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Topic Last Updated on 09-07-2024
Where Do the Ocean’s Waves Come From?
Post on topic: The Physics of Surfing.
Waves are born far away from the coast, in the open ocean where storms and strong winds blow. Wind travels from high-pressure zones to low-pressure zones. In the ocean, these areas are separated by many miles, so the wind blows over a very large area. Where the air touches the surface of the water, the atmospheric molecules make contact with the liquid molecules and transmit some of the wind energy to them.
At first, they create only a tiny disturbance— capillary waves that can be calmed by the surface tension of the water alone. But if the wind continues to blow, the capillary waves work like sails, collecting more and more energy and increasing in size. At a certain point, they become so large that the perturbation resists not the tension of the water surface but gravity itself; such waves are called gravitational. In a storm zone, the ocean is like a boiling soup in which gravitational waves of different sizes move randomly.
Understanding Energy Accumulation and Direction
The energy that the water receives does not stand still but moves in the same direction as the wind blows. Additionally, the energy builds up. Its final value in the ocean depends on three factors: the stronger the wind, the larger the area over which it blows, and the longer this effect lasts, the more energy the water will receive as a result. For example, if the storm itself moves in the same direction as the wind, it follows the movement of energy in the water, fueling and increasing it. When the storm subsides or turns, the energy of the ocean continues its movement in the specified direction, and this is when a swell begins to form.
Surfing | Understanding the Mechanics of Swell
A swell is a series of mechanical waves on the surface of the water in the absence of direct environmental impact. That is, the wind no longer blows, but the waves continue to move forward. It is important to understand that the water itself does not move anywhere — only the energy accumulated during the storm moves. This is due to the collision of molecules, following the same principle as in a game of billiards.
When a molecule hits a neighboring one, it changes its trajectory, collides with the next one, and so on. As a result, the molecules move in a circle, and the water surface creates the visual effect of waves moving forward. The energy is transmitted not only forward, but also downward, partially dissipating at the same time. The depth of penetration of the disturbance is calculated at half of the wavelength. At deeper points, the ocean is relatively calm.
The Dynamics of Wave Interaction
During their journey, waves formed under the influence of storms interact with each other: they compound and grow larger; others, on the contrary, are mutually destroyed. Moreover, waves of different lengths move at different speeds: some begin to lag behind while others run ahead. Due to the huge size of the ocean, waves travel a long way and are consequently “filtered out,” and what is called groundswell — a series of smooth waves of approximately the same size — reaches the shore. In addition, the waves usually travel in groups of three to nine called sets, with a small interval of calm between them. If the distance from the storm to the shore isn’t very large and waves of different sizes don’t have time to separate from each other, this is called wind swell. In this case, the sets are not so pronounced. More often, waves come one after another without interruption.
How Swell Becomes Waves for Surfing
Not every swell is destined to become a surfing wave— at least, not everywhere. In order to ride a wave, it has to have a certain shape. Wave formation for surfing depends on the structure of the bottom of the coastal zone.
How Waves Appear
Since the ocean is very deep, and the energy from the swell moves only in a small layer of thickness at the surface, it reaches the shore without hindrance. Reaching shallow water, the lower molecules begin to touch the bottom and slow down first before being forced up. The wave length shortens, and the crest grows, meaning the wave grows higher. Where the depth, or rather shallowness, reaches a critical value, the grown ridge overtakes the base of the wave, and in the absence of support, collapses on itself. The wave is destroyed. The place where this happens is called a break; that’s where the surfers sit, waiting for the right wave. Depending on the structure of the bottom, the wave may collapse immediately along its entire length, or it may close gradually, creating a wall along which the surfer rides.
The shape of the wave directly depends on the shape of the bottom: the steeper the shallows, the more sharply the wave grows. Usually, the sharpest and most powerful waves are born where the differences in height change almost instantaneously, for example, if there is a huge rock at the bottom or a reef plateau begins. Where the drop is gradual, for example, on beaches with a sandy bottom, the waves are more gentle and close more slowly. In this way, to form good waves for surfing, you need a combination of many factors: a storm passes off in the distance, a good swell blows in the right direction, there aren’t any obstacles in the way, and the bottom of the shore has a certain shape.
