We are taking a look back at stories from Cosmos Magazine in print. In March 2024, coastal scientist Rob Brander tackled the physics of everything from wave formation to staying on a board to spending time in the green room.
On October 29, 2020, German surfer Sebastian Steudtner rode a 26.2 metre-high wave (86 foot) at Nazaré, Portugal. It’s considered to be the biggest wave ever surfed. Aside from the obvious questions of Why would you surf a six-storey high wave? and Where do you learn to surf in Germany?, you might be wondering how a wave can become that big, and how it’s even possible to surf one.
The remarkable answer is that whether a wave is 20 metres or two metres high, the physics of surfing follows the same principles.
So what’s the science behind this activity that millions of people around the world love?
Surfing is the act of riding a wave, but the creation of a ridable wave involves a complex combination of meteorology, wave physics, hydrodynamics, coastal geology and nearshore bathymetry.
It all starts with wind.
The ocean surface is rarely flat. As wind blows across the surface, energy is transferred from the wind to the water, making any bumps or ripples grow exponentially and move in the direction of the wind, creating waves. The faster and longer the wind blows and the greater the distance it blows across the water (known as the fetch), the larger the resulting waves will be.
Most waves are formed by low pressure systems, which create a chaotic cocktail of waves of all shapes and sizes that travel in all directions thanks to their newly acquired energy.
Once formed, all waves travel at a speed dictated by their wavelength – the distance between wave crests – and their period: the time it takes two wave crests to pass the same point. Waves with longer wavelengths travel faster and eventually turn into swell waves with periods of between eight and 20 seconds. Long, clean and powerful lines of swell are the waves that surfers dream of.
Conversely, waves with periods of less than eight seconds are often formed by locally generated onshore winds and tend to be messier and choppier. These wind waves, often referred to by surfers as being junky – or just plain mush – are not great for surfing.
It might be obvious, but surfing is largely an ocean phenomenon. Even large bodies of water like the Mediterranean Sea and the Great Lakes of North America do not have sufficient fetch to generate swell waves. Surfing in those environments is largely restricted to strong wind events, which create short, choppy wind waves.
Breaking waves
As swell waves propagate into shallower water, the configuration of the coastline causes them to bend or refract around headlands, rock platforms or coral reefs. This creates point breaks, which can give surfers longer rides. Then, the underlying morphology of the beach profile – like the gradient of the beach and patterns of sandbars – controls where a wave breaks: either as a ‘close out’ where waves break simultaneously along its length, or as peaks within the surf zone which provide left handers or right handers as waves break (or peel) towards left or right.
It doesn’t matter what the height of the incoming wave is; what determines how it breaks is what lies underneath.
Surfers need to reach a speed close to the speed the wave is travelling, otherwise it will pass them by. The wave will also need to be of just the right steepness.
The ultimate goal of experienced surfers is to find themselves tucked into a tube. This occurs when swell moves from deep to shallow water over a very short distance, such as at the shoreline on very steep beaches or at sudden drop offs associated with sandbars, rocks and reefs. Friction with the sea bed causes the wave to slow down quickly, and the orbital motion of water particles within the wave overtakes the speed of the wave motion, thus creating a circular barrel shape (nicknamed the ‘green room’) as the wave crest curls over and breaks. These plunging waves are easily the most spectacular and dangerous type of surfing wave. They don’t call it the Banzai Pipeline on Oahu’s North Shore for nothing.
But for the beginners in the room, fear not! Not every breaking wave is a plunging wave. Beaches with gradual offshore slopes will cause swell waves to slow down and steepen gently, ultimately breaking as a spilling wave, which are more easily rideable. As their energy is dispersed over a wider area, experienced surfers can catch a spilling wave just before it breaks, while beginners can learn by riding the whitewater of the broken wave moving towards the shore. They can be big, but spilling waves are (mostly) harmless.
Surf’s up!
The science doesn’t stop with wave formation. There’s also physics involved in actually catching and riding a wave. Regardless of whether they’re a shortboard or longboard rider, boogie (body) boarder or bodysurfer, all surfers need to reach a speed close to the speed the wave is travelling, otherwise it will pass them by. Most people paddle onto waves, but on big waves like Nazaré surfers need to be towed on by a jetski. The wave will also need to be of just the right steepness to allow surfers to catch the wave just before it begins to break.
Once on the wave, you maintain your momentum by essentially riding downhill on the face of the wave through the force of gravity, while feeding off the kinetic energy of both the wave motion and the orbital water particle motions.
The buoyancy of your surfboard also allows you to stay on top of the wave, and various body positioning and actions – plus the assistance of stabilising fins – allow you to control the direction you’re surfing, as well as being able to move up and down the front face of the wave as it propagates and begins to break.
Surf’s not up…?
It may surprise you to know that surfers are quite a patient bunch. They can wait a long time for the right wave during a session, and sometimes have to wait weeks or even months for the right conditions. You can blame it on the surf god Huey – or the wave climate, which is basically the same thing depending on who you talk to. What’s clear is that there’s a fair bit of temporal and spatial nuance when it comes to surfing waves.
Waves are formed by wind and wind patterns that are dictated by larger-scale meteorological forcing, which can be seasonal in nature or linked to longer-term phases like the El Niño Southern Oscillation. And while some surf breaks are fixed in place by geology and others shift in position along with shifting sand bars, all surf breaks are modulated over minutes and hours by what the tide is doing.
As water depth changes with the rising and falling tide, some breaks become tuned to the water level and only ‘work’ for a very short period of time. The wave at Nazaré, for example, is not mountainous all the time. It requires extremely specific wind and wave characteristics interacting with the unique offshore bathymetry. It takes a special type of person to willingly ride those waves. But for most of us, crumbly two footers will do just fine.