What actually makes a wave break

Out in deep water, the water in a wave is not travelling with it. Each parcel of water moves in a circle and returns roughly to where it started, and it is the disturbance — the energy — that moves along. That is why a gull sitting on the surface bobs up and down as a swell passes instead of being carried off toward the beach.

Those circles get smaller with depth. The motion reaches down to about half the wave's own wavelength, and below that the water barely notices there is a wave overhead at all. A fifteen-second swell has a wavelength around 350 metres, so it is still stirring the water 175 metres down. A six-second windswell reaches barely 28 metres.

This is the whole reason depth matters. As long as the seafloor is deeper than that reach, the wave passes over it unaffected. The moment it isn't, the seafloor starts taking energy out of the bottom of the wave, and the wave starts to change shape.

When the bottom of a wave starts dragging on the seafloor, the wave slows down. The top does not — it is still in open water — so the crest begins to lean forward over its own base. At the same time the wave gets shorter and, to conserve its energy, taller. It stands up.

It falls over when the crest is moving faster than the wave form itself. In practice that happens at a fairly reliable ratio of wave height to water depth: a wave breaks in water roughly three-quarters of its own height. A four-foot wave breaks in about three feet of water. That single rule of thumb explains more about surf than any other sentence in this article.

It also explains why a spot has a personality. The depth contours off your beach are fixed. Feed them a bigger wave and it breaks further out; feed them a longer-period wave and it starts feeling the bottom sooner and spends longer organising itself on the way in. Same ground, different wave, different result — every time, and predictably.

Most descriptions of breaking stop at 'and then it breaks'. What happens next has been measured in detail, and it is quicker and stranger than people expect.

The lip detaches and becomes a projectile. It falls at about three-quarters of gravity — air drag and surface tension slow it — and lands on the face at roughly one and a half times the speed the crest itself was travelling. Then the wave does the thing that no one draws: instead of fading, the crest jumps. It hops from the original crest to the first splash-up, then to the second, stepping down in height at each jump.

Two things in that figure are worth carrying around. The first is how brief the overturn is — a few percent of the period, and it scales with the square root of the wave height rather than with the period at all. A four-metre, sixteen-second wave has its lip in the air for about one second out of sixteen. The second is that getting from there to whitewater takes most of a period: five to eight breaker depths from where it broke, the wave is a bore, a travelling step of foam, and from that point on it decays the same way whether it spilled or pitched.

Waves break in three recognisable ways, and which one you get is not a matter of size. A spilling wave crumbles down its own face and gives you a soft, forgiving shoulder. A plunging wave throws its lip clear of the face and encloses air — the barrel. A surging wave never really breaks at all; it rushes up a steep beach and drains back.

Remarkably, one number predicts which you get. It compares how steep the seafloor is against how steep the wave is, and it is built from three things you already know: the slope of the bottom, the height of the wave, and its period.

The non-monotonic part catches people out. It is tempting to read the scale as 'bigger number, hollower wave', and that is wrong past about two: a very steep bottom does not give you a heavier barrel, it gives you a wave that never forms one. The hollowest surf in the world sits in the middle of that range, not at the end of it.

One more caution, because it is the most common overreach with this number: it tells you the type of break, not the shape of the barrel. When researchers went looking for a relationship between the surf-similarity number and barrel proportions at Duck, North Carolina, they found none. Barrel shape tracks the seabed gradient directly.