Let’s start at the beginning. Most waves are created by wind, which develops ripples, then small waves, and finally larger ones as it increases in force and persists in time. But the direction from which winds blow changes quite frequently, so that waves of different heights run in different directions, riding on each other’s backs and forming a complex system of high waves that we call a “sea.”
The characteristic of a sea is that it is irregular, producing crests of varying heights and lengths. The waves have steep surfaces and break frequently, bearing white crests. Scientists have shown that a sea consists of complicated layers of waves of different heights, different periods, and different velocities, although certain heights and periods will be dominant.
What’s interesting about a sea is that it’s impossible to estimate, from the height of the present crest, the height of the next crest likely to come along, because you can’t know whether it will ride on top of a small wave or a big one.
Now, as the wind decreases, the various waves forming the sea will decay and become more rounded. Usually, the steepest and largest waves will decay the quickest. They are the ones most recently formed in the direction of the prevailing wind. The other, smaller waves, will continue to trundle along in their various directions, as before.
So now the seas’ motion becomes more regular. It loses height in comparison with length. Its surfaces lose their steepness and become smoother and more rounded. And so, finally, we have a swell.
The characteristic of a swell is that you can rely on it for some short-term regularity. Unlike a wave, which is unpredictable, one swell is pretty much like any other swell in the same train. However, it seems to be true that every fifth or ninth swell is a little different from the common herd, either bigger or smaller, and often has a couple of similar companions dragging along behind it. Surfers know this well.
It’s also true that a swell generated by a large storm can run for many hundreds of miles after the storm has blown itself out. In some large sea areas there is an almost continually present swell. For example, the Cape Rollers, off the southern coast of Africa, come spinning off the Roaring Forties, a world-girdling belt of westerly winds hundreds of miles to the south.
There. Now you know all you need to know about waves, seas, and swells. Class dismissed.
Great seas have little seas upon their backs to bite 'em,
And little seas have lesser seas, and so ad infinitum.
— With apologies to Augustus de Morgan
A news item in the Walnut Street Gazeout (should be Gazette) says:
“Next Friday night’s concert in the main cell block will be performed by the pop group Heavy Lift, and their supporting group, The Truss.”
(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)