Surfing Explained: Ep14 How an Ocean Wave Forms

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How does the wind form a wave far out at sea?

Continuing on our Surf Forecasting journey, in this episode we go through the interesting science behind how a wave forms out in open ocean, before arriving at shore for us to surf on.

Ep 14: How Waves are Created

Waves are created by wind blowing over the surface of the ocean. Initially, these small waves are known as Capillary Waves. It is not fully understood how this formation occurs as the surface tension of water makes it difficult observe at a molecular level.

There are 2 methods by which wind energy is transferred from the air to the water.

First, small changes in air pressure created by the wind will either push down on the ocean surface during high air pressure, or lift up on the ocean surface during low air pressure.

Second, when the wind blows directly across the ocean surface the shear force between the stationary and water molecules and the moving air molecules cause the incompressible water molecules to move into each other in the same direction as the wind.

Water molecules’ strong attraction to each other creates a surface tension that reduces the energy within the capillary waves, which are often just a few millimeters high. This causes the low energy capillary waves to decay soon after the wind stops.

If the wind continues though, capillary waves can grow larger into Gravity waves, where they then have enough energy to over come the surf tension, and the controlling force then becomes gravity, attempting to pull the now larger waves downwards.

As wind blows over the wave shape from behind, air is directed upwards, also pulling the water and wave upwards with it. Low pressure and eddies in the wind on the downwind wave face then also draws the leeward side up.

For waves to absorb enough energy to reach shore will depend on three factors during this process.

The wind speed, the duration of time the wind blows, and the area over which the wind blows (known as the fetch).

In order to create the biggest waves we need a strong wind to blow over a large area for a long period of time. An average hurricane is around 500km in diameter with wind speeds of well over 200kph that can blow for many days.

A theoretical wave height maximum does exist, which depends on the wind speed, duration and distance over which it was blown. When a wave reaches its wave height maximum it will topple over into what’s called a white cap.

Wave height increases faster than the wave period increases, so white capping can occur quite quickly during a storm. If a wave reaches its maximum, energy can still be added but it will do so by increasing the wave energy below the surface, and thus increasing the wave period. And as the wave period increases, so does the maximum wave height before the wave topples into a white cap.

As wave period increases, so does the speed at which the wave travels. Once the speed of the wave is faster than the wind speed, no more energy will be added to the wave and it will travel out away from the fetch.

Waves within the fetch area are called wind swell. This area is often bumpy and irregular as wind gusts and changes direction creating a mix of small and larger waves traveling over each other in all directions.

Once the waves travel out of the fetch area, they are known as ground swell. Ground swell waves can have enough energy to cross an entire ocean, which leads us to next weeks episode. Thanks for watching, and we’ll see you then.








Waves are created by wind blowing over the surface of the ocean
The initial formation of tiny Capillary Waves or "Cats Paws" on a totally smooth body of water is not fully understood, as the surface tension of water makes it hard to observe at the molecular level, and the ocean is never totally flat.
There are 2 methods by which wind energy is transferred from the air to the water
Small changes in air pressure created by the wind either push down or lift up on the surface of the water
Shear forces between the stationary water molecules and the moving air molecules drag the water molecules up and over their downwind neighbours (important here is that water is incompressible) 

Capillary waves are controlled by surface tension of the water, and die quickly if the wind stops
Capillary waves have a maximum wavelength of 17mm and are normally only a few millimetres high
Capillary waves can grow into larger waves called Gravity Waves where they overcome the surface tension of the water and the controlling force becomes gravity, pulling them back down.
As the wind is deflected around the shape of the wave, there will be an upward flow of air on the windward side which pulls the water upwards. (This is a continuation of the Shear force from before) (see photo below)

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