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	Short Description	Specifications for use on Sea/Land	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>