Beaufort scale

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Ship in Force 12 storm at sea Beaufort scale 12 notext.jpg
Ship in Force 12 storm at sea

The Beaufort scale /ˈbfərt/ 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 scale was devised in 1805 by the Irish hydrographer Francis Beaufort (later Rear Admiral Sir Francis Beaufort), 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. [1] 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.

Sir Francis Beaufort FrancisBeaufort.jpg
Sir Francis Beaufort

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". [2]

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. [3] 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. [1] The measures were slightly altered some decades later to improve its utility for meteorologists. Nowadays, meteorologists typically express wind speed in kilometers per hour or miles per hour, but Beaufort scale terminology is still used for weather forecasts for shipping [4] and the severe weather warnings given to the public. [5]

Extended scale

The Beaufort scale was extended in 1946 when forces 13 to 17 were added. [3] 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. [6]

Wind speed on the 1946 Beaufort scale is based on the empirical relationship: [7]

v = 0.836 B3/2 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.

data graphic showing Beaufort wind force scale in scale units, knots, and meters/second. Beaufort wind scale.png
data graphic showing Beaufort wind force scale in scale units, knots, and meters/second.

Today, hurricane-force winds are sometimes described as Beaufort scale 12 through 16, very roughly related to the respective category speeds of the Saffir–Simpson hurricane scale, by which actual hurricanes are measured, where Category 1 is equivalent to Beaufort 12. However, the extended Beaufort numbers above 13 do not match the Saffir–Simpson 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. [8]

Wave heights in the scale are for conditions in the open ocean, not along the shore.

Modern scale

Beaufort Scale [9] [10] [11]
Beaufort numberDescriptionWind speedWave heightSea conditionsLand conditionsSea conditions (photo)Associated warning flag
0Calm< 1  knot
< 1  mph
< 2  km/h
< 0.5  m/s
0 ft (0 m)Sea like a mirrorSmoke rises vertically. Beaufort scale 0.jpg
1Light air1–3 knots0–1 ftRipples with appearance of scales are formed, without foam crestsDirection shown by smoke drift but not by wind vanes. Beaufort scale 1.jpg
1–3 mph
2–5 km/h0–0.3 m
0.5–1.5 m/s
2Light breeze4–6 knots1–2 ftSmall wavelets still short but more pronounced; crests have a glassy appearance but do not breakWind felt on face; leaves rustle; wind vane moved by wind. Beaufort scale 2.jpg
4–7 mph
6–11 km/h0.3–0.6 m
1.6–3.3 m/s
3Gentle breeze7–10 knots2–4 ftLarge wavelets; crests begin to break; foam of glassy appearance; perhaps scattered white horsesLeaves and small twigs in constant motion; light flags extended. Beaufort scale 3.jpg
8–12 mph
12–19 km/h0.6–1.2 m
3.4–5.5 m/s
4Moderate breeze11–16 knots3.5–6 ftSmall waves becoming longer; fairly frequent white horsesRaises dust and loose paper; small branches moved. Beaufort scale 4.jpg
13–18 mph
20–28 km/h1–2 m
5.5–7.9 m/s
5Fresh breeze17–21 knots6–10 ftModerate waves taking a more pronounced long form; many white horses are formed; chance of some spraySmall trees in leaf begin to sway; crested wavelets form on inland waters. Beaufort scale 5.jpg
19–24 mph
29–38 km/h2–3 m
8–10.7 m/s
6Strong breeze22–27 knots9–13 ftLarge waves begin to form; the white foam crests are more extensive everywhere; probably some sprayLarge branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty. Beaufort scale 6.jpg Gale pennant.svg
25–31 mph
39–49 km/h3–4 m
10.8–13.8 m/s
7High wind,
moderate gale,
near gale
28–33 knots13–19 ftSea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind; spindrift begins to be seenWhole trees in motion; inconvenience felt when walking against the wind. Beaufort scale 7.jpg Gale pennant.svg
32–38 mph
50–61 km/h4–5.5 m
13.9–17.1 m/s
8 Gale,
fresh gale
34–40 knots18–25 ftModerately high waves of greater length; edges of crests break into spindrift; foam is blown in well-marked streaks along the direction of the windTwigs break off trees; generally impedes progress. Beaufort scale 8.jpg Gale pennant.svg
Gale pennant.svg
39–46 mph
62–74 km/h5.5–7.5 m
17.2–20.7 m/s
9Strong/severe gale41–47 knots23–32 ftHigh waves; dense streaks of foam along the direction of the wind; sea begins to roll; spray affects visibilitySlight structural damage (chimney pots and slates removed). Beaufort scale 9.jpg Gale pennant.svg
Gale pennant.svg
47–54 mph
75–88 km/h7–10 m
20.8–24.4 m/s
10 Storm, [12]
whole gale
48–55 knots29–41 ftVery 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 affectedSeldom experienced inland; trees uprooted; considerable structural damage. Beaufort scale 10.jpg Storm warning.svg
55–63 mph
89–102 km/h9–12.5 m
24.5–28.4 m/s
11Violent storm56–63 knots37–52 ftExceptionally 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 affectedVery rarely experienced; accompanied by widespread damage. Beaufort scale 11.jpg Storm warning.svg
64–72 mph
103–117 km/h11.5–16 m
28.5–32.6 m/s
12 Hurricane force [12] ≥ 64 knots≥ 46 ftThe air is filled with foam and spray; sea is completely white with driving spray; visibility very seriously affectedDevastation. Beaufort scale 12.jpg Storm warning.svg

