable height of waves* metres
 Force  Description  Specification for use at sea* knots metres per second
 
knots metres per second
0 Calm
 
Sea like a mirror. 0 0.0 <1 0.0-0.2 Calm Calm 0.0
1 Light air Ripples with the appearance of scales are formed, but without foam crests. 2 0.8

1-3

0.3-1.5 Light Calm 0.1 (0.1)
2 Light breeze Small wavelets, still short but more pronounced. Crests have a glassy appearance and do not break. 5 2.4 4-6 1.6-3.3 Light Smooth 0.2 (0.3)
3 Gentle breeze Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses. 9 4.3 7-10 3.4-5.4 Light Smooth 0.6 (1.0)
4 Moderate breeze Large waves begin to form; the white foam crests are more extensive everywhere. Probably some spray. 13 6.7 11-16 5.5-7.9 Moderate Slight 1.0 (1.5)
5 Fresh breeze Moderate waves, taking a more pronounced long form; many white horses are formed. Chance of some spray 19 9.3 17-21 8.0-10.7 Fresh Moderate 2.0 (2.5)
6 Strong breeze Large waves begin to form; the white foam crests are more extensive everywhere. Probably some spray. 24 12.3 22-27 10.8-13.8 Strong Rough 3.0 (4.0)
7 Near gale Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind. 30 15.5 28-33 13.9-17.1 Strong Very rough 4.0 (5.5)
8 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. 37 18.9 34-40 17.2-20.7 Gale High 5.5 (7.5)
9 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 affect visibility. 44 22.6 41-47 20.8-24.4 Severe gale Very high 7.0 (10.0)
10 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 takes a white appearance.The 'tumbling' of the sea becomes heavy and shock-like. Visibility affected. 52 26.4 48-55 24.5-28.4 Storm Very high 9.0 (12.5)
11 Violent storm Exceptionally high waves (small and medium-sized ships might be lost to view for a time 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. 60 30.5 56-63 28.5-32.6 Violent storm Phenomenal 11.5 (16.0)
12 Hurricane The air is filled with foam and spray. Sea completely white with driving spray; visibility seriously affected. - - 64 and over 32.7 and over Hurricane force Phenomenal 14.0 (-)

* These columns are a guide to show roughly what may be expected in the open sea, remote from land. Figures in brackets indicate the probable maximum height of waves. In enclosed waters, or when near land with an offshore wind, wave heights will be smaller and the waves steeper.
 

 

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7 Feb 2017
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