ESTIMATING THE WIND AT SEA - The master and mates on board ships at sea can determine the speed of the wind by estimating its Beaufort Force. Through experience ships officers have various methods of estimating this force. The effect of the wind on the observer himself, the ship's rigging, flags, etc, is used as a criterion. Estimates on these give the relative wind which must be corrected for the motion of the ship before an estimate of the true wind speed can be found.
The most common method is the appearance of the sea surface. The state of the sea disturbance, the height of the waves, noticeing of white caps, foam or spray, depends on three factors.
1. The wind speed - The higher the speed of the wind, the greater is the sea desturbance.
2. The duration of the wind - At any point on the sea, the disturbance will increase the longer the wind blows at a given speed, until a maximum state of disturbance is reached.
3. The Fetch - This is the length of the stretch of water over which the wind acts on the sea surface from the same direction. For a given wind speed and duration, the longer the fetch, the greater is the sea disturbance. If the fetch is short, say a few miles a disturbance will be small no matter how great the wind speed is or how long it has been blowing.
There are other factors which can modify the appearance of the sea surface caused by wind alone. These are strong currents, shallow water, swell, precipitation, ice, and wind shifts.
A wind of a given Beaufort Force will produce a appearance of the sea surface provided that it has been blowing for a length of time, and over a long fetch. The effects of currents, shallow water, swell, precipitation, etc., should also be absent. The use of the sea criterion has the advantage that the speed of the ship need not be considered. The mariner observes the sea surface, noting the size of the waves, the white caps, etc., and then finds the criterion which describes the sea surface as he saw it. This criterion is associated with a Beaufort number, for a mean wind speed and range in knots are given. There are other factors besides the duration of the blow and the fetch that affect the appearance of the sea surface and these should be considered if they are present.
