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  The Beaufort scale
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It's often said that Francis Beaufort, of the British Royal Navy, was the first to devise a scale of wind force, towards the start of the 19th century. You might be surprised to learn that he was not, in fact, the originator of such a scale. A similar one was actually in use at least a century earlier - and probably long before that.
1. Thar she blows!
2. The 'table of degrees'
3. Who was Beaufort?
4. A 'private' scale
5. An evolving system
6. Observers on land
 
Thar she blows!

We don't know who first devised a scale of wind force. But it would be surprising if medieval Arab seafarers didn't use one because they had, by the late 15th century, classified in detail virtually every aspect of the weather that had any navigational significance.

It would be surprising, too, if the mariners of ancient times didn't use such a scale - but as they left so few records, we can only speculate.

The scale we all know - the one that bears Beaufort's name - was formulated at the start of the 19th century. But accounts from 1704 show that a similar scale was in use a century earlier.

The 'table of degrees'

In his account of the dreadful tempest that visited the British Isles on 26-27 November 1703, Daniel Defoe referred to a 12-point scale that he called a 'table of degrees'. This comprised, as he put it, "bald terms used by our sailors":

Stark calm, Calm weather, Little wind, A fine breeze, A small gale, A fresh gale, A topsail gale, Blows fresh, A hard gale of wind, A fret of wind, A storm, and A tempest.

By the beginning of the 19th century, a quantitative version of a wind scale had been devised, as a work by Colonel Capper of the East India Company shows. In his Observations on the winds and monsoons, 1801, he reproduced 'A table of the different velocities and forces of the winds, constructed by Mr Rous, with great care, from a considerable number of facts and experiments'.

Terms of the wind
Velocity of wind
Perpendicular force on one square foot in Avoirdupois pounds
Miles in one hour
Feet in one second
Almost calm
1
1.47
0.005
Just perceptible
2
3
2.93
4.40
0.020
0.044
Gentle breeze
4
5
5.87
7.33
0.079
0.123
Fresh breeze
10
15
14.64
22.00
0.492
1.107
Fresh gale
20
25
29.34
36.67
1.968
3.075
Strong gale
30
35
44.01
51.34
4.429
6.027
Hard gale
40
45
56.68
66.01
7.873
9.963
Storm
50
75.35
12.300
Violent hurricanes, tempests, etc.
60
80
100
88.02
117.36
146.70
17.715
31.490
49.200

Fig 1: Velocities and forces of the wind by Mr Rous

From 1660 onwards, keeping weather records at places on land became increasingly popular, and as early as 1723 Secretary of the Royal Society James Jurin recommended a scale for observers to estimate and record wind strength.

Sixty years later, in the Ephemerides published in the 1780s by the Palatine Meteorological Society of Mannheim - the world's first meteorological society - there appeared the following scale, in which halves were used to denote intermediate strengths.

Number
Specification
0
Calm
1
Leaves rustle
2
Small branches move
3
Large branches in motion and dust swirls up from the ground
4
Twigs and branches break off trees

Fig 2: Wind scale as used in 1780

Francis Beaufort devised his scale of wind force in 1805, when serving aboard HMS Woolwich, and first mentioned it in his private log on 13 January 1806, stating that he would "hereafter estimate the force of the wind according to the following scale"…
Category
Description
0
Calm
1
Faint air just not calm
2
Light airs
3
Light breeze
4
Gentle breeze
5
Moderate breeze
6
Fresh breeze
7
Gentle steady gale
8
Moderate gale
9
Brisk gale
10
Fresh gale
11
Hard gale
12
Hard gale with heavy gusts
13
Storm

Fig 3: Beaufort's scale from 1806.

Beaufort modified his scale in 1807, when he decided to combine categories 1 and 2 and thereafter use a scale extending from 0 to 12.

The same year, he added a description of the canvas that could be carried by a fully rigged frigate in different wind conditions. Like the observers of the Palatine Meteorological Society, he frequently used halves, which suggests he was confident he could estimate wind force accurately

Who was Beaufort?

Francis Beaufort was born in Ireland in 1774, and went to sea in 1787. He took command of HMS Woolwich in 1805. His seagoing career ended in 1812 when he was severely wounded in an encounter with Turks while surveying the coast of Asia Minor.

After convalescence, he pursued a number of scientific interests until, in 1829, he was appointed Hydrographer of the Navy. At the time, he held the rank of captain. In 1831, Beaufort commissioned the celebrated voyage of the Beagle. During the voyage (December 1831 to October 1836), Beaufort's scale of wind force was used officially for the first time. Beagle's commander, Robert FitzRoy, subsequently became, in 1854, the first director of the body now known as the Met Office. He and Beaufort were close friends for many years.

Beaufort was made a rear-admiral on the retired list in 1846, served as Hydrographer until 1855 and died in 1857.

Fig 4
Fig 4: Admiral Beaufort photo © Crown

A 'private' scale

For many years, Beaufort's scale of wind force was used only in his private logs. There is no mention of it in the official logs of HMS Woolwich or any other ships in which he served. Nor is there any mention of his scale of weather notation, also devised in 1805. In this notation, he assigned letters to weather types, for example b for blue sky, r for rain, h for hazy, fg for foggy, sq for squally, and so on.

