Observing ocean waves

Wave characteristics to be observed 11.3.1

The wave characteristics that are reported in the weather message are:

 Wave period

 Wave height

 Direction from which the waves are coming from (for swell only)

If more than one system of waves is present at the same time (e.g. a sea and a swell, or no sea and two swells), and can be distinguished, then the characteristics of each wave system should be reported.

To the experienced mariner ocean waves are part of their life, and they have learned to judge accurately their characteristics through acquaintanceship. However because of the complex nature of ocean waves, a new observer may regard the sea surface as being in a state of great confusion. For example, are all waves to be considered on an equal footing, or are only the big waves to be counted? Since the wave characteristics vary so much, what average values should be taken? It is obvious that if comparable results are to be obtained the observer must follow a definite procedure. The flat and badly formed waves between the wave groups cannot be observed accurately by eye and different observers would

undoubtedly obtain different results if an attempt were made to include them. The best method therefore is to observe only the well-formed waves near the center of the wave groups.

Reliable values of period and height can only be obtained by observing at least 15–20 well-formed wave groups. Of course these waves cannot be consecutive. A few must be selected from each succeeding group until the required number has been observed.

Measurements or quite good estimates are required; rough guesses have little value and should not be recorded.

Observing the direction of waves 11.3.2

Wave direction, like wind direction is the direction from which the waves are coming. The wave direction is determined with reference to True North (not magnetic), and is recorded to the nearest 10° (e.g., 010°, 020°, 030°, right around to 360°).

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The direction of the sea waves is not required, as their direction is normally within 10° of the wind direction. Only the direction of the swell waves is required.

The direction is most easily found by sighting along the wave crests, then turning through 90° to face the advancing waves. The direction you are then facing will be the direction from which the waves are coming.

Observing the wave period 11.3.3

For measurements of period a stop watch is invaluable. If this is not available, an ordinary watch with a second hand may be used, or a practiced observer may count seconds.

The average value of the periods of the larger well-formed waves near the center of the wave groups are reported. To measure the period, an object floating on the water at some distance from the ship must be available. This can be a piece of wood thrown overboard from the bow by a crew member while you observe from the bridge. A piece of seaweed or a bird floating on the sea will also serve, or if nothing better is available a distinct patch of foam that remains identifiable for a minute or so will do. When the object reaches the crest of the first well-formed wave start the stop watch and count the number of crests which passed under the object. Note the time elapsed and the number of crests that passed under the object (do not count the first crest). Repeat the procedure until at least 15 well-formed waves have been timed. A new floating object will usually be necessary for each wave group, as the original object will probably have passed out of sight by the time the next wave group arrives. Finally add the elapsed time for the various wave groups together and divide the total by the number of crests counted to obtain the average period.

Example: An observer might make the observations shown in Table 11—1 while determining the wave period.

Table 11—1: Example of observations made to determine the average wave period

Wave group number Crests counted Time elapsed (seconds)

1 3 36

The wave period would be recorded to the nearest second; in this case 10 seconds.

MANMAR — Manual of Marine Weather Observations 11—153

Observing the wave height 11.3.4

There is no inexpensive device yet available for measuring the height of waves on a moving ship, so the height must be estimated by the observer.

When estimating wave height, consider only the larger well-formed waves near the center of the wave group. Estimate the average height of these larger waves, and disregard the lesser waves between the groups.

Waves of shorter length than the ship 11.3.5

If the length of the waves is short in comparison to the ship’s length (i.e., if the ship spans two or more wave crests), then the observer should take up a position on the deck

amidships where the effect of pitching is least, and on the side of the ship towards which the waves are coming. Look over the side of the ship and watch the waves as they travel the length of the ship at times when the rolling is least. Estimate the height (vertical distance between trough and crest) using as a yardstick the relative heights of two known points along the side, for example, the sea ladder fittings, the loading ports, or the rubbing strake.

Try to estimate the height to the nearest half metre (or one foot).

Waves of greater length than the ship 11.3.6

The recommended method of estimating height in this situation is that the observer takes up a position in the ship so that their eyes are just in line with the advancing wave crest and the horizon, when the ship is vertical in the trough. The height of the eye above the waterline is then the height of the wave. If the ship is rolling heavily it is particularly important to make the observation at the instant when it is upright in the trough, otherwise the estimate of the height will be too large.

Wave observation at night or in low visibility 11.3.7

On very dark nights or in dense fog, the most that the observer can hope for is an estimate of the height of the sea waves, which would be based on the last clear sighting of the waves, and whether in the observer’s judgment the waves have since increased or decreased, as might be indicated by the ship’s motion, or by a change in wind speed. Swell waves are difficult to observe at any time, but at night it becomes almost impossible. It is only on very bright moonlit nights that the height, and perhaps the period, could be observed.

11—154 MANMAR — Manual of Marine Weather Observations