The term ‘sea’ is used to refer to the system of waves raised by the local wind blowing at the time of observation. The term ‘swell’ is used for waves which are either due to winds blowing at a distance or to winds which were blowing previously but have now ceased.
The following definitions are used:
1. The speed (c) is the speed at which individual waves travel.
2. The length (L or ) is the horizontal distance between successive crests or troughs.
3. The period (T) is the time interval required for the passage of successive crests (or troughs) past a given point.
4. The height (h) is the vertical distance between the top of a crest and the bottom of a trough.
Waves tend to travel in groups with patches of relatively dead water in between (in fact, individual waves travel through a group at about twice the speed of the wave group).
The characteristics which are measured are direction, period and height. Reliable values of period and height can only be found by taking the average of the values of at least twenty waves. Observations are made of a few waves in the middle of a number of wave groups until the necessary total is obtained. Rough guesses are of no value and should not be recorded.
It is often found that waves are coming from more than one direction. The observer should distinguish between sea and swell and report them separately. Additional groups may be used if there are more than two sets of swell waves.
If sea and swell cannot be differentiated when there is a strong wind blowing, it is best to treat their combined effect as ‘sea’ waves.
a. Direction
This is easily found either by sighting directly along the wave crests (and adding or subtracting 90°) or by sighting directly into or with the sea. The direction reported is the True direction from which the waves are coming.
b. Period
Either a stopwatch or any digital watch with a stopwatch function may be used.
The observer selects a patch of foam or a small object floating on the water some distance away. The time when it is on the crest of each successive wave is noted and the procedure is repeated for the larges waves of each successive group until at least twenty waves have been observed. The average time for a complete oscillation is then taken as the period.
In a fast ship the patch of foam will probably not be identifiable for more than one observation and each wave may have to be observed separately. An alternative is to use an empty beer can, which should be thrown as far outboard and for’ard as possible.
c. Height
Some vessels (mainly research ships) have wave recorders fitted. For merchant ships, wave height is found by estimation. The procedure used depends on the wavelength of the waves compared to the length of the ship.
i. Wavelength shorter than the ship’s length
If the ship spans two or more wave crests, the height should be estimated from the appearance of the waves at or on the side of the ship, at times when the rolling and pitching is least. For best results the observer should use a position as low as possible on deck, and preferably amidships on the side of the ship towards which the waves are coming.
ii. Wavelength longer than the ship’s length
In this case the ship will rise bodily with the passage of each wave crest. The observer should take up a position at a height so that his eye is just in line with the advancing wave crest and the horizon when the ship is vertical in the trough. The height of eye above the waterline is then the height of the wave. Again the best results will be obtained from observations made amidships.
If the height of the waves is small only a rough estimate is possible. An attempt should be made to fix a standard of height in terms of the height of a man or the height of a bulwark, fo’csle or other suitable object. There is a tendency to overestimate the height of short waves and underestimate the height of long waves.
Estimating wave height from a high bridge on a fast ship is difficult and much depends on the skill and ingenuity of the observer. Often all one can hope for is a rough estimate.
All estimates should preferably be made when the vessel is on an even keel.
Under these conditions the most that an observer can hope to record is the direction and possibly a rough estimate of the height. Such observations are particularly valuable in tropical waters in the hurricane season.
The study of ocean waves is of practical importance. The forecasting of swell is important for exposed coasts and for oil and gas installations. Better wave records are important for weather routing and the statistics are of value to naval architects for the design of vessels.
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Meteorology 1
dariusz.lipinski