September 24th 2010. By Staff.

The Beaufort Scale: Judging the Wind in Your Sails

Most sailors know that meteorologists derive wind speeds in knots from the spacing of the isobars. But why do they describe the wind using a scale derived over 200 years ago?

Before Beaufort
In 1704, after he had witnessed the Great Storm of 1703, Daniel Defoe suggested a scale of winds comprising eleven points and using words normally used in the English language. Even before that – certainly during the late 17th century – sailors were using a fairly standard set of names to describe winds. In a book written in 1697 after a circumnavigation, it is clear that William Dampier, the privateer, was using the same general words to describe winds.

The Beaufort scale remains valid for sailors of modern yachts who battle the elements.

In 1759 a British civil engineer named John Smeaton designed an early anemometer as part of his study on windmill design. From readings and his experiments he defined an 8-point scale of winds. This had descriptions in terms of movement of leaves, branches and trees as well as the speed of rotation of a windmill. Some twenty years later, Alexander Dalrymple, hydrographer of the Honourable East India Company with which Beaufort had served his seagoing apprenticeship, produced a scale of winds for use by company captains. Similar words were used also by Archibald Menzies on Vancouver’s circumnavigation from 1791 to 1795.

In 1801 Colonel Capper, also of the Honourable East India Company, calibrated a list of the common terms in terms of miles per hour and feet per second. Apart from Smeaton and Capper, nobody else seems to have tried to define the terms used in ship’s logs to describe wind. Even Dalrymple seems to have taken it for granted that his captains would all work to a common, but unwritten standard.

The original scale description, and its namesake.

Commander Beaufort
When on 13th January 1806, Commander Francis Beaufort of His Majesty’s Sloop Woolwich wrote in his logbook that he would use a 14-point scale (0-13) to which he gave descriptive names, he was only following the custom of the sea at that time. Not surprisingly, Beaufort was highly influenced by Dalrymple; in his 1806 scale he used very similar descriptive names and added an extra, fifth, point which he called a “Moderate Breeze”. It is clear that Beaufort was being evolutionary rather than revolutionary and at that stage he did not include definitions of the points. Probably these were well known to his officers.

In 1810, when Captain of His Majesty’s Sloop Blossom, Beaufort wrote in his personal log the first (known to me) descriptions of his scale, now reduced to thirteen points. Twenty-one years later, he sent a refined form of the scale to Commander Fitzroy, Captain of HMS Beagle (later to become the first Head of the Meteorological Office). No doubt the revisions were in the light of further experience, changes to ships and their rigging.

Before the days of wind measuring instruments there were several descriptive scales of wind force in use in Europe with numbers of points varying from 4 to 20. Beaufort was both a good, practical seaman and a good scientist. He recognised the need to be able to define, sensibly and in a reproducible fashion, the state of the wind in ways that could be understood unequivocally by his fellow seamen.

Use of the Beaufort scale became mandatory in 1838 for all Royal Navy vessels. Until then, each captain could use his own method of describing the wind. The scale has remained in wide use by sailors ever since.

Wind conditions were originally ascribed according to the effect upon 19th-century ships like this vintage frigate.

How the Scale Works
After Force 0 for a calm and Force 1 for a wind that could just give steerageway, Beaufort described Forces 2, 3 and 4 in terms of the speed made by a fully rigged man-of-war. In his 1831 version he talks about the vessel being “clean full.” This meant that the ship was off the wind, with the sails bellying out and probably at its fastest point of sailing. The nearest equivalent for a modern yacht would be a beam reach.

Forces 5 to 9 refer to the sail that the same ship could carry, “in chase, full and by”. In other words the ship would be chasing the enemy, as hard on the wind as possible, without pinching, in order to get to windward of the other vessel. In such circumstances, it would be carrying as much sail as the captain dared. His rigging might fail were he carrying too much sail for the wind conditions. He might not be carrying enough sail and so let the enemy escape. This served to sharpen the mind of the captain, encouraging fine judgement and good seamanship. But, importantly, the descriptions were such that other seamen would recognise what was meant by each force. This would prove most important in the event of a court-martial.

There was no need to consider wind speed, even if it had been measurable. The whole emphasis was on the effect of the wind, and there was no need for any finer resolution; Beaufort was as well aware as we are today of the inherent variability in the wind. Anything stronger than hurricane force was irrelevant – if you are struggling to survive then it does not really matter whether the wind is 65 or 95 knots. Anything over 60 knots is as extreme as the immortal phrase for force 12 says – ”that which no canvas could withstand.”

Beaufort Today
Nowadays, the Beaufort scale is defined for seamen in terms of sea state. So much is historical fact. But why are such methods of measuring wind relevant today or, more pointedly, are they? Why do forecasts for seafarers use the Beaufort scale and not give speeds in knots or some other recognised unit of speed?

The concept of the wind force being observed as an effect is just as valid today for us in our modern yachts or dinghies. For the most part, ships react to the average wind and so, for much of the time, do yachts although the smaller the yacht the more reactive it will be to gusts. Dinghy helmsmen accept this as a fact of life and go to great lengths to de-power their rig for the gusts. Skippers of small yachts may find it better to take in an extra reef when the gusts are very much stronger than the average wind.

The Heron Dinghy Class magazine used to contain a “Heron” version of the Beaufort Scale. Force 2 was “Helmsman only sits on weather side”. Force 5 was “Have to ease sheets in heavier gusts when beating”.

In my Firefly days, I knew that I could plane at the top end of a Force 3/bottom of a Force 4.

In his Art of Coarse Sailing, Michael Green has seagoing Coarse Sailor’s and Landsman’s versions of the Beaufort scale. Force 2 is “Tea towels blow off rigging” or “Public Houses close one window”. He was following the spirit of Beaufort in emphasising the effect of the wind.

The concept of the wind force being observed as an effect is just as valid today.

Pragmatic Realism
The continued use of the Beaufort scale in weather forecasts is partly a matter of pragmatism and partly realism. The wind is never steady and the forecaster, working in terms of pressure gradient and resultant wind speed, is only too happy to express that variability and the general uncertainty in an economic way through the Beaufort scale.

Hearing a forecast on the radio it is easier to distinguish between winds of Forces 4 and 7 than it might be between speeds such as, say, 13 and 30 knots. I know that I find it easier to understand the forecasts broadcast by the French CROSS radio stations when they use Beaufort forces rather than equivalents in knots. It also takes fewer words. This is particularly important in the time-constrained shipping forecast on Radio 4. Even the more relaxed Marinecall texts and the Inshore waters forecasts read out by the Coastguard on VHF benefit from the brevity of the Beaufort Force.

Yachtsmen should learn to determine the wind force from the sea state. There are some excellent wall charts and other publications for this purpose. With a wind speed indicator on the boat it is possible to check your estimates but remember to allow for the boat speed and to take an average over several minutes rather than just the highest speeds. The yachtsman should become aware of what the various forces mean to his boat and crew. In this equation will come time of day, tiredness, experience and fitness of those on board. A lightweight boat might bounce around and give an uncomfortable sail in a Force 5 or 6 hard on the wind but be easier off the wind. A heavy displacement keelboat is more likely to chomp through the waves and be easier for all.

The Beaufort scale concept is still valid. All the versions share a common feature: they all relate the wind to its effects on boats, the sea, people and the world around us. It is that that makes the Beaufort Scale still the most appropriate way to describe wind in weather forecasts for seafarers. Long live the good Admiral or, at least, his scale.

View a cartoon graphic explaining the Beaufort scale