The Beaufort scale

wind speed

Beaufort number name of wind knots km/hr description of sea surface

0 calm \\<1 \\<1 like a mirror

1 light air 1–3 1–5 ripples with the appearance of scales are formed, but without foam crests

2 light breeze 4–6 6–11 small wavelets, still short but more pronounced; crests have a glassy appearance and do not break

3 gentle breeze 7–10 12–19 large wavelets; crests begin to break; foam of glassy appearance, perhaps scattered white horses

4 moderate breeze 11–16 20–28 small waves, becoming longer; fairly frequent white horses

5 fresh breeze 17–21 29–38 moderate waves, taking a more pronounced long form; many white horses are formed (chance of some spray)

6 strong breeze 22–27 39–49 large waves begin to form; the white foam crests are more extensive everywhere (probably some spray)

7 moderate gale 28–33 50–61 sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind

8 fresh gale 34–40 62–74 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

9 strong gale 41–47 75–88 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

10 whole gale 48–55 89–102 very high waves with 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 shocklike; visibility affected

11 storm 56–65 103–117 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

12–17 hurricane above 65 above 117 the air is filled with foam and spray; sea completely white with driving spray; visibility very seriously affected