Beginner 9 minutes
Swell: how wind turns into waves
Energy that travels without the water traveling, and what happens when it reaches shore
A buoy in open water rises and falls with each wave, but it never goes anywhere. It's the best demonstration of what actually happens: what travels thousands of kilometers across the ocean isn't the water, it's the energy passing through it.
In this article you'll see how wind transfers energy to the surface, what the water under a wave actually does, what swell becomes once it moves away from the storm that generated it, and what happens to it when it reaches shallow water and breaks.
Swell decides entries, exits, visibility, and surface comfort. The wind article covers how to read a forecast and which coast stays sheltered; this one covers the physics behind it, which is what lets you anticipate what the sea will do as it approaches the coast.
From wind to wave
A gradual transfer, not a single blow.
It all starts with wind friction on a smooth surface. The first irregularities are tiny, barely a ripple, but they change the geometry of the surface: from there the wind has something to push against, and the energy transfer becomes much more efficient.
The process feeds back on itself: the bigger the wave, the more surface it offers the wind, and the more energy it absorbs. That's why the sea doesn't grow linearly but develops, and why duration and fetch matter just as much as speed, as detailed in the wind article.
While the wind keeps acting, the result is a disorganized sea, with waves of many sizes and overlapping periods, short crests, and occasional breakers. This is wind sea, and its defining feature is chaos.
Every wave is described by three numbers: the height between trough and crest, the length between two crests, and the period, which is the time it takes two consecutive crests to pass the same point. Length and period are related, and almost everything else depends on them.
And there's a fourth, implicit quantity: the energy the wave carries, which grows very fast with height. Doubling a wave's height multiplies its energy by considerably more than two, which explains why a sea that looks only a bit bigger can be disproportionately more uncomfortable.
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