Storm warning.svg
≥ 73 mph
≥ 118 km/h≥ 14 m
≥ 32.7 m/s

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 scale is used in the Shipping Forecasts broadcast on BBC Radio 4 in the United Kingdom, and in the Sea Area Forecast from Met Éireann, the Irish Meteorological Service. Met Éireann issues a "Small Craft Warning" if winds of Beaufort force 6 (mean wind speed exceeding 22 knots) are expected up to 10 nautical miles offshore. Other warnings are issued by Met Éireann for Irish coastal waters, which are regarded as extending 30 miles out from the coastline, and the Irish Sea or part thereof: "Gale Warnings" are issued if winds of Beaufort force 8 are expected; "Strong Gale Warnings" are issued if winds of Beaufort force 9 or frequent gusts of at least 52 knots are expected.; "Storm Force Warnings" are issued if Beaufort force 10 or frequent gusts of at least 61 knots are expected; "Violent Storm Force Warnings" are issued if Beaufort force 11 or frequent gusts of at least 69 knots are expected; "Hurricane Force Warnings" are issued if winds of greater than 64 knots are expected.

This scale is also widely used in the Netherlands, Germany, [13] Greece, China, Taiwan, Hong Kong, Malta, and Macau, although with some differences between them. Taiwan uses the Beaufort scale with the extension to 17 noted above. China also switched to this extended version without prior notice on the morning of 15 May 2006, [14] and the extended scale was immediately put to use for Typhoon Chanchu. Hong Kong and Macau retain force 12 as the maximum.

In the United States, winds of force 6 or 7 result in the issuance of a small craft advisory, with force 8 or 9 winds bringing about a gale warning, force 10 or 11 a storm warning ("a tropical storm warning" being issued instead of the latter two if the winds relate to a tropical cyclone), and force 12 a hurricane-force wind warning (or hurricane warning if related to a tropical cyclone). A set of red warning flags (daylight) and red warning lights (night time) is displayed at shore establishments which coincide with the various levels of warning.

In Canada, maritime winds forecast to be in the range of 6 to 7 are designated as "strong"; 8 to 9 "gale force"; 10 to 11 "storm force"; 12 "hurricane force". Appropriate wind warnings are issued by Environment Canada's Meteorological Service of Canada: strong wind warning, gale (force wind) warning, storm (force wind) warning and hurricane-force wind warning. These designations were standardized nationally in 2008, whereas "light wind" can refer to 0 to 12 or 0 to 15 knots and "moderate wind" 12 to 19 or 16 to 19 knots, depending on regional custom, definition or practice. Prior to 2008, a "strong wind warning" would have been referred to as a "small craft warning" by Environment Canada, similar to US terminology. (Canada and the USA have the Great Lakes in common.)

See also

Related Research Articles

The Saffir–Simpson hurricane wind scale (SSHWS), formerly the Saffir–Simpson hurricane scale (SSHS), classifies hurricanes – Western Hemisphere tropical cyclones – that exceed the intensities of tropical depressions and tropical storms – into five categories distinguished by the intensities of their sustained winds.

Gale strong wind

A gale is a strong wind, typically used as a descriptor in nautical contexts. The U.S. National Weather Service defines a gale as 34–47 knots of sustained surface winds. Forecasters typically issue gale warnings when winds of this strength are expected. In the United States, a gale warning is specifically a maritime warning; the land-based equivalent in National Weather Service warning products is a wind advisory.

Small craft advisory

A small craft advisory is a type of wind warning issued by the National Weather Service in the United States. In Canada a similar warning is issued by Environment Canada. It is issued when winds have reached, or are expected to reach within 12 hours, a speed marginally less than gale force. A Small Craft Advisory may also be issued when sea or lake ice exists that could be hazardous to small boats.

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Gale warning

Weather services issue a gale warning for maritime locations currently or imminently experiencing winds of Gale Force on the Beaufort scale. Gale warnings allow mariners to take precautionary actions to ensure their safety at sea or to seek safe anchorage and ride out the storm on land. Though usually associated with deep low-pressure areas, winds strong enough to catalyze a gale warning can occur in other conditions too, including from anticyclones, or high-pressure systems, in the continental interior. The winds are not directly associated with a tropical cyclone.

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Cyclone Ofa Category 4 South Pacific cyclone in 1990

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  12. 1 2 The names "storm" and "hurricane" on the Beaufort scale refer only to wind strength, and do not necessarily mean that other severe weather (for instance, a thunderstorm or tropical cyclone) is present. To avoid confusion, strong wind warnings will often speak of e.g. "hurricane-force winds".
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