The Beaufort scale

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.

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'.

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.

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

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.

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'.

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: 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

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.