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The science of surfing


Surfing is a cool way to spend a hot day—but there’s much more to riding waves than just balancing on a surfboard. 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!

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 conversation 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.

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. 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.

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. The 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.

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 9000 miles across the open ocean before they reach their eventual destination. 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.

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, its upper layers are traveling faster than its lower layers. 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 beach 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.

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.

Why do you have to paddle?

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 you have to paddle…

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.

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.

What about tides?

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.

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.