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Beaufort scale

A numerical scale for relating wind speed and sea state, devised by the British Admiral Sir Francis Beaufort in 1805. Beaufort numbers (or forces) range from force 0 (calm) to force 12 (hurricane). Sometimes (and originally) called the Beaufort scale of wind forces.

See WXTL RES-21 for numerical values of the Beaufort scale. Sample pictures of the various sea states are in the new edition of Bowditch, although we must remember that photographs never seem to capture the perspective of waves that one sees underway unless they are actually breaking over the vessel.

The scale relates the appearance of the sea to the strength of the wind. Originally there was no actual mention of numerical wave heights nor of wind speeds, these were all added later. The "force" part meant to imply the effect of that wind on specific sailing craft originally a fishing smack and later to British war ships.

Despite its quantitative limitations, however, which still remain, the Beaufort scale is very much more than just a quasi-scientific way to describe the wind in terms of the sea's appearance. It has remained active and useful among mariners for hundreds of years because of its practical value. It is also unlikely that we will ever do much better; there are too many variables involved.

But from a mariner's point of view, it is essentially the full description of the environment that we care about, wind and sea. It is our understanding of this relationship that is one key to good seamanship and navigation underway. By knowing what wind should produce what sea state we have one way of judging what is taking place now and what might happen later, especially since we now have access to accurate wind instruments. In other words, by noting whether the seas match the winds (according to this scale) we can judge the presence of current, whether the seas will build, or what they might build to, or how they might decay if the wind speed or direction changes, and so forth.

It also in principle facilitates communications among mariners, since saying we had "30 knots of wind" is not really communicating the crucial elements of the conditions, whereas Force 7, if understood, is more specific. The British tend to retain this valuable aspect of the scale more so than seems the case in modern American writing about the sea. The Met Office, for example, gives wind forecasts in terms of force numbers and not knots. The official GMDSS reports from the WMO also use that system.

Inherent, but often unstated, is an assumption that we have some understanding about the effect of current flow on wave steepness. Sea state can be dramatically influenced by the current present, but this point is not addressed in the Beaufort scale itself. See WXTL ART-20 on wave steepness.

The original scale was based on a force 1 to 8 scheme of Charles Tomlinson which dates to the 1600s. Force 1 was not enough wind to move a fishing smack, force 8 was time to go home. The Beaufort scale did not have wind speed in it for many years since there simply was no way to quantitatively measure it.

Historical notes might be found in articles by B. Kinsman: "Historical notes on the original Beaufort scale," in The Marine Observer 39, 116-24, 1969 and "Who put the wind speeds in Admiral Beaufort's scale?" in Oceans 2, 18-25, 1969. See also H.L. Crutcher "Winds, numbers, and Beaufort" in Weatherwise 28, 260-71, 1975.

There are those such as Lyall Watson in his book Heaven's Breath who have assigned "profound" numerological significance to the scale and its correlation to actual force of the wind, but I can see no significance or even validity to the claims. Indeed, the "Force scale" itself is misnamed in light of scientific knowledge. The Beaufort scale numbers increase as roughly the wind speed to the power of 2/3, (wind = kxN^1.4) whereas the force of the wind and the development of wave height (in fully developed seas) increases as the wind speed squared. See wind force. The word "force" was originally used simply as a general way to describe how certain vessels responded to the wind indeed, the word was used in this application before Newton defined force for us.


The author's personal abbreviated scale:

1 or 2 knots = see wind with light smoke but not feel it

4 knots = can first feel the wind

10 knots = can first see isolated white caps (in still water, change to 8 knots in a contrary current or 12 knots in a following current)

20 knots = a bed spread of white caps are the first thing that catches the eye

30 knots = first see spindrift. Usual first clear signs of spindrift (blowing foam or spray) are in gusts of 30 in winds of 25+ or so. Also seen flying off of wave crests. It's a good way to judge the gust speed, for gusts seen away from your location.

40 knots = need goggles if it is raining, and when sustained, what looked like fish scales on wave surfaces at lower speeds now looks like deep gouged out roof tiles.

Author's shortcut way to remember the scale since we do not use it often here: Mean wind (kts) = 6 x N - 10. (see http://davidburchnavigation.blogspot.com/2017/08/wind-speed-from-beaufort-force-number.html). This might be helpful to those who only use it periodically. It is pretty close in all cases except 1 (1-3 kts) and 2 (4-6 kts). Note that wind feathers on pilot charts mark the Beaufort Number. GMDSS reports and forecasts use Beaufort Numbers as well.




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