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How the Beaufort scale came into being and why it is a meaningful way of describing wind - even in these days of computerised forecasts and observing systems.


Anyone who has taken the RYA Yachtmaster or Day Skipper courses will know that meteorologists derive wind speeds in knots from the spacing of the isobars. But, why do they describe the wind using the Beaufort scale derived over 200 years ago? I know that many sailors, whether deep sea merchant seamen or cross-channel yachtsmen, have a conservative streak. Even so, to use a scale devised nearly 200 years ago seems inordinately, even perversely archaic.

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Commander Beaufort

It was in 1806 that the Captain of HMS Woolwich, Commander Beaufort, said that his log would describe winds using a 14 point scale (0 to 13) with names such as Light Air, Moderate Breeze, Brisk Gale and so on. 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 13 points. Twenty one years later, he sent a refined form of the scale to Commander Fitzroy, Captain of HMS Beagle and 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. 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. 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? 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.

The Scale

After Force 0 for a calm and Force 1 for a wind that could just give steerage way, 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 to be on 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 seaman would recognise what was meant by Force 5, or whatever. Important in the event of a subsequent 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. There was no need for any finer resolution; Beaufort was as well aware as we are today of the inherent variability in the wind.

There was no need for anything stronger than hurricane force - 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 extreme as the immortal phrase for force 12 says -"that which no canvas could withstand".

Beaufort Today

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 "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 sea going 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". Force 6 is "Coarse sailors try to double reef and go aground" or "Elderly customers have difficulty in leaving pub". He was following the spirit of Beaufort in emphasising the effect of the wind. "Helmsman only sits on weather side". Force 5

Pragmatic Realism

The continued use of the Beaufort scale in weather forecasts is partly a matter of pragmatism and partly for 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 broadcasts 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 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 him, his boat and crew. In this equation will come time of day, tiredness, experience and fitness of those on board. At night, with a tired novice crew it might be better to take in an extra reef. 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, long keel boat is more likely to chomp through the waves and be easier for all.

As ever, the effect of the wind force will depend upon the vessel and some personal calibration is needed. Nevertheless, the Beaufort scale concept is still valid. Long live the good Admiral or, at least, his scale.

For various versions of the Beaufort Scale, historical, current and tongue-in-cheek click here. See also measuring visibility and measuring wind from a yacht.]

The section below is a summary of modern day reporting practice on ships that provide weather observations.

How are Winds Reported at Sea Nowadays?

For weather forecasting purposes, meteorologists use wind data averaged over 10-minutes and professional observers are trained to take such averages. This is built in to the automatic systems increasingly coming into use. Waves result from the average wind at the time. (Swell is caused by winds that have now died away or from winds occurring some distance away.) The wind deduced from the sea state is the nearest practical equivalent to the 10-minute representative measurement made at land stations.

Since 1855, ships officers have reported the weather on behalf of Meteorological Services. Nowadays, some 7000 merchant ships transmit their reports in real time to provide valuable and otherwise unobtainable data for use by weather forecasters. Most of the wind reports from these ships are still made by visual observations of the sea state and, no doubt, partly on the behaviour of the vessel. Even a large tanker will have a motion that depends upon the wind, or more correctly, the sea generated by that wind.

A few ships do use anemometers but these, surprisingly at first sight, do not give such good observations as the visual estimates. The officer of the watch on the bridge is likely to take a snapshot look at a wind indicator, be influenced by the gust speeds and tend to report exaggerated wind speeds. That is unless he has some form of averaging device. Allowance has to be made for the speed and direction of the ship and there are great difficulties in positioning anemometers on ships so that the measurements are not affected by the superstructure.

Visual observations by seamen of wind have been remarkably consistent over the past 140 years and form the basis of the climatology of the oceans. The Meteorological Office, through exchange of data agreements with other Met Services now has a computer based archive of nearly 80 million ship weather reports over all the world's oceans and seas.