Wind creates waves

Waves: How they arise, travel and break

Papillon stands on the cliffs of Devil's Island and counts waves. The Île du Diable is one of a group of three small islands off the coast of French Guiana that serve as a prison and are considered escape-proof. After a few weeks of observations, he comes to the conclusion that waves always appear in groups, usually of seven, and that the middle waves of these groups are the largest and most powerful. So Papillon decides to jump on a self-made raft made of tied sacks of coconuts during a particularly large swell.

He counts the waves, chooses the fourth, jumps into the roaring white water and lets himself be pulled out to sea. The adventure ends well for Papillon, the story of his eternal escape is a success as a book and film. But his friend Dega stays behind, the escape plan was too risky for him. It is quite possible that even the daring Papillon would have discarded the plan if he had been able to deal with the knowledge of waves in a little more detail.

Because wave sets do not always consist of 7 waves. And to deduce the size of the next from one set remains a vague forecast. The Formation, their journey and the refraction behavior of waves are highly complex and not yet fully understood.

Your surfing can get better on every turn, on every wave you catch. Learn to read the ocean better. A big part of my success has been wave knowledge.

Kelly Slater

Every surfer understands the end of the journey: The waves come in clear lines with some distance, bend around the point and break with precision and reliability - or not. This judgment, made in a few minutes, quickly reveals the complexity of the prediction a day ago or the question: why are you breaking so hot today? to forget. At most, an irritation remains, after all, the app had given 5 stars, but the waves were confused and unsurfable.

The most important factor to be able to predict the quality of the waves is the knowledge of the local characteristics of a break: How is it oriented? How is the underground? Are there upstream shallow water zones or a deep sea trench? How are the local winds?

The second most important factor is to be able to read, understand and interpret the data provided by the prediction tools. And although you can learn to read a forecast in two minutes, understanding it is a whole different story.

The real wow factor only comes when you look at the overall picture: The journey of an energy package from the sun to our surf spot. And that's what this article is about. The key questions are as follows:

  • How do waves come about?
  • How do waves travel?
  • How do waves break?

All explanations relate to waves in Europe, and although the principles would apply anywhere, some details and examples are given for the Spots in Northern Spain or the Costa de la Luz relevant, while for Peru they are completely superfluous.

Wave formation

The emergence of low pressure areas

At the beginning of an energy package that we will call a wave set, there is the sun. The sun warms the earth and, through its energy, makes life possible and then also meaningful: through the formation of surfable waves.

However, the sun does not heat the earth in the same way everywhere: the equator is closer to it than the poles and is therefore warmed up more. In addition, the earth rotates, which brings the all-influencing Coriolis force into play. And because the earth also rotates slightly inclined around the sun, there are seasonal fluctuations in warming. However, these do not apply to the equatorial regions, which always have similar temperatures. On the other hand, the temperature difference between these regions and the poles is particularly large in winter. Two thirds of the earth's surface is also covered with water. The greater part of the land mass is in the northern hemisphere. And because water stores heat much better than land, this results in a further increase in the temperature gradient in winter. So what happens in the Northern Hemisphere in terms of waves in the fall?

The sun heats the atmosphere: more over the equator and less over the North Pole. The heated air has a lower density and rises. Cold air tries to push in. Because of the Coriolis force, it is pushed to the right on its way - which leads to the breakup of the system into three smaller circles, which run between the equator and the 30 ° north latitude, 30 ° and 60 ° as well as 60 ° and 90 °.

Because there is a lot of land mass in the northern hemisphere (see above), the temperature gradient between the northern latitudes and the equator is particularly high in autumn and winter. The air circulates faster and as a result, larger areas of low pressure arise: On the polar front, a band where cold and warm air masses meet, the warm air pushes over the cold one. This sometimes leads to disturbances, mostly caused by the Coriolis force. If the warm air moves upwards faster at a specific point, then the system begins to rotate at this point. Subsequent cold air trying to equalize the low pressure is deflected and circulates counterclockwise around the low pressure area. At the same time, the low pressure area migrates, often from the American east coast, with the jet stream towards the northeast. The deeper the atmospheric pressure in the center of a storm drops and the greater the pressure gradient, the faster the winds become. This is easy to see from the densely packed isobars, each of which shows a pressure value in mb or hPa.

And the bigger the swell that makes its way to us. Low pressure areas often reach a size of several hundred kilometers. The wind direction is counterclockwise along the isobars and slightly inclined towards the low. Waves that arise grow with the wind and continue to travel radially in the direction of the wind.

