This scale of wind speeds was devised in 1805 by Sir Francis Beaufort.
Force |
Speed |
Conditions |
|
|---|---|---|---|
knots |
km/h |
||
| 0 |
<1
|
<1
|
Calm, sea like a mirror. |
| 1 |
1-3
|
1-5
|
Light air, ripples only. |
| 2 |
4-6
|
6-11
|
Light breeze, small wavelets (0.2m). Crests have a glassy appearance. |
| 3 |
7-10
|
12-19
|
Gentle breeze, large wavelets (0.6m), crests begin to break. |
| 4 |
11-16
|
20-29
|
Moderate breeze, small waves (1m), some white horses. |
| 5 |
17-21
|
30-39
|
Fresh breeze, moderate waves (1.8m), many white horses. |
| 6 |
22-27
|
40-50
|
Strong breeze, large waves (3m), probably some spray. |
| 7 |
28-33
|
51-61
|
Near gale, mounting sea (4m) with foam blown in streaks downwind. |
| 8 |
34-40
|
62-74
|
Gale, moderately high waves (5.5m), crests break into spindrift. |
| 9 |
41-47
|
76-87
|
Strong gale, high waves (7m), dense foam, visibility affected. |
| 10 |
48-55
|
88-102
|
Storm, very high waves (9m), heavy sea roll, visibility impaired. Surface generally white. |
| 11 |
56-63
|
103-118
|
Violent storm, exceptionally high waves (11m), visibility poor. |
| 12 |
64+
|
119
|
Hurricane, 14m waves, air filled with foam and spray, visibility bad. |
Wave heights quoted are approximately those that may be expected in the open sea. In enclosed waters the waves will be smaller and steeper. Fetch, depth, swell, heavy rain and tide will also affect their height, and there will also usually be a time lag between any increase in the wind and the consequent increase in the sea.