Beaufort scale

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Force 12 at sea Beaufort scale 12 notext.jpg
Force 12 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.

Wind speed

Wind speed, or wind flow velocity, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Note that wind direction is usually almost parallel to isobars, due to Earth's rotation.



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, later to set up the first Meteorological Office (Met Office) in Britain giving regular weather forecasts. [1] In the early 19th 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.

Francis Beaufort Irish hydrographer and naval officer

Rear Admiral Sir Francis Beaufort, KCB, FRS, FRGS, FRAS, MRIA was an Irish hydrographer and officer in the Royal Navy. Beaufort was the creator of the Beaufort Scale for indicating wind force.

Rear admiral (Royal Navy) flag officer rank of the British Royal Navy

Rear admiral (RAdm) is a flag officer rank of the British Royal Navy. It is immediately superior to commodore and is subordinate to vice admiral. It is a two-star rank and has a NATO ranking code of OF-7.

Royal Navy Maritime warfare branch of the United Kingdoms military

The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by the English kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years War against the Kingdom of France. The modern Royal Navy traces its origins to the early 16th century; the oldest of the UK's armed services, it is known as the Senior Service.

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]

Frigate Type of warship

A frigate is a type of warship, having various sizes and roles over the last few centuries.

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 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 measure was slightly altered some decades later to improve its utility for meteorologists. Today, many countries have abandoned the scale and use the metric system based units, m/s or km/h, instead,[ citation needed ] but the severe weather warnings given to the public are still approximately the same as when using the Beaufort scale.

Anemometer meteorological instrumentation used for measuring the speed of wind

An anemometer is a device used for measuring wind speed, and is also a common weather station instrument. The term is derived from the Greek word anemos, which means wind, and is used to describe any wind speed instrument used in meteorology. The first known description of an anemometer was given by Leon Battista Alberti in 1450.

George Simpson (meteorologist) British meteorologist

Sir George Clarke Simpson KCB CBE FRS HFRSE LLD was a British meteorologist. He was President of the Royal Meteorological Society 1940/41.

Metric system decimal system of units of measurement

The metric system is an internationally recognised decimalised system of measurement. It is in widespread use, and where it is adopted, it is the only or most common system of weights and measures. It is now known as the International System of Units (SI). It is used to measure everyday things such as the mass of a sack of flour, the height of a person, the speed of a car, and the volume of fuel in its tank. It is also used in science, industry and trade.

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. [4]

Tropical cyclone Is a rotating storm system

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".

Typhoon type of tropical cyclone

A typhoon is a mature tropical cyclone that develops between 180° and 100°E in the Northern Hemisphere. This region is referred to as the Northwestern Pacific Basin, and is the most active tropical cyclone basin on Earth, accounting for almost one-third of the world's annual tropical cyclones. For organizational purposes, the northern Pacific Ocean is divided into three regions: the eastern, central, and western. The Regional Specialized Meteorological Center (RSMC) for tropical cyclone forecasts is in Japan, with other tropical cyclone warning centers for the northwest Pacific in Hawaii, the Philippines and Hong Kong. While the RSMC names each system, the main name list itself is coordinated among 18 countries that have territories threatened by typhoons each year A hurricane is a storm that occurs in the Atlantic Ocean or the northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a tropical cyclone occurs in the South Pacific or the Indian Ocean.

Wind speed on the 1946 Beaufort scale is based on the empirical relationship: [5]

In science, an empirical relationship or phenomenological relationship is a relationship or correlation that is supported by experiment and observation but not necessarily supported by theory.

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. Category 1 tornadoes on the Fujita and TORRO scales 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. [6]

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

Modern scale

Beaufort numberDescriptionWind speedWave heightSea conditionsLand conditionsSea state photoAssociated warning flag
0Calm< 1  knot 0  ft Sea like a mirrorSmoke rises vertically. Beaufort scale 0.jpg
< 1  mph
< 2  km/h 0  m
< 0.5  m/s
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, [7]
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 [7] ≥ 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
References: Met Office, [8] Royal Meteorological Society, [9] Encyclopædia Britannica [10]

The reason is that 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, [11] 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, [12] 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.)

