When Sir Francis Beaufort first devised the scale in 1805 it was simply his assessment of the wind strength, based on the observed sea conditions, so that a mariner could decide how much sail to carry. His scale was intended to describe the conditions under which various amounts of sail could be carried by a man-o-war, the principle warship of the time.
The scale ran from a Force 0, dead calm in which all sail would be flown, to a Force 12 in which the winds were “…. such that no canvas could withstand”.
In 1831, when anemometers had been around a bit, wind speeds were applied to each of Sir Francis’ 13 levels of wind force. A Force 6 was described as a fresh breeze of 22 to 27 knots “or that in which a well-conditioned man-o-war could carry, in chase, full and by, single reefed topsails and top gallant sails”. Very evocative if you know your top gallants from your tam o’shanters.
Over time the scale was further modified and modernised. Wind speeds were added, as I’ve said, and a ‘state of sea card’ was produced bearing photographs of the sea state to be expected for each Beaufort force. Further Forces were added to cover the conditions that might prevail in tropical cyclonic storms. Wave heights are now seen on many versions of the scale.
The wind speeds which were applied to each of the Forces were, presumably, those that most closely related to the conditions that Sir Francis described. For instance, F0, dead calm, is given a wind speed of less than 1 knot, something of a no-brainer, but F5 is 17 to 21 knots – it must have taken some serious debate to arrive at that range of figures. And, inevitably, the progression of wind speeds up the scale is not linear, reflecting the exponentially increasing force on the sails as the wind speed climbs. F5 is 17 to 21 knots, whilst F10 is 48 to 55 knots – an F10 is not merely twice an F5.
The Beaufort scale is seen as an anachronism by many sailing newbies. There is a temptation to assume the Beaufort scale is simply an illogical grouping of wind speeds with no obvious conversion rate to anything else. Why not, they might think, devise some logical groupings: 0-9 knots, 10-19 knots and so on, if it’s necessary to group wind speeds at all. Such logic is all very well if you think of Beaufort Force as simply another form of wind speed measurement such as knots, miles per hour or meters per second, for which there is a mathematical conversion.
But that isn’t where it came from; it might have been diverted to that use, but what Sir Francis Beaufort devised was a means of establishing the force of the wind by looking at the sea, a reference source to tell mariners how much sail to risk in any given condition.
merchant ships at sea still determine true wind speed from sea conditions – and
they supply this information to the MET office. The reason they do this is that
anemometers mounted on large fast moving ships don’t tell the true wind speed,
they measure apparent wind speed – the wind speed modified by the ships own,
often very high, speed and by the effect of the ships superstructure. To get
the true wind speed they rely on their deck officers who are skilled at
estimating it from the sea state. Sir Francis Beaufort would be proud of them.