The first published reference to Beaufort's scales of wind force and weather notation came in 1832, when the Nautical Magazine carried an article entitled 'The Log Board'. In this article, formulation of the scales was attributed to Beaufort, and the versions of the scales discussed were identical to those introduced later by the Admiralty in a memorandum issued in December 1838 to 'all Captains and Commanding Officers of Her Majesty's Ships and Vessels'.

Admiralty, Dec 28th, 1838

M E M O R A N D U M.

THE Lords Commissioners of the Admiralty having had under consideration the general utility of recording with clearness and precision, in the Log Books of all Her Majesty's Ships and Vessels of War, the actual State of the Winds and Weather, have thought fit to order that henceforward in each page of the Log Book two columns should be introduced, wherein the force of the Wind and the appearance of the Atmosphere shall be every hour registered according to the annexed scheme, a copy of which shall be pasted into each book and painted on the back of every Log Board or Log Slate and two more columns shall likewise be given for the purpose of entering the heights of the Barometer or Sympiesometer, and Thermometer, when such instruments may be on board.

By Command of their Lordships,

C. WOOD

Fig 5: Admiralty memorandum, 28 December 1838

To denote the force of the wind and the state of the weather, Royal Navy officers were ordered to use the scales below.
Beaufort Number General Description Beaufort's Criterion
0 Calm Calm
1 Light Air Just sufficient to give steerage way
2 Light Breeze With which a well-conditioned man of war, under all sail, and ‘clean full’, would go in smooth water from 1 to 2 knots
3 Gentle Breeze 3 to 4 knots
4 Moderate Breeze 5 to 6 knots
5 Fresh Breeze In which the same ship could just carry close hauled... royals etc.
6 Strong Breeze single-reefs and top-gallant sails
7 Moderate Gale double-reefs, jib, etc.
8 Fresh Gale triple-reefs, courses, etc.
9 Strong Gale close-reefs and courses
10 Whole Gale With which she could only bear close-reefed maintop–sail and reefed fore-sail
11 Storm With which she would be reduced to storm staysails
12 Hurricane To which she could show no canvas

Fig 6: Beaufort's criterion 1832

An evolving system

Beaufort's scale of wind force was revised in 1874 to reflect changes in the rig of warships, and expanded two decades later to include particulars of the sail required by fishing smacks. A scale of equivalent wind speeds was introduced in 1903, its basis being the formula:

V = 1.87 x square root (B3)

… where B is the Beaufort number, and V the corresponding wind speed in miles per hour 30 feet above the surface of the sea.

By the early 20th century, the passing of sail made a specification based on the canvas carried by a sailing ship impractical. British meteorologist George Simpson proposed an alternative, a scale of wind force based on the sea's appearance. It was devised in 1906 and soon accepted by mariners and meteorologists, but it was not adopted by the International Meteorological Organization until 1939.

The Beaufort scale was extended in 1944, when Forces 13 to 17 were added. Hitherto, Force 12 (Hurricane) had been the highest point on the scale, referring to a sustained wind speed of 64 knots (32.7 m/s) or more - that is, the wind speed averaged over a period of 10 minutes.

The additional five points extended the scale to 118 knots (61.2 m/s), with Force 12 referring only to speeds in the range 64 to 71 knots (32.7-36.9 m/s). However, Forces 13 to 17 were intended to apply only to special cases, such as tropical cyclones. They were not intended for ordinary use at sea - indeed, it's impossible to judge Forces 13 to 17 by the appearance of the sea. For all normal purposes, the Beaufort scale extends from Force 0 (Calm) to Force 12 (Hurricane), with Force 12 defined as a sustained wind of 64 knots (32.7 m/s) or more.

Fig 7
Fig 7:
Sea appearance in winds of Force 8.

Fig 8
Fig 8: Sea appearance in winds of Force 10.

Fig 9
Fig 9: Sea appearance in winds of Force 11
photos © G Allen

Beaufort's scale of wind force assumed its present form around 1960, when probable wave heights and probable maximum wave heights were added. The latter is the height of the highest wave expected in a period of 10 minutes, and wave heights refer to the open sea, well away from land.
Observers on land
George Simpson devised a scale for land-based observers in 1906. Similar in concept to the scale used by the Palatine Meteorological Society, it has subsequently been altered very little.
Fig 10
Fig 10:
Simpson's scale for land-based observers
Soon after its introduction, Simpson's version of the Beaufort scale was illustrated in a humorous but effective way.

The scale for observers on land is a useful and reasonably accurate tool for estimating wind strength. The scale for seafarers, however, is no more than 'a guide to show roughly what may be expected on the open sea, remote from land' - to quote from the warning that used to be attached to the copies of the scale issued to marine observers.

Strictly, it applies only when the sea is fully developed; that is, when waves have reached their maximum height for a particular wind speed. Care must be exercised when the fetch and duration of the wind are limited (the fetch is the distance over which the wind has blown, and the duration the time it has been blowing). And it's also worth remembering that the appearance of the sea's surface is influenced not only by wind but also by swell (waves from far away), precipitation, tidal streams and other currents in the sea.

 
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