 The
Beaufort scale |
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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
7. Key Stage 3 Relevance |
| 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. |
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We don't know who first devised a scale of wind force.
But it would be surprising if medieval Arab seafarers
of 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.
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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".
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| Terms of the wind |
Velocity of wind
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Perpendicular
force on one sqaure foot in Avoirdupois pounds |
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Miles in one hour
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Feet in one second
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| Almost calm |
1
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1.47
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0.005
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| Just perceptible |
2
3
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2.93
4.40
|
0.020
0.044
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| Gentle breeze |
4
5
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5.87
7.33
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0.079
0.123
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| Fresh breeze |
10
15
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14.64
22.00
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0.492
1.107
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| Fresh gale |
20
25
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29.34
36.67
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1.968
3.075
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| Strong gale |
30
35
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44.01
51.34
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4.429
6.027
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| Hard gale |
40
45
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56.68
66.01
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7.873
9.963
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| Storm |
50
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75.35
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12.300
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| Violent hurricanes, tempests,
etc. |
60
80
100
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88.02
117.36
146.70
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17.715
31.490
49.200
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Fig 1: Velocities and forces of the wind by
Mr Rous
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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.
60 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.
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Number
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Specification |
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0
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Calm |
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1
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Leaves rustle |
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2
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Small branches move |
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3
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Large branches in motion and dust swirls up
from the ground |
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4
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Twigs and branches break off trees |
Fig 2: Wind scale as used in 1780
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| 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"… |
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Category
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Description |
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0
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Calm |
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1
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Faint air just not calm |
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2
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Light airs |
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3
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Light breeze |
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4
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Gentle breeze |
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5
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Moderate breeze |
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6
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Fresh breeze |
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7
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Gentle steady gale |
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8
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Moderate gale |
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9
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Brisk gale |
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10
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Fresh gale |
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11
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Hard gale |
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12
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Hard gale with heavy gusts |
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13
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Storm |
Fig 3: Beaufort's scale from 1806.
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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
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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 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.
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Fig 4: Admiral Beaufort photo © Crown
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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".
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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. W O O D
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Fig 5: Admiralty memorandum, 28 December 1838
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| 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
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Fig 6: Beaufort's criterion 1832
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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 20 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.
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Fig 7: Sea appearance in winds of Force 8.
Fig 8: Sea appearance
in winds of Force 10.
Fig 9: Sea appearance
in winds of Force 11
photos © G Allen
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| 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. |
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| 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: 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. |
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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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Geographical enquiry and skills
- Appropriate fieldwork techniques
- Use of secondary sources of evidence
- Use of scales
Knowledge and understanding of
patterns and processes
- Physical processes and their impact on places and
environments
Breadth of study
- Causes and effects of a hazard
- Knowledge of weather and climate
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