Exceptionally high waves. The sea is completely covered in long white patches of foam, and visibility is reduced.
Widespread damage. Very rarely experienced.
Force 11, Violent storm, is a wind of 56–63 kn, which builds 11.5 m waves in the open sea.
| Force | Description | Wind | Waves | Sea state |
|---|---|---|---|---|
| 0 | Calm | < 1 kn | 0.0 m waves | Sea like a mirror. |
| 1 | Light air | 1–3 kn | 0.1 m waves | Ripples with the appearance of scales, but without foam crests. |
| 2 | Light breeze | 4–6 kn | 0.2 m waves | Small wavelets, still short but more pronounced. Crests look glassy and do not break. |
| 3 | Gentle breeze | 7–10 kn | 0.6 m waves | Large wavelets. Crests begin to break, with scattered white horses. |
| 4 | Moderate breeze | 11–16 kn | 1.0 m waves | Small waves becoming longer, with fairly frequent white horses. |
| 5 | Fresh breeze | 17–21 kn | 2.0 m waves | Moderate waves taking a longer form. Many white horses, and some spray. |
| 6 | Strong breeze | 22–27 kn | 3.0 m waves | Large waves begin to form. White foam crests are extensive everywhere, with some spray. |
| 7 | Near gale | 28–33 kn | 4.0 m waves | The sea heaps up, and white foam from breaking waves blows in streaks along the wind. |
| 8 | Gale | 34–40 kn | 5.5 m waves | Moderately high waves of greater length. Crest edges break into spindrift, and foam blows in well-marked streaks. |
| 9 | Strong gale | 41–47 kn | 7.0 m waves | High waves with dense streaks of foam. Crests topple, tumble and roll over, and spray can reduce visibility. |
| 10 | Storm | 48–55 kn | 9.0 m waves | Very high waves with long overhanging crests. The sea surface looks white, and visibility is affected. |
| 11 | Violent storm | 56–63 kn | 11.5 m waves | Exceptionally high waves. The sea is completely covered in long white patches of foam, and visibility is reduced. |
| 12 | Hurricane force | 64+ kn | 14.0 m waves | The air is filled with foam and spray. The sea is completely white with driving spray, and visibility is very seriously affected. |