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The science of surfing

by Chris Woodford. Last updated: January 13, 2014.

Surfing is a cool way to spend a hot day—but there's much more to "hanging ten" than just balancing on a board. Mastering surfing is all about mastering science: you need to know how waves travel across the ocean carrying energy as they go, and how you can capture some of this energy to move yourself along. Whether you're surfing or bodyboarding, riding a longboard or whizzing on a skimboard, you're using cool science in a very cool way. Let's take a closer look!

Photo: Surfer Dustin Ray "D-Ray" catching a wave in Morro Bay, California. By very kind permission and courtesy of Mike Baird and Bairdphotos.com, published on Flickr under a Creative Commons licence. Sadly, Dustin died on September 9, 2012 aged only 39. You can read more about his life chasing waves in Dustin Ray RIP.

What are waves?

Waves are always the first thing you notice about the ocean. Except on very calm days, there are always waves skimming across the surface of the sea. What exactly are they doing there? We usually find waves in a place where energy has appeared. A basic law of physics called the conservation of energy says that energy can't be created or destroyed; it can only ever be converted into other forms. When energy suddenly appears, concentrated in one place, something has to happen to it. Usually, energy doesn't stay put: it tends to travel out in all directions to other places that don't have as much.

Supernova

Picture: Energy likes to travel outwards! This bubble-shaped explosion of gas and dust is 14 light-years wide and is expanding at 4 million miles per hour (2,000 kilometers per second). It's the remnant of a supernova (exploding star). Photo courtesy of NASA Jet Propulsion Laboratory (NASA-JPL).

Think about a couple of familiar examples. Suppose you bang a kettle drum in the middle of a football stadium. As your arm beats the stick, you make the drum skin vibrate up and down with kinetic energy (the energy of movement). As the drum skin bounces back and forth, it produces waves of sound energy. These travel through the air, making air molecules vibrate in sympathy, carrying sound energy in all directions until it dissipates and gradually disappears. Something similar happens if you switch on a lamp in the middle of a dark room. This time, waves of light energy travel out from the lamp in all directions. Why can't you see sound and light traveling in the same way that you can see waves on the ocean? Sound travels at over 1000 km/h (600 mph)—so it is both quick and invisible. Light travels much faster than sound at 300,000 km per second (186,000 miles per second)—so it carries electromagnetic energy between two places virtually instantaneously. Even though you can see light, you cannot see it traveling.

A man surfing a breaking wave

The great thing about ocean waves is that you can see them coming. If you're surfing, even fast-moving waves are slow enough to catch and carry you along. Try doing that with sound or light! The properties of an ocean wave are also very easy to see. You can estimate its amplitude (height) just by looking out to the horizon. Its wavelength (the distance from one wave crest to the next) and frequency (the number of waves that travel past in a certain amount of time) are also very easy to see.

If sound waves come from people beating drums, and light waves come from people switching on lamps, where do ocean waves come from? If you live in the northern hemisphere, far from the equator, you've probably noticed that there are more waves around in the fall (autumn) or spring than in the summer. In the UK, for example, there's most wind in the fall and winter—and that's usually the best time of year for surfing in places like Cornwall. Wind is important because it's what puts energy into the ocean: it makes ocean waves in more or less exactly the same way as you make sound waves when you bang the skin of a drum.

Photo: The curling wave behind this surfer provides all the kinetic energy that drives him forward. No waves, no energy, no surfing! Photo by very kind permission and courtesy of Mike Baird and Bairdphotos.com, published on Flickr under a Creative Commons licence.

Make waves! (for younger readers)

NASA JPL satellite image of Hurricane Isobel, 2003.