  • The sun heats the atmosphere to different degrees
  • The air flow and the ocean currents try to compensate for this inequality
  • This creates low pressure areas
  • The stronger the wind and
  • the bigger the depth, the bigger the waves

The creation of waves

While we can now predict the probable occurrence of low pressure areas and their behavior very well with the help of computers, it is still not entirely clear how these show themselves to be responsible for the formation of waves. It seems clear that two processes are involved: First, the wind causes small irritations in the sea surface because it does not blow completely horizontally. These irritations are called capillary waves and their growth is linear with time.

When the sea is so rough, a second process comes into play. Larger turbulences adhere to the capillary waves, the turbulent eddies. They enlarge the waves now Gravity waves hot, exponential. This means that the waves grow faster in the second hour of the storm than in the first. However, gravity waves cannot grow indefinitely either. They are limited by gravity. As the wind blows over the waves, they grow until they reach their maximum size for that wind force. That's called a fully developed lake. When the waves leave the storm and their home behind, they are called traveling swell designated.

  • Waves occur when wind brings energy into the water
  • Surfable waves are created when a low is consistently above water
  • first capillary waves arise, then gravity waves
  • The greater the distance the wind blows and the strength of the wind, the bigger the waves will be
  • After the formation, the waves spread and are now on the way as a “traveling swell”

The journey of the waves

While all sorts of waves criss-cross in the center of the storm bottom, where the waves are formed, the journey of the waves becomes clearer once they have left their origin. On their journey, which can be several thousand kilometers long, the waves sort themselves into groups. The longer waves (i.e. those with the larger period) are faster and hit the spot first. And the waves get smaller the further they travel. Nevertheless, surprisingly little energy is lost on the way and what the waves lose in size, they often make up for with their energy and the special refraction of long-period waves.

  • The wave packets are sorted on the way: Fast waves arrive first
  • The wave packets also get wider and at the same time the wave size decreases
  • The number of waves per set tends to increase with the distance covered

When waves travel nothing more than information travels: namely that the water particles have to make a round up-and-down movement and pass it on. Just like when you lift a carpet in one flowing movement and then pull it downwards: A wave moves from one end of the carpet to the other without the carpet parts themselves moving. A floating object, a ship or a tree trunk, for example, would be lifted by the wave, shifted slightly backwards, lowered and then brought back to the front. This orbital motion of the wave is irrelevant in the open ocean, but is critical when the waves hit flat water.

Individual waves don't get very far. Waves therefore travel in groups. Every single wave travels at twice the speed of the group. The waves arise behind the group, run through them and disappear again when they have reached the front. There are many such wave packets in the center of a storm.

Because the wavelengths and thus the speed of waves are different, they sometimes overtake each other. If two wave troughs or mountains coincide, the two waves add up: One speaks of one constructive interference. Of course there is also the opposite of that destructive interference. The waves subtract because the wave crest meets the wave trough. In practice, this grouping of waves is chaotic, complex and difficult to understand because often more than two wave trains are involved.

The result is that the waves are sorted into groups of now larger waves. And it is possible that some sets on the beach are half the size of others. One then speaks of freaksets. In addition to the appearance of sets, there is again the effect of wind. If he blows against the waves, he can blow out the small and slow waves. Due to their shorter length, they are steeper out of the water and offer more surface to attack. A local offshore wind also has such a sorting function. This is especially important when surfing near the storm center.

  • Waves sort themselves into groups
  • Waves with a period of 15s travel around 750km per day
  • Short length / high frequency waves do not travel very far
  • Long period waves can travel thousands of kilometers

How waves break

Refraction

When the waves reach shallow water, they change their speed. The waves, which are now very wide, slow down where they meet an obstacle and otherwise continue to run at undiminished speed. That's why the waves for example wrap around a point break. In English the phenomenon is called Refraction. There are two main types of refraction: The Focus and the Defocus. When focusing, the flatter point that slows the shaft is exposed and in front of it. On both sides of the reef, the water is deeper and the waves are faster. It therefore bends towards the reef from both sides and a large part of the power is focused on this peak. That's what makes a swell magnet.

The opposite case is that the wave is braked on its flank and bends convex around the point. Here the energy is not suddenly chased out over a punctual reef, but runs evenly along the point. This process makes the waves less powerful and larger than when focusing. To do this, they sometimes break over hundreds of meters with constant force.

The third variant of refraction is the beach break. On long, open beaches there is usually a fairly regular pattern of sandbanks and channels. So the swell line bends like the Focus on the peaks, only at the same time in many places. Hossegor is known for these A-frame peaks.

If a bay is too narrow and not deep enough in the middle, then the wave defocuses on both sides, it never breaks and runs in a crescent shape to the beach where it suddenly collapses. Not very good for surfing! The otherwise beautiful bay of Torimbia in Asturias is such a case:

Interestingly, the refraction depends on the period. Swells with a longer period bend significantly more than short-period swells. This refraction of the long (and fast) waves is also an indicator of an incoming new swell. In general, that's a nice thing: On point breaks, the waves run longer, reef breaks are brought to life by the smallest swells, if only the period is long enough, and beach breaks usually benefit from the effect - admittedly, large periods are desirable for other reasons .

