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Wind force and direction

Wind force is expressed numerically on a scale from 0 to 12. This scale, which originally defined the wind force in terms of the canvas carried by a frigate, was devised by Captain, afterwards Admirals Sir Francis Beaufort in the year 1806 for use in vessels of the Royal Navy. Since Admiral Beaufort's time, however, so many changes had taken place in the build, rig, and tonnage of seagoing vessels that in 1874 Beaufort's scale was adapted to the full- rigged ship with double topsails of that period. With the passing of sail, this specification meant very little to those who had no experience in square-rigged ships, and the practice arose of judging wind force from the state of the sea surface. In 1939 the International Meteorological Organization, now the WMO, agreed to the use of a sea criterion by which the wind force was judged from the appearance of the sea surface. Photographs showing the appearance of the sea corresponding to each Beaufort force are given below.

Force 0
Force 1
Force 2
Force 3
Force 4
Force 5
Force 6
Force 7
Force 8
Force 9
Force 10
Force 10
Force 10
Force 11
Force 11
Force 12
Force 12

In using this specification it is assumed that the observation is made in the open ocean and that the wind has been blowing long enough to raise the appropriate sea. The possibility of a lag between the wind getting up and the sea increasing must be considered. The appearance of the sea surface also depends on many other factors such as the fetch of the wind (i.e. distance from weather shore), the swell, the presence of tides, and whether or not precipitation is occurring. These effects should be allowed for before deciding the appropriate number on the scale. Experience is the only sure guide but the following remarks may be of some use:

(a) A discrepancy between wind and sea occurs frequently close inshore where winds of a local character are likely.

(b) An off-shore wind does not produce its appropriate sea close inshore but requires a certain fetch before its full effect is produced.
(c) Swell is the name given to waves, generally of considerable length, raised by winds at a considerable distance from the point of observation. Swell is not taken into account when estimating wind.
(d) Tides or strong currents affect the appearance of the sea surface, a wind against tide or current causing more 'lop' – a weather tide – and the wind in the same direction as a tide or current producing less disturbance of the sea surface – a lee tide.

(e) Precipitation, especially if heavy, produces a smoothing effect on the sea surface.

(f) There is evidence that the height of the sea disturbance caused by a wind of a particular force is affected by the difference between sea and air temperatures, the sea being the warmer medium. If this difference increases, there is an appreciable increase in the sea disturbance, and vice versa.

Beaufort force can be transformed approximately into wind speed by means of a table of equivalents

The International Code (used for making meteorological reports by radio) makes provision for the reporting of wind speed in knots (or metres per second). The observer may derive this from the table of equivalents, taking the mid-point of the range corresponding with the observed Beaufort force; or, better still, he may interpolate according to his own judgement. For example, if the wind is estimated to be over Beaufort 5 but not quite Beaufort 6, it might be reported as having a mean speed of 21 knots.

Wind direction is logged as the true direction and is given to the nearest ten degrees. The exposed position that a ship's standard compass usually occupies gives a clear all-round view and from it the observer takes a compass bearing, noting the tops of the waves, the ripples, the spray and the faint lines that generally show along the wind. It is usually best to look to windward in judging wind direction, but in some lights the direction is more evident when looking to leeward.

Meteorologists as well as seamen use the term 'veering' to indicate a change of wind in a clockwise direction and the term 'backing' to denote a change in an anticlockwise direction. Estimation of wind force and direction can often be made in the same way at night but sometimes on very dark nights it is impossible to see the effect of the lighter winds on the sea surface. In such cases, the apparent or relative wind force and direction must be estimated by their effect, i.e. by the 'feel' upon the face or upon a moistened finger, or by the direction in which the smoke is blowing. Allowance must then be made for the ship's course and speed. In a fast ship considerable difference exists between the apparent and true wind directions. When the wind is astern and of the same velocity as the ship there is apparent calm on board the ship. In a calm, a ship steaming at 10 knots will have an apparent head wind of velocity 10 knots, but as soon as the wind blows from any direction out of the fore and aft line, the difference between the apparent and true directions will vary with each angle on the bow, and with each force of the wind. The true wind may be obtained from the apparent wind by use of the parallelogram of velocities, or Table 10 as explained below. In Figure 19 if, for example, the ship is travelling along the line AB with speed 15 knots and the wind appears to be coming from the direction DA with speed 29 knots (Beaufort scale 7), the true direction of the wind is along CA and its speed 18 knots.

This result is easily obtained graphically by drawing the figure, making BA proportional to 15 and DA proportional to 29, and then measuring DB which is equal to CA, where ABDC is a parallelogram. The angle CAD, which is the same as BDA, is measured with a protractor and gives the difference between the true and apparent directions of the wind. Table 10 enables the conversion from apparent to true wind to be made by inspection.

In fast vessels the task of estimating accurately the true wind force and direction is no easy one and special care is required; this applies particularly to occasions when the wind is very light, and on dark nights.

Anemometers have as yet found only limited use at sea, the chief problem being to achieve a suitable exposure. Estimation of wind force and direction from careful observation of the sea state is the method preferred. The ship disturbs the airflow in its vicinity with the result that the wind measured by the instrument is not representative of the true airflow over the open sea. If a portable cup-anemometer is used, the exposure may be varied at will and the best position chosen for any particular wind direction. The instrument measures 'apparent' wind speed. To determine the true value, the wind direction must first be estimated and then allowance made for the speed of the ship.

Figure 19. Wind, parallelogram of velocities,

Wind force and direction, taken alone, do not completely specify the character of the wind. It is well known that on occasions the wind is particularly gusty, as in showery weather. Less frequently, definite squalls may occur. The difference between a gust and a squall is essentially one of time-scale, a gust being momentary, whereas a squall may last several minutes. It is important when making the observations to note any unusual gustiness and the occurrence of squalls. When the latter occur it is of advantage if the time be noted together with any sudden change in wind direction. It is of interest to note that gusts have no appreciable effect in raising waves, whereas squalls may act for a sufficient length of time to raise a group of waves which tend to travel with the squall.