The Beaufort Scale for wind and sea state

Beaufort
number
Description Sea State Velocity in knots Probable wave height (m) Probable max. wave height (m)  
0 Calm Sea like a mirror less than 1 0 0
1 Light air Ripples formed, no foam crests 1 - 3 0 0  
2 Light breeze Small wavelets, short but more pronounced. Crests glassy but do not break. 4 - 6 0.1 0.3
3 Gentle breeze Large wavelets, crests start to break. Foam glassy, scattered white horses. 7 - 10 0.4 1  
4 Moderate breeze Small waves becoming longer, fairly frequent white horses. 11 - 16 1 1.5
5 Fresh breeze Moderate waves, more pronounced long form, many white horses, some spray. 17 - 21 2 2.5  
6 Strong breeze Large waves start to form, extensive white foam crests, spray. 22 - 27 3 4  
7 Near gale Sea heaps up, white foam from breaking waves starts blowing in streaks along wind direction. 28 - 33 4 5.5  
8 Gale Moderately high waves in greater length, crests break off in spindrift, foam blown in well marked streaks. 34 - 40 5.5 7.5  
9 Severe Gale High waves with tumbling crests; dense streaks of foam; spray may affect visibility 41 - 47 7 10  
10 Storm Very high waves with long overhanging crests; dense streams of foam make surface of sea white. Heavy tumbling sea; visibility affected. 48 - 55 9 12.5  
11 Violent storm Exceptionally high waves; sea completely covered with long white patches of foam; edges of wave crests blown into froth. Visibility affected. 56 - 63 11 16  
12 Hurricane Air filled with foam and spray; sea completely white with driving spray; visibility very seriously affected. 64+ 14 -  

Note that gusts may be up to 50% higher the average wind strength (i.e. wind force) forecast.

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