The depth of the water

While the waves propagate unhindered in deep water, they are slowed down and bent in shallow water. If the water depth falls below a critical value, the wave breaks. And this is how it works: We have seen that an object on the surface of the water is hit by a wave and lifted back into place in a circle (viewed in cross section). This circular motion is faster in its upper part because there is more water down to the sea floor. If the water becomes even shallower, the crest of the wave overtakes the retrograde movement and the wave breaks. A rule of thumb for the water depth at which this happens is 1.3 x wave height.

However, there are numerous factors that change this value and as varied as the underwater topography shows, waves break as variably. When the water depth changes very quickly, the waves swell by leaps and bounds and break in very shallow water. Offshore wind also keeps the waves from breaking. A third factor is - once again - the period. The longer it is, the farther apart the waves are (logically) and the flatter they are - until they suddenly pile up to their original size and break.

Waves break earlier (further out), though

  • the period is short
  • The underwater topography is flat
  • Onshore wind blows

Waves break later, though

  • The wind blows offshore
  • The water suddenly becomes shallow
  • The waves are long period

Of course, these factors also interact with each other. And the local wind has even more influence, namely either killing the short-period waves completely (offshore) or allowing even more of them to arise (onshore). So while the water depth at which waves break is quite difficult to generalize, one or the other can be said about the waves breaking in shallow or deep water. Since the speed of the waves in shallow water depends directly on the water depth, the crests of waves in very shallow water are suddenly much faster than their bottom. That's why they break hollow, steep and fast. Waves that break in deep water, on the other hand, are more or less equally fast and break everywhere mushy. For these reasons, long-period waves are not as susceptible to local winds, especially when they suddenly pop over a reef: they sneak into the shallow water and then break themselves, terrified and inevitable. It cannot be said often enough: knowing the local break peculiarities is the most important factor if you want to hit good waves.

In many cases the journey of the solar energy package is over at this point. However, if it is a beach break, not all of the energy is lost in the turbulence of the break. Part of it moves the sediment. Sandbanks are forming or being relocated. These Coastal sediment morphology is probably the least studied part of the wave journey. For surfers, however, it is essential whether a sandbar is parallel to the approaching waves, or slightly crooked.

In any case, some of the sand is trapped in a feedback loop and migrates under the wave. The sandbanks are created where the waves break. The water runs out again through channels and thus reinforces the structure. In some regions such as the Aquitaine these sandbar structures are quite stable. The reason for this are the constant swell patterns (WNW) and the Gulf Stream. But here, too, there is a difference, for example between the sandbanks in winter and those in summer. Because bigger waves break in winter, an upstream bank forms further out. While many beach breaks work well in summer for this very reason, there are others who prefer the winter benches. And we haven't even talked about the tides yet.

What can Papillon take with me now?

This text is intended to provide background knowledge, although it is pretty superficial. This makes it easier to classify how likely a certain forecast is: If you know that waves are formed by wind and then spend a maximum of 4-5 days on the Atlantic before they hit European coasts, you see that a wave forecast is very unreliable for two weeks from now: The low has yet to arise, then it is open how it behaves and only then can we determine the wave size, the period and the swell direction with some degree of determination. For this reason, some swells disappear completely from the forecasts.

This knowledge takes away some of your dependence on swell forecasts. Not that you know better now, or that you should calculate the occurrence of a swell yourself. But it explains, for example, the extraordinary importance of the wave period for surfing and also the seasonal swell probability differences.

Because the waves were already several thousand kilometers up to Papillon's Devil's Island, only the particularly long waves are left. And indeed: Long breaks in set are followed by several strong waves. This scenario is also common in winter in the Canary Islands, for example. And the well-traveled waves suddenly hit the exposed reef and rear up to their full size. A foot of swell can turn into head-high waves.

But what is a big period now, for example? Or the wave size? Why can it be that a perfect storm brings waves to my spot and the conditions are still bad? For the transfer of knowledge into practice, I have written another article, namely "How do you read a wave forecast?

Additional information

The standard work on wave science for surfers is the book Surf Science* by Tony Butt and Paul Russel.

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While some surf guides now have much more up-to-date spot information, the Stormrider Surfguide Europe* still the pillar of a good surf trip. In addition to the points discussed here, the tides and other surfing knowledge are also conveyed in a profound introduction. That the British are interested in the nature of things can already be seen from the publisher's name: Low Pressure

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A wealth of different surf spots and their conditions, mainly surfed at a time when forecasts were still handmade, describes Wiliam Finnegan in Barbarian days* - which earned him a Pulitzer Prize.

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