How Waves Form: Understanding Swell Before Reading a Forecast

This is the first article in a three-part series on how waves form. It covers their origin: how they're born out at sea. The second will look at the seabed, which turns that energy into a surfable wave. The third will cover wind and tide, which decide at the last moment whether a session turns out good or bad.

A wave is not water moving forward

This is the most common misunderstanding, and everything else follows from it.

When a wave crosses the ocean, the water doesn't travel with it. Water molecules rotate in place, in circles, passing the motion on to their neighbors. What moves is energy, not matter.

The clearest image is a taut rope shaken at one end: a ripple travels its full length, but the rope stays where it is. The ocean works exactly the same way. A cork floating on the water doesn't drift toward the beach as a swell passes: it rises, falls, and ends up more or less where it started.

This might seem like a minor detail. It's actually the key to everything else. If waves are energy in transit, the real question isn't "how big are they?" but "how much energy are they carrying, and how long have they been traveling?"

It all starts with a storm, far away

Swell is born under a low-pressure system, out on the open sea. Wind blows across the surface, deforms it, creates ripples that become small waves, then proper waves. Three factors determine how much energy is produced:

• Wind strength. The harder it blows, the more energy it transfers.
• Duration. A violent wind for two hours produces far less than a moderate wind for two days.
• Fetch. This is the distance over which the wind blows unobstructed, in the same direction. It's the most underestimated factor, and often the most decisive.

A thousand-kilometer fetch under a well-organized storm generates swell capable of crossing the Atlantic. The same wind over a lake will never produce more than disorganized chop, no matter how strong it is.

For the Landes coast, these storms most often form in the North Atlantic, between Newfoundland, Iceland, and Ireland. The swells that make Hossegor's reputation in autumn are born three or four days' travel from there.

Wind sea and swell: the distinction that explains everything

Under the storm, the sea state is chaotic. Waves of every size, every length, in every direction, colliding with each other. This is called wind sea. It's messy, and it's unsurfable.

Then the swell leaves the storm zone and starts traveling. Something remarkable happens there: the waves sort themselves out.

Long waves travel faster than short ones. Over thousands of kilometers, they pull ahead and group together. The shorter ones wear themselves out along the way and dissipate. By the time it arrives, all that's left are regular, evenly spaced wave trains arriving in sets.

This is called groundswell. This sorting-by-travel turns storm chaos into clean, parallel lines along the beaches. A quality wave is a wave that has traveled a long way.

This is why a low-pressure system sitting right off the Bay of Biscay often produces mediocre conditions despite impressive numbers: the swell hasn't had time to organize itself. It still arrives as wind sea.

Period: the number everyone ignores

A swell forecast shows three numbers. Height gets all the attention. That's a mistake.

Period is the interval, in seconds, between two successive wave crests. It measures wavelength, and therefore the depth at which the swell acts beneath the surface. It is by far the most important indicator.

In practice:

• 6 to 8 seconds: short-period swell, born close to shore, low energy. Often messy, soft chop with no real power.
• 9 to 12 seconds: decent swell, already organized. Most ordinary good sessions fall here.
• 13 to 16 seconds: long-period swell, born far away, highly energetic. It "feels" the bottom well before the beach and breaks with force.
• Above 16 seconds: exceptional swell that has crossed an entire ocean. Considerable power, often best left to experienced surfers.

One rule of thumb worth remembering: a 1 m swell at 15 seconds is far more powerful than a 1.5 m swell at 7 seconds. It will produce bigger, hollower, faster waves at the beach — even though the announced number is smaller.

This is the number one reason for parking-lot disappointment. The forecast promised good height, but at a 7-second period: energy that never traveled, collapsing weakly onto the sand.

The energy carried increases with the square of the height and with the period. Doubling the period is far more significant than doubling the impact power actually felt.

Why the Landes get such good swell

This coast combines three advantages that few places share.

It faces due west onto the North Atlantic. Swells generated by Icelandic low-pressure systems arrive almost unobstructed. No islands, no wide continental shelf to absorb them.

The continental shelf is narrow. Along much of the French Atlantic coast, swell crosses tens of kilometers of shallow water before reaching the beach, losing much of its energy to friction along the way. In the Landes, the ocean turns deep very close to shore: swell arrives nearly intact.

The Capbreton canyon. An underwater canyon that rises to within a few hundred meters of the shore, with depths of several hundred meters. It focuses and redirects swell energy toward the neighboring beaches. This is the main reason for the singular power of Hossegor and La Gravière — a textbook case covered in detail in the second article of this series, on the seabed.

Reading a forecast without getting fooled

Given all this, here's the order in which to read forecast information:

1. Period first. Below 9 seconds, don't expect much, whatever the height.
2. Direction second. A west or northwest swell enters most Landes beaches cleanly. A swell too far north or south will be blocked or poorly angled depending on the spot.
3. Height last. It's a consequence, not a cause.

One more useful habit: look at the swell's birth date, not just its state on arrival. Forecast models show the low-pressure system that generated it. A swell born four days earlier, three thousand kilometers away, will be clean and organized. One born the day before, two hundred kilometers away, will be messy even with good numbers.

What swell doesn't tell you yet

At this point, the origin of the energy and how to judge its quality are both clear. But two major unknowns remain, and they're what decide the final result.

The seabed, which turns that energy into a wave. An identical swell can produce a perfect barrel over a well-shaped sandbank, and a shapeless mess two hundred meters away. That's the subject of the next article.

Local wind and tide, which can ruin a perfect swell within an hour. That's the subject of the third.

Understanding all three means no longer just enduring forecasts, and starting to actually choose sessions.

Surf sessions on the Landes coast can be filmed by drone and in the water, for a surfer, a school, or a club. Discover my work as a Videographer.