Beaufort Wind Scale
CalmLight AirLight BreezeGentle BreezeModerate BreezeFresh BreezeStrong BreezeNear Gale Gale Strong Gale Storm Violent Storm Hurricane Force
Light WindsAdvisory-forceGale-forceStorm-forceHurricane-force
<1 mph
<1 knot
<0.3 m/s
1–3 mph
1–3 knots
0.3–1.5 m/s
4–7 mph
4–6 knots
1.6–3.3 m/s
8–12 mph
7–10 knots
3.4–5.5 m/s
13–18 mph
11–16 knots
5.5–7.9 m/s
18–24 mph
17–21 knots
8.0–10.7 m/s
25–31 mph
22–27 knots
10.8–13.8 m/s
31–38 mph
28–33 knots
13.9–17.1 m/s
39–46 mph
34–40 knots
17.2–20.7 m/s
47-54 mph
41–47 knots
20.8–24.4 m/s
55–63 mph
48–55 knots
24.5–28.4 m/s
64–72 mph
56–63 knots
28.5–32.6 m/s
≥73 mph
≥63 knots
≥32.7 m/s

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.

Tropical cyclones and subtropical cyclones are named by various warning centers to provide ease of communication between forecasters and the general public regarding forecasts, watches, and warnings. The names are intended to reduce confusion in the event of concurrent storms in the same basin. Generally once storms produce sustained wind speeds of more than 33 knots, names are assigned in order from predetermined lists depending on which basin they originate. However, standards vary from basin to basin: some tropical depressions are named in the Western Pacific, while tropical cyclones must have a significant amount of gale-force winds occurring around the centre before they are named in the Southern Hemisphere.

Small craft advisory

A small craft advisory is a type of warning issued by the National Weather Service in the United States. 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.

Tropical cyclone warnings and watches are two levels of alert issued by national weather forecasting bodies to coastal areas threatened by the imminent approach of a tropical cyclone of tropical storm or hurricane intensity. They are notices to the local population and civil authorities to make appropriate preparation for the cyclone, including evacuation of vulnerable areas where necessary. It is important that interests throughout the area of an alert make preparations to protect life and property, and do not disregard it on the strength of the detailed forecast track. Tropical cyclones are not points, and forecasting their track remains an uncertain science.

Joint Typhoon Warning Center

The Joint Typhoon Warning Center (JTWC) is a joint United States Navy – United States Air Force command located in Pearl Harbor, Hawaii. The JTWC is responsible for the issuing of tropical cyclone warnings in the North-West Pacific Ocean, South Pacific Ocean, and Indian Ocean for all branches of the U.S. Department of Defense and other U.S. government agencies. Their warnings are intended for the protection of primarily military ships and aircraft as well as military installations jointly operated with other countries around the world.

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.

1910 Atlantic hurricane season hurricane season in the Atlantic Ocean

The 1910 Atlantic hurricane season was the period during the summer and fall of 1910 in which tropical cyclones formed in the North Atlantic Ocean. The season was fairly inactive, with only five storms; however, three grew into hurricanes and one became a major hurricane. The season got off to a late start with the formation of a tropical storm in the Caribbean Sea on August 23. September saw two storms, and the final tropical cyclone—Hurricane Five—existed during October. All but one of the storms made landfall, and the only cyclone which remained at sea had some effects on the island of Bermuda.

Tropical cyclones are unofficially ranked on one of five tropical cyclone intensity scales, according to their maximum sustained winds and which tropical cyclone basin(s) they are located in. Only a few scales of classifications are used officially by the meteorological agencies monitoring the tropical cyclones, but some alternative scales also exist, such as accumulated cyclone energy, the Power Dissipation Index, the Integrated Kinetic Energy Index, and the Hurricane Severity Index.

Cyclone Percy Category 5 South Pacific cyclone in 2005

Cyclone Percy was the seventh named storm of the 2004–05 South Pacific cyclone season and the fourth and final severe tropical cyclone to form during the 2004–05 South Pacific cyclone season.

Tropical cyclone windspeed climatology

Tropical cyclone windspeed climatology is the study wind distribution amongst tropical cyclones, a significant threat to land and people. Since records began in 1851, winds from hurricanes, typhoons and cyclones have been responsible for fatalities and damage in every basin. Major hurricanes usually cause the most wind damage. Hurricane Andrew for example caused $45 billion in damage, most of it wind damage.