It's easy to see how wind makes waves in this simple activity:

  1. Fill your sink or wash basin with a few inches of water.
  2. Bend down so you get as low as you can and as close as possible to the surface of the water.
  3. Take a deep breath.
  4. Blow as hard as you can directly across the surface of the water.
  5. You should see waves traveling over the water. If you keep blowing in a regular pattern, it's possible to send streams of waves—which look very much like ocean waves—across the surface.
  6. As the waves reflect back and forth in your sink, if you blow at exactly the right time, you'll find you can add more energy to the existing waves and make them bigger—and bigger. This is effectively how the wind keeps adding energy to ocean waves.
  7. Now imagine this experiment scaled up a few million times. Imagine your basin replaced by the Pacific Ocean and a gale-force wind replacing your breath—and you can see exactly how ocean waves are created.

Photo: Hurricanes aren't much fun if you're in the line of fire, but the energy they pump into the oceans is often great news for surfers. This is a satellite photo of the 200km/h (120mph) winds produced by Hurricane Isobel in September 2003, courtesy of NASA Jet Propulsion Laboratory (NASA-JPL).

What's the difference between wind swell and groundswell?

The waves that arrive at your beach are not necessarily created anywhere nearby. Back in the 1950s, an ocean scientist named Walter Munk conducted an amazing series of experiments with ocean waves. He managed to prove that some waves travel over 15,000 km (over 9000 miles) across the open ocean before they reach their eventual destination. That's like traveling across the United States, from New York City to California, four times! Generally, the more widely spaced and the cleaner waves are when they roll up on the shore, the further they have traveled.

Waves like this are known as swell (or groundswell) and they make the best waves for surfing. Groundswell is the reason you can have quite large waves washing up on your beach even when there's little or no wind blowing. Waves generated nearby (by winds blowing in the local area) are known as wind swell. They are usually choppier, smaller, messier, harder to surf, and less interesting to surfers than groundswell. Often the waves in a particular place are a mixture of groundswell and wind swell—a random collection of waves that have traveled from far away mixed with waves that have come a much shorter distance.

Small groundswell

Photo: Small groundswell rolling in. Note how the waves are almost evenly spaced. The distance between one wave crest and the one following is the wavelength of the wave. These waves have a wavelength of about 15 meters (50 ft).

Why is groundswell cleaner than windswell? When the wind blows on the sea, it produces all kinds of waves of different wavelengths, frequencies, and speeds. As the waves travel, the faster waves gradually separate out from the slower waves. The further the waves go, the more chance they have to sort themselves into an orderly pattern. Groundswell has more time to get itself into shape than windswell. Eventually, the waves form into distinct little groups called sets: when they finally arrive at their destination, a little group of good waves will arrive at once. Then there will be a pause. Then the next group of waves will arrive a bit later.

When and why do waves break?

Swell is only one of the ingredients for great surfing. Surfers don't like any old waves: they want waves that peel (break gradually to the left or right along the wave crest) rather than close out (where the crest folds over and smashes to pieces all in one go). When a wave is peeling, you can ride back and forth across the crest as it slowly breaks; with a wave that's closing out, there's nowhere much to go. In surfing slang, waves that close to the right are called, not surprisingly, "righthanders", while left-breaking waves are "lefthanders". The angle at which the wave peels makes it more or less interesting to surf. The steeper the angle, the harder it is to surf and the more interesting moves you can pull.

What makes a wave break... and peel rather than closing out? When water flows, in the ocean or in a river, its upper layers are traveling faster than its lower layers (indeed, the water is usually stationary on the ocean floor or on a river bed). Think about waves arriving at a beach. As they  travel from the open ocean to the shore, they move up a gradual sandy incline, and start to slow down. The bottom of a wave slows more quickly than the top. So instead of a wave moving forwards as one, we have a whole series of water layers sliding past one another, with the top layers moving fastest and the bottom moving slowest. A wave breaks when the top part of the wave goes so far over the bottom part that the wave can no longer support itself—so it completely collapses. A wave peels when this process happens gradually along the length of the wave rather than all at once. If you like, a peeling wave is breaking in two dimensions: along the crest of the wave as the wave advances up the beech or reef.

