SEA STATE and the BEAUFORT SCALE

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Sea state refers to the height, period, and character of waves on the surface of a large body of water. The large number of variables involved in creating the sea state cannot be quickly and easily summed, so simpler scales are used to give a rough description of current conditions, primarily for reporting in a ship's log or similar record.

Beaufort scale force 11 seas with waves 11-16 metres high and wind 56-63 knots

Sea states in marine engineering

In engineering applications, sea states are often characterized by the following two parameters:

• The significant wave height H_1/3 — the mean height of the one third highest waves.

• The mean wave period, T₁.

The sea state is in addition to these two parameters (or variation of the two) also described by spectrum S(ω,Θ) which is the product of a wave height spectrum S(ω) and a wave direction spectrum f(Θ). Some wave height spectra are listed below. The dimension of the wave spectrum is {S(ω)} = {Length²Time}, and many interesting properties about the sea state can be found from the spectrum.

Long term sea state statistics are often given as a joint frequency table of the significant wave height and the mean wave period. From the long and short term statistical distributions it is possible to find the extreme values expected in the operating life of a ship. A ship designer can find the most extreme sea states (extreme values of H_1/3 and T₁) from the joint frequency table, and from the wave spectrum the designer can find the most likely highest wave elevation in the most extreme sea states and predict the most likely highest loads on individual parts of the ship from the response amplitude operators of the ship. Surviving the once in 100 years or once in 1000 years sea state is a normal demand for design of ships and offshore structures.

Beaufort scale force 12 winds 64 knots waves over 16 metres

World Meteorological Organization sea state code

• 0 Calm (glassy) 0 m

• 1 Calm (rippled) 0 to 0.1 m

• 2 Smooth (wavelets) 0.1 to 0.5 m

• 3 Slight 0.5 to 1.25 m

• 4 Moderate 1.25 to 2.5 m

• 5 Rough 2.5 to 4 m

• 6 Very rough 4 to 6 m

• 7 High 6 to 9 m

• 8 Very high 9 to 14 m

• 9 Phenomenal Over 14 m

 

 

Direction from which swell is coming should be recorded. Confused swell should be recorded as "confused northeast," if coming from the direction of northeast.

THE BEAUFORT SCALE

The Beaufort scale is an empirical measure for describing wind intensity based mainly on observed sea conditions. Its full name is the Beaufort wind force scale.

History

The scale was created in 1805 by Irishman Sir Francis Beaufort, a British admiral and hydrographer. At that time naval officers made regular weather observations, but there was no scale and so they could be very subjective - one man's "stiff breeze" might be another's "calm conditions". The initial scale from zero to 12 did not reference wind speed numbers, but related qualitative wind conditions to effects on the sails of a man of war, 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.

The scale was adapted to non-naval use from the 1850s, with scale numbers corresponding to cup anemometer rotations. In 1906, with the advent 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, Director of the UK Meteorological Office, was responsible for this and for the addition of the land-based descriptors. The measure was slightly altered some decades later to improve its utility for meteorologists.

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.

Wind speed on the 1946 Beaufort scale is defined by the empirical formula:

v = 0.836 B^3/2 m/s

 

where v is the equivalent wind speed at 10 metres above the 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".

Today, hurricanes are sometimes described as Beaufort scale 12 through 16, very roughly related to the standard Saffir-Simpson Hurricane Scale where Category 1 is equivalent to Beaufort 12. However, the Saffir-Simpson Scale does not match the extended Beaufort numbers above 13. Category 1 tornadoes on the Fujita and TORRO scales also begin roughly at the end of level 12 of the Beaufort scale but are indeed independent scales.

Note that wave heights in the scale are for conditions in the open ocean, not along shore.

The scale is used in, and may be most recognizable to some from, the Shipping Forecasts broadcast on BBC Radio 4 in the United Kingdom.

This scale is also widely used in China. Taiwan uses the Beaufort scale extended in 1946 with Forces 13-17 to better represent the wind caused by typhoons. On the morning of May 15, 2006, mainland China suddenly introduced the extended scale to Force 17 without any prior notice. [1] This extended scale was immediately put into use for Typhoon Chanchu. Hong Kong and Macau keep using Force 12 as the maximum and adopted a set of simpler descriptions for public. The descriptions used in Hong Kong are shown here. Macau used similar descriptions except the term gentle is retained for Force 3.

In the United States, winds of Beaufort 6 or 7 result in the issuance of a small craft advisory, with force 8 or 9 winds bringing about a gale warning, 10 or 11 a storm warning (or "tropical storm warning" for 8 to 11 if related to a tropical cyclone), and anything to 12 a hurricane warning.

LINKS and REFERENCE

• Bowditch, Nathaniel original; H.O. pub No. 9: American Practical Navigator, Revised Edition 1938; United States Hydrographic Office; Not Copyrighted 1938.

• Faltinsen, O. M. (1990). Sea Loads on Ships and Offshore Structures. [Cambridge University Press]. ISBN 0-521-45870-6. 

• Scott Huler, Defining the Wind : The Beaufort Scale, and How a 19th-Century Admiral Turned Science into Poetry, Crown, 2004, ISBN 1-4000-4884-2hi

• UK Meteorological Office: The Beaufort Scale

• Investigating Clouds : A lesson plan from the National Science Digital Library that uses the Beaufort Scale.

• National Public Radio : The Beaufort Scale on NPR - interview with Scott Huler.

• OceanWeather.com : Current Ocean Weather Graphics for Wind, Waves and Temp

• Open Source Textbook at Oceanworld.tamu.edu : Cite the original definition formula in chapter 4

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