Where waves come from

The waves you'll surf this weekend are already out there. A storm you'll never see made them days ago, maybe on the other side of an ocean. This page follows them from the storm to your beach.

It starts with wind

Wind dragging over water makes ripples. The ripples give the wind something to push on, so they grow. Blow harder, or for longer, and you get bigger waves that are longer from crest to crest. They also need room. The stretch of sea the wind blows over is called the fetch, and once the waves reach the end of it they stop growing. That's why a gale on a lake makes nasty chop but never a long swell, however long it blows.

Those heights are the sea out where the wind blows, not surf. Forecasters size up a sea by the average of its biggest third of waves, and in a big storm that can be as tall as a three-storey building. The wind here also blows perfectly steadily, which real winds never quite do, so real seas tend to come out a little smaller.

Only part of that sea reaches your beach as swell. On the way it shrinks in three ways. It spreads out over a wider and wider stretch of ocean. Its periods string out, so only a slice of the storm's waves goes past at any one time. And the short waves die off. The swell that's left keeps roughly the storm's period, but after crossing an ocean it's usually a third to half the storm's height, and less from a small storm. You can try it in the ocean scene below.

A real storm blows like this for a day or two over a patch of ocean the size of France. What it leaves is a mess: waves of every size, running every which way, piling into each other.

Then the sea sorts itself out

Long waves travel faster than short ones. So once the waves leave the storm, they spread out: the longest race ahead, the short ones fall behind, and the shortest just die. A few days out, the mess has turned into neat lines of waves that are all about the same size. That's a swell.

Surfers who watch buoys know this pattern. A new swell shows up as a few long, slow waves, 18 or 20 seconds apart, often hardly any height at all. Over the next day the period drops and the waves fill in. The farther away the storm, the longer that takes.

Across an ocean

Swell goes a very long way. Out in deep water it loses surprisingly little, and a 15-second swell covers about 1,000 km a day. In 1963 the oceanographer Walter Munk and his team followed swells from storms in the Southern Ocean all the way up the Pacific to Alaska, more than 10,000 km.

A storm doesn't send swell equally all round, though. Most of it goes the way the wind blows, in a fan, so a beach off to one side gets a smaller swell, or none.

This is why a beach only gets swell from some directions: it needs open water between it and the storm. It's also why some spots wake up only in winter, when the storms are in the right place, and why a south swell in California can come from a storm south of New Zealand a week and a half earlier: storms down there mostly blow towards the east and north-east, aiming their swell up the Pacific.

Why waves come in sets

Sit on the beach and you'll see the waves don't come evenly: a few big ones, then a lull. That's because a swell is never one clean period. It's a bundle of close ones, say 12, 12.5 and 13 seconds, that drift in and out of step. When they line up, their waves add up into a set. When they fall out of step, they cancel out and you get the lull. Two swells are the simplest version of that, so here are two.

So "every seventh wave" isn't a rule. How many waves come in a set depends on how close the periods are, and it changes from one swell to the next.

What's at your beach right now

Every swell at your beach left a storm somewhere. Here are the ones there right now, each with the line it came in on. Pick a spot, use where you are, or tap the globe.

Want to go further? Swell period, explained shows why a smaller swell with a longer period is often the bigger day.