Waves can break in many different ways, and that largely depends on the profile of the seabed underneath them (known as the bathymetry). All waves will break eventually, but major features like rock or coral reefs, ledges, and sandbars will make one side break before another, causing waves to peel. Nearby groins (sea fences), piers, and jetties can also make waves peel. Different shapes of reef produce different breaking effects. Over the last decade or so, surf science has become advanced enough for engineers to start designing artificial reefs. Rocks or ballast are buried at a key point offshore to give the waves a helping hand in breaking and peeling early in places where they might otherwise simply close out.

A surfer lying on his board and paddling hard to catch a wave

Swell and bathymetry are not the only things that affect the quality of your surfing. How the wind is blowing on your beach will make a big difference too. Waves are obviously always traveling from the open ocean towards the beach, like scaled-up versions of ripples on a pond, but the wind can be blowing in any direction. If the wind is blowing directly out to sea, it is known as an offshore wind. As it blows, it will naturally tend to prop up the waves, stopping them from breaking so quickly, cleaning out some of the smaller choppier waves, and making the waves finally break with greater intensity in shallower water. A combination of strong ground swell and a light offshore wind is always best for surfing, especially if the wind has been blowing for a few days (both to create groundswell and to give it time to travel to your beach). If the wind blows in the opposite direction, so it is onshore, it will make the waves collapse much too soon—spoiling your fun! A strong wind that is blowing directly onshore (at right angles to the beach) can produce a very random, choppy sea that is impossible to surf, but fun to mess about in with a bodyboard.

Tell me why...

Science (and physics in particular) can explain most of the strange things you'll notice when you're riding along on your surfboard. Questions like...

Why do you have to paddle?

Whether you're on a surfboard or a bodyboard, if a great wave is heading towards you, you have to paddle like mad to be able to catch it. In other words, you have to be traveling with some speed and momentum as the wave hits you to stand any chance of riding along with it. Why is that? To travel with a wave, you have to accelerate to the speed it's traveling. In other words, you have to gain a certain amount of kinetic energy very quickly. If you've already got some kinetic energy to start with—if you're already moving when the wave catches up with you—it's much easier for the wave to accelerate you a little bit more. Or in simple terms, the faster you paddle, the more likely you are to catch your wave.

Typical blue and white bodyboarding fins.

Photo: Right This surfer is paddling like mad with his arms to gain speed. When the huge wave catches him up, he'll have enough momentum to leap to his feet and surf. Photo by very kind permission and courtesy of Mike Baird and Bairdphotos.com, published on Flickr under a Creative Commons licence.

Left: If you want to catch waves on a bodyboard, fins like these are essential: by kicking with fins, you can accelerate your body much faster and greatly improve your chance of catching waves. However, it cuts both ways: once you've caught a wave, you need to lift your fins clear of the water to stop them acting like brakes and slowing you down!

Why can small kids ride small waves?

Have you noticed how young kids can ride almost any waves—but older ones can't? It's back to momentum again. To move you forward, a wave has to give you a certain amount of momentum and energy. Both of these depend on your mass (how much "stuff" your body is made from). The bigger you are, the more energy you need to travel at a certain speed. So the older and bigger you are, the bigger the waves you need for surfing—because bigger waves can supply you with more energy. If you're younger or smaller, you need less energy to move at the same speed, so a smaller wave will do the job.

Why do waves suck you backwards?

At school you learn about two kinds of wave. There are waves like sound, which travel by a sort of push-pull process, making patterns of alternately squeezed up dense air (compressions) and thinned-out, less-dense air (rarefactions). Sounds wave are called compression waves; they're also called longitudinal waves, because the air molecules that carry sound energy move in the same direction as the wave travels. Then there are waves like light that travel in a familiar, up-and-down pattern. These are known as transverse waves, because they vibrate at right angles to the direction in which the wave travels.

