The Beaufort scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort wind force scale.
The Beaufort scale is not an exact nor an objective scale; it was based on visual and subjective observation of a ship and of the sea. The corresponding integral wind speeds were determined later, but the values in different units were never made equivalent.
The Beaufort scale was extended in 1946 when forces 13 to 17 were added. However, forces 13 to 17 were intended to apply only to special cases, such as tropical cyclones. Nowadays, the extended scale is only used in Taiwan and mainland China, which are often affected by typhoons. Internationally, WMO Manual on Marine Meteorological Services (2012 edition) defined the Beaufort Scale only up to force 12 and there was no recommendation on the use of the extended scale.
Wave heights in the scale are for conditions in the open ocean, not along the shore.
| Beaufort | Description | Wind speed | Wave height | ||||
|---|---|---|---|---|---|---|---|
| In KNOTS | >In MPH | >In KM/h | >In M/S | >In FT | >In mt | ||
| 0 | Calm | < 1 kn | < 1 mph | < 2 km/h | < 0.5 m/s | 0 ft | 0 mt |
| 1 | Light air | 1–3 kn | 1–3 mph | 2–5 km/h | 0.5–1.5 m/s | 0–1 ft | 0–0.3 mt |
| 2 | Light breeze | 4–6 kn | 4–7 mph | 6–11 km/h | 1.6–3.3 m/s | 1–2 ft | 0.3–0.6 mt |
| 3 | Gentle breeze | 7–10 kn | 8–12 mph | 12–19 km/h | 3.4–5.5 m/s | 2–4 ft | 0.6–1.2 mt |
| 4 | Moderate breeze | 11–16 kn | 13–18 mph | 20–28 km/h | 5.5–7.9 m/s | 3.5–6 ft | 1–2 mt |
| 5 | Fresh breeze | 17–21 kn | 19–24 mph | 29–38 km/h | 8–10.7 m/s | 6–10 ft | 2–3 mt |
| 6 | Strong breeze | 22–27 kn | 25–31 mph | 39–49 km/h | 10.8–13.8 m/s | 9–13 ft | 3–4 mt |
| 7 | High wind, moderate gale, near gale | 28–33 kn | 32–38 mph | 50–61 km/h | 13.9–17.1 m/s | 13–19 ft | 4–5.5 mt |
| 8 | Gale, fresh gale | 34–40 kn | 39–46 mph | 62–74 km/h | 17.2–20.7 m/s | 18–25 ft | 5.5–7.5 mt |
| 9 | Strong or severe gale | 41–47 kn | 47–54 mph | 75–88 km/h | 20.8–24.4 m/s | 23–32 ft | 7–10 mt |
| 10 | Storm, whole gale | 48–55 kn | 55–63 mph | 89–102 km/h | 24.5–28.4 m/s | 29–41 ft | 9–12.5 mt |
| 11 | Violent storm | 56–63 kn | 64–72 mph | 103–117 km/h | 28.5–32.6 m/s | 37–52 ft | 11.5–16 mt |
| 12 | Hurricane-force | ≥ 64 kn | ≥ 73 mph | ≥ 118 km/h | ≥ 32.7 m/s | ≥ 46 ft | ≥ 14 mt |
| Beaufort | Description | Wind Speed | Wave Height | Sea conditions | Land conditions | |
|---|---|---|---|---|---|---|
| IN KNOTS | IN FT | IN MT | ||||
| 0 | Calm | < 1 kn | 0 ft | 0 mt | Sea like a mirror | Smoke rises vertically. |
| 1 | Light air | 1–3 kn | 0–1 ft | 0–0.3 mt | Ripples with appearance of scales are formed, without foam crests | Direction shown by smoke drift but not by wind vanes. |
| 2 | Light breeze | 4–6 kn | 1–2 ft | 0.3–0.6 mt | Small wavelets still short but more pronounced; crests have a glassy appearance but do not break | Wind felt on face; leaves rustle; wind vane moved by wind. |
| 3 | Gentle breeze | 7–10 kn | 2–4 ft | 0.6–1.2 mt | Large wavelets; crests begin to break; foam of glassy appearance; perhaps scattered white horses | Leaves and small twigs in constant motion; light flags extended. |
| 4 | Moderate breeze | 11–16 kn | 3.5–6 ft | 1–2 mt | Small waves becoming longer; fairly frequent white horses | Raises dust and loose paper; small branches moved. |
| 5 | Fresh breeze | 17–21 kn | 6–10 ft | 2–3 mt | Moderate waves taking a more pronounced long form; many white horses are formed; chance of some spray | Small trees in leaf begin to sway; crested wavelets form on inland waters. |
| 6 | Strong breeze | 22–27 kn | 9–13 ft | 3–4 mt | Large waves begin to form; the white foam crests are more extensive everywhere; probably some spray | Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty. |
| 7 | High wind, moderate gale, near gale | 28–33 kn | 13–19 ft | 4–5.5 mt | Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind; spindrift begins to be seen | Whole trees in motion; inconvenience felt when walking against the wind. |
| 8 | Gale, fresh gale | 34–40 kn | 18–25 ft | 5.5–7.5 mt | Moderately high waves of greater length; edges of crests break into spindrift; foam is blown in well-marked streaks along the direction of the wind | Twigs break off trees; generally impedes progress. |