This article describes severe weather terminology used by the Meteorological Service of Canada, a branch within Environment and Climate Change Canada. The article primarily describes various weather warnings, and their criteria. Related weather scales and general weather terms are also addressed in this article. Some terms are specific to certain regions.

Outline of meteorology Overview of and topical guide to meteorology

The following outline is provided as an overview of and topical guide to meteorology:

The maximum sustained wind associated with a tropical cyclone is a common indicator of the intensity of the storm. Within a mature tropical cyclone, it is found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are determined via their sampling and averaging the sampled results over a period of time. Wind measuring has been standardized globally to reflect the winds at 10 metres (33 ft) above the Earth's surface, and the maximum sustained wind represents the highest average wind over either a one-minute (US) or ten-minute time span, anywhere within the tropical cyclone. Surface winds are highly variable due to friction between the atmosphere and the Earth's surface, as well as near hills and mountains over land.

The following outline is provided as an overview of and topical guide to tropical cyclones:

Glossary of tropical cyclone terms

The following is a glossary of tropical cyclone terms.

A hurricane force wind warning is a warning issued by the National Weather Service of the United States when sustained winds or frequent gusts of 64 knots or greater are either being observed or are predicted to occur. The winds must not be directly associated with a tropical cyclone, or a hurricane warning will be issued. If winds are lighter than 64 knots, a storm warning or gale warning will be issued. In US maritime warning flag systems, two red square flags with a black square taking up the middle ninth of each flag is used to indicate a hurricane force wind warning.

Cyclone Ofa Category 4 South Pacific cyclone in 1990

Severe Tropical Cyclone Ofa was considered to be the worst tropical cyclone to affect Polynesia since Cyclone Bebe. The system was first noted on January 27, 1990, near Tuvalu, as a shallow tropical depression that had developed within the South Pacific Convergence Zone. The cloud pattern slowly organized, and on January 31, while located east of Tuvalu, Ofa attained cyclone intensity. Moving slowly southeast, Ofa developed storm-force winds. It attained hurricane-force winds on February 2. Cyclone Ofa reached peak intensity on February 4. Shortly after, its peak Ofa began to weaken over a less favourable environment. Ofa was declared an extratropical cyclone on February 8, though the system was still tracked by meteorologists until February 10.

Cyclone Ernie Category 5 Australian region cyclone in 2017

Severe Tropical Cyclone Ernie was one of the quickest strengthening tropical cyclones on record. Ernie was the first Category 5 severe tropical cyclone in the Australian region since Cyclone Marcia in 2015, and also the strongest tropical cyclone in the Australian region since Cyclone George in 2007. Ernie developed from a tropical low into a cyclone south of Indonesia in the northeast Indian Ocean on 6 April 2017, and proceeded to intensify extremely rapidly to a Category 5 severe tropical cyclone. A few days later, on 10 April, the system was downgraded below cyclone intensity following a period of rapid weakening, located southwest of its original position. Ernie had no known impacts on any land areas.


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  2. Oliver, John E. (2005). Encyclopedia of world climatology. Springer.
  3. Walter J. Saucier (1955). Principles of Meteorological Analysis. Retrieved on 2009-01-09.
  5. Tom Beer (1997). Environmental Oceanography. CRC Press. ISBN   0-8493-8425-7.
  6. Maiden, Terence. "T-Scale: Origins and Scientific Basis". TORRO. Archived from the original on 5 February 2012. Retrieved 4 January 2012.
  7. 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".
  8. "Beaufort wind force scale". Met Office. Retrieved November 27, 2015.
  9. "Beaufort Scale". Royal Meteorological Society. Retrieved November 27, 2015.
  10. "Beaufort Scale". Encyclopædia Britannica. Retrieved November 27, 2015.
  11. "Wetterlexikon - Beaufort-Skala" (in German). Deutscher Wetterdienst. Archived from the original on 12 December 2013. Retrieved 2014-02-14.
  12. "昨日实行新标准"珍珠"属强台风_新闻中心_新浪网".