As I sit watching the waves on a beach at the less trendy end of Grand Cayman, I got to wondering about the truth about the mythical (pr perhaps not) "seventh wave".
The truth, as these things usually turn out, is complicated.
The short answer is that the old saw is nonsense: ocean waves are generated by the wind way out to sea before they crash onto the beach, and the wind is capricious and unpredictable. Why then, should we expect waves to be regular?
However, once a few waves get going and begin to feed off and amplify each other, they do become a bit more coherent and regular, and in practice they do tend to form packs or groups of between 12 and 16 waves, with the biggest waves in the centre of the pack. So, the group of waves typically goes from small to big and back to small again, with the largest wave of the group right around number seven or eight.
However, that does still mean that the largest waves only occur every 12, 14 or 16 waves, not the 7 of folklore. Sorry.
And just while we are about it, why do waves break anyway? Waves out at sea are caused by the wind imparting its energy and friction to the waves it comes into contact with. But waves out at sea usually don't break - they just cause bigger and bigger swells.
When the waves approach a coastline, though, the water is shallower and there is less water to draw up into the wave. At the same time, there is more friction acting on the bottom of the wave (where it meets the seabed) than there is at the top. So, the bottom is slowed down more than the top until eventually the top of the wave kind of overbalances and topples over itself due to the force of gravity. This is what we see as the wave breaking. So, there you go.
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