Beaufort Scale
The Beaufort Scale was devised by Admiral Sir Francis Beaufort in 1805/06. It is a way of estimating the wind strength according to the appearance of the sea (or on land, largely by the response of trees).
| Force / Warning | Short Description | Specifications for use on Sea/Land | Mean wind speed* at 10m above ground level | ||
|---|---|---|---|---|---|
| Knots | mph | km/h | |||
| 0 | Calm | Smoke rises vertically | < 1 | < 1 | < 1 |
| 1 | Light air | Ripples/Direction of wind shown by smoke but not by wind vanes | 1–3 | 1–3 | 1–5 |
| 2 | Light breeze | Small wavelets/Wind felt on face, leaves rustle, ordinary vanes moved by wind | 4–6 | 4–7 | 6–11 |
| 3 | Gentle breeze | Large wavelets, crests begin to break/Leaves and small twigs in constant motion, wind extends light flag | 7–10 | 8–12 | 12–19 |
| 4 | Moderate breeze | Small waves, becoming larger; fairly frequent white horses/Raises dust and loose paper, small branches are moved | 11–16 | 13–18 | 20–28 |
| 5 | Fresh breeze | Moderate waves, taking a more pronounced, longer form; many white horses are formed. Chance of some spray/Small trees in leaf begin to sway, crested wavelets form on inland waters | 17–21 | 19–24 | 29–38 |
6  |
Strong breeze | Large waves begin to form; the white foam crests are more extensive everywhere. Some spray/Large branches in motion, whistling heard in electricity wires; umbrellas used with difficulty | 22–27 | 25–31 | 39–49 |
| 7 |
Near gale | Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind/Whole trees in motion, inconvenience walking against the wind | 28–33 | 32–38 | 50–61 |
8  |
Gale | Moderately high waves of greater length; edges of crests begin to break into spindrift. The foam is blown in well-marked streaks/Breaks off twigs, generally impedes progress | 34–40 | 39-46 | 62–74 |
| 9 |
Strong gale | High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over/Slight structural damage occurs (chimney pots and slates removed) | 41–47 | 47–54 | 75–88 |
10  |
Storm | Very high waves with long overhanging crests. The resulting foam, in great patches, is blown in dense white streaks, along the direction of the wind. The whole surface of the sea takes on a white appearance. Visibility affected/Seldom experienced inland, trees uprooted, considerable structural damage occurs | 48–55 | 55–63 | 89–102 |
11  |
Violent storm | Exceptionally high waves (small and medium ships might be, for a time, lost to view behind the waves). The surface is covered with long white patches of foam lying along the direction of the wind. Everywhere, the edges of the wave crests are being blown into the froth. Visibility affected/Very rarely experienced, accompanied by widespread damage | 56–63 | 64–72 | 103–117 |
| 12 |
Hurricane | The air is filled with foam and spray. Sea completely white with driving spray. | 64> | 73> | 117> |
*Note: Beaufort Scale wind speed are mean speeds (i.e. the wind speed averaged over 10 minutes).