Knots

Bft.

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