Sucking wave on a beach

But ocean waves are not like the waves you learn about at school. They move the water surface round in circles as they travel along. Watch a seagull sitting on the ocean as a wave approaches. The gull is sucked backwards up the front of the wave, lifted onto the wave's crest, pushed forward as the crest passes by, and then lowered down to pretty much the same place it started off in. This happens because ocean wave energy is not traveling purely on the surface of the ocean: it also affects the layers of water underneath. You'll have noticed this sucking effect if you've ever caught waves on a bodyboard. As you lie on the sea surface, you'll feel yourself being pulled backwards as a wave approaches. This is another reason why you have to be paddling forwards to catch a wave. If you're not paddling forwards, you're definitely going to get sucked backwards!

Photo: Layers of water slide past one another at the shore. At the top of the beach, water is sucking back down into the sea. Just offshore, a small wave is breaking inwards and up the beach. In between, another small wave has just broken and is coming to a halt. There are at least three layers of water sliding over one another here. In these calm conditions, the liquid layers are moving in what physicists would call laminar flow.

Why does a surfboard have a curved front edge?

Closeup of the front of a Manta Elite bodyboard showing the curved nose.

Everything from the biggest ocean liner to the smallest surfboard has a curved front edge. Why? If you push a curved edge over water, the curve makes water travel more quickly underneath than on top. This generates an upward force called lift that moves you up and slightly out of the water—an effect called planing. Because you're partly out of the water, there's less drag (water resistance) and you go faster. You can see planing happening on almost any boat as it picks up a bit of speed. With a hydrofoil—a kind of "surfing boat"—the planing is so spectacular that the entire craft lifts up out of the water. The same science is at work on a surfboard, only not quite so dramatic!

Photo: The curved front edge of my Manta bodyboard. If I lay this board down on a flat floor, it has a very noticeable—amost banana-like—curve to it.

What about tides?

Low tide with small sea swell

Tides have nothing to do with waves. Tides are caused by the Moon and the Sun working together to "pull" the sea back and forth with their gravity, rather like a giant blanket moving up and down a bed. Tides change the depth of the water on your beach. When the tide is "in", the waves come in further and break later; when the tide is "out", the waves break further out. Depending on the profile of the seabed, a rising tide (one coming in) or a falling tide (one going out) will make the waves tend to break somewhat better or somewhat worse than usual, depending on the local seabed. There is no absolute rule that works everywhere: some places work well as high tide approaches; some work best when the tide is going out.

When the Moon and Sun line up, twice a month, they make higher tides than usual called spring tides, which give deeper water during high tides (when the tide is in) and shallower water at low tide (when the tide is out). In between the spring tides are neap tides, when the sea moves back and forth less than usual, high tides are less deep, and low tides are less shallow. Again, depending on the seabed, high and low tides, and spring and neap tides, will make the surfing better or worse—but it varies wildly from place to place. If you're in a place that needs deep water to make the waves break properly, the highest spring tides are going to be better than the lowest neap tides. But elsewhere, the opposite may be true.

Photo: Low tide with onshore wind and little swell. Even if you could catch "waves" this small, science tells us they have too little energy to take us anywhere. This water's definitely not for surfing, but you might get a few short rides on your boogie board if you're really lucky. Not that it matters. These guys are having more fun in the water than I am taking their photo: the number one rule of surfing is that there's always more fun in the water than watching from the beach.

Why wear a wetsuit?

The synthetic rubber traps water next to your body, which provides a useful layer of insulation to keep you warm. Read our article about wetsuits to find out more.

Can science make you a better surfer?

Of course! It won't make you stand on the board any better. But if you understand what waves are, how they are made, and where they come from, you'll have a much better idea of when the surf's going to be up. And if you can predict when the waves are ready to ride, you're halfway there already. If surfing is a quest for the perfect wave, science can at least point you in the right direction. It might not make you a better surfer, but it certainly won't make you any worse!

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Text copyright © Chris Woodford 2007, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2007) The science of surfing. Retrieved from http://www.explainthatstuff.com/surfingscience.html. [Accessed (Insert date here)]

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