A ship in a force 12 ("hurricane-force") storm at sea, the highest rated on the Beaufort scale
TheBeaufort scale (/ˈboʊfərt/BOH-fərt) is anempirical measure that relateswind speed to observed conditions at sea or on land. Its full name is theBeaufort wind force scale. It was devised in 1805 byFrancis Beaufort, ahydrographer in theRoyal Navy. It was officially adopted by the Royal Navy and later spread internationally.
The scale that carries Beaufort's name had a long and complex evolution from the previous work of others (includingDaniel Defoe the century before). 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 a scale.[1] The scale was devised in 1805 byFrancis Beaufort (laterRear Admiral), ahydrographer and aRoyal Navy officer, while serving onHMS Woolwich, and refined until he wasHydrographer of the Navy in the 1830s, when it was adopted officially. It was first used during the 1831-1836"Darwin voyage" ofHMSBeagle under CaptainRobert FitzRoy, who was later to set up the firstMeteorological Office in Britain giving regular weather forecasts.[2]
The initial scale of 13 classes (zero to 12) did not referencewind speed numbers, but related qualitative wind conditions to effects on the sails of afrigate, then the main ship of theRoyal Navy, from "just sufficient to give steerage" to "that which no canvas sails could withstand".[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.Anemometer rotations to scale numbers were standardised only in 1923.George Simpson,CBE (later Sir George Simpson), director of theUK Meteorological Office, was responsible for this and for the addition of the land-based descriptors.[2] The measures were slightly altered some decades later to improve its utility formeteorologists. 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[4] and thesevere weather warnings given to the public.[5]
wherev is the equivalent wind speed at 10 metres above the sea surface andB 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. F1tornadoes on theFujita scale and T2TORRO 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.[7]
The table below details the contemporary use of the scale.Wave heights in the scale are for conditions in the open ocean, not along the shore.
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 knots 64–72 mph 103–117 km/h 28.5–32.6 m/s
37–52 ft 11.5–16 m
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
The air is filled with foam and spray; sea is completely white with driving spray; visibility very seriously affected
Devastation
The Beaufort scale is neither 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 conversions have not been made official.
The Beaufort scale was extended in 1946 when forces 13 to 17 were added.[1] However, forces 13 to 17 were intended to apply only to special cases, such astropical cyclones. Nowadays, the extended scale is used in Taiwan,[13] mainland China[14] and Vietnam,[15] which are often affected bytyphoons. Internationally, theWorld Meteorological Organization 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.[16]
The scale is used in theShipping Forecasts broadcast onBBC Radio 4 in the United Kingdom, and in the Sea Area Forecast fromMet 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.[citation needed]
This scale is widely used in the Netherlands, Germany,[18] Greece, China, Taiwan, Hong Kong, Malta, Macau, and the Philippines, 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,[19] and the extended scale was immediately put to use forTyphoon Chanchu.Hong Kong andMacau retain force 12 as the maximum.[citation needed]
In the United States of America, winds of force 6 or 7 result in the issuance of asmall craft advisory, with force 8 or 9 winds bringing about agale warning, force 10 or 11 astorm warning ("atropical storm warning" being issued instead of the latter two if the winds relate to atropical cyclone), and force 12 ahurricane-force wind warning (orhurricane 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.[citation needed]
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 standardised 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.)[citation needed]
In this scale the weather designations could be combined, and reported, for example, as "s.c." for snow and detached cloud or "g.r.q." for dark, rain and squally.[20]
^Hay, William W. (2016).Experimenting on a Small Planet: A History of Scientific Discoveries, a Future of Climate Change and Global Warming (second ed.). Cham, Switzerland: Springer Verlag. p. 26.ISBN978-3-319-27402-7.
^"Beaufort Scale". Royal Meteorological Society. Retrieved27 November 2015.
^"Beaufort Scale".Encyclopædia Britannica. Retrieved27 November 2015.
^abThe names "storm" and "hurricane" on the Beaufort scale refer only to wind strength, and do not necessarily mean that other severe weather (for instance, athunderstorm ortropical cyclone) is present. To avoid confusion, strong wind warnings will often speak of e.g. "hurricane-force winds".