| 9 | Strong or severe gale | 41–47 kn | 23–32 ft | 7–10 mt | High waves; dense streaks of foam along the direction of the wind; sea begins to roll; spray affects visibility | Slight structural damage (chimney pots and slates removed). |
| 10 | Storm, whole gale | 48–55 kn | 29–41 ft | 9–12.5 mt | Very high waves with long overhanging crests; 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; rolling of the sea becomes heavy; visibility affected | Seldom experienced inland; trees uprooted; considerable structural damage. |
| 11 | Violent storm | 56–63 kn | 37–52 ft | 11.5–16 mt | Exceptionally high waves; small- and medium-sized ships might be for a long time lost to view behind the waves; sea is covered with long white patches of foam; everywhere the edges of the wave crests are blown into foam; visibility affected | Very rarely experienced; accompanied by widespread damage. |
| 12 | Hurricane-force | ≥ 64 kn | ≥ 46 ft | ≥ 14 mt | The air is filled with foam and spray; sea is completely white with driving spray; visibility very seriously affected | Devastation. |
Data graphic showing Beaufort wind force in scale units, knots and metres/second
Wind speed on the 1946 Beaufort scale is based on the empirical relationship:
v = 0.836 B(elevated 3/2) m/s
where v is the equivalent wind speed at 10 metres above the sea surface and B is Beaufort scale number. For example, B = 9.5 is related to 24.5 m/s which is equal to the lower limit of “10 Beaufort”.
Using this formula the highest winds in hurricanes would be 23 in the scale. F1 tornadoes on the Fujita scale and T2 TORRO scale also begin roughly at the end of level 12 of the Beaufort scale, but are independent scales – although the TORRO scale wind values are based on the 3/2 power law relating wind velocity to Beaufort force.
History
The scale was devised in 1805 by the Irish hydrographer Francis Beaufort (later Rear Admiral), a Royal Navy officer, while serving on HMS Woolwich.
The scale that carries Beaufort’s name had a long and complex evolution from the previous work of others (including Daniel Defoe the century before) to when Beaufort was Hydrographer of the Navy in the 1830s, when it was adopted officially and first used during the voyage of HMS Beagle under Captain Robert FitzRoy, who was later to set up the first Meteorological Office (Met Office) in Britain giving regular weather forecasts.
In the 18th century, naval officers made regular weather observations, but there was no standard scale and so they could be very subjective – one man’s “stiff breeze” might be another’s “soft breeze”. Beaufort succeeded in standardising the scale.
The initial scale of thirteen classes (zero to twelve) did not reference wind speed numbers but related qualitative wind conditions to effects on the sails of a frigate, then the main ship of the Royal Navy, from “just sufficient to give steerage” to “that which no canvas sails could withstand”.
The scale was made a standard for ship’s log entries on Royal Navy vessels in the late 1830s and was adapted to non-naval use from the 1850s, with scale numbers corresponding to cup anemometer rotations. In 1853, the Beaufort scale was accepted as generally applicable at the First International Meteorological Conference in Brussels.
In 1916, to accommodate the growth of steam power, the descriptions were changed to how the sea, not the sails, behaved and extended to land observations. Rotations to scale numbers were standardized only in 1923. George Simpson, CBE (later Sir George Simpson), director of the UK Meteorological Office, was responsible for this and for the addition of the land-based descriptors.
The measures were slightly altered some decades later to improve its utility for meteorologists. Nowadays, meteorologists typically express wind speed in kilometres or miles per hour or, for maritime and aviation purposes, knots; but Beaufort scale terminology is still sometimes used in weather forecasts for shipping and the severe weather warnings given to the public.
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