Tuesday, March 11, 2008

Ocean Science 101: How our surf is made (Part 2: Wave Creation and Swell Generation)

For surfing wind waves are the end-all be-all of the sport. Without wind waves there would be no surfing at all…so everything else like conditions, tides, beach shape, it is all just window dressing.

Generally wind waves have 3 different phases in their life-cycle; Generation, Swell, and breaking waves.

The first is generation, which occurs underneath a consistent area of winds going the same direction, called a Fetch.

Fetch can occur in a variety of situations and can be found in anything from a frontal storm, hurricane, trade winds, or even just consistent winds around an area of high-pressure. Fetch is measured in 5 ways.

1. Intensity – the strength of the winds in the fetch area

2. Length – how long is the fetch? This can affect the swell strength…think of the fetch-length as a rifle barrel…the longer a swell can stay in the “barrel” the more energy that the winds will be able to add to it.

3. Width – How wide is the fetch? The wider an area of fetch the broader the swell direction will be…this is particularly important for SoCal where a spot can get totally skunked by a swell that is just a few degrees outside of its swell window.

4. Duration – How long have the winds been blowing on a particular area of ocean? The more time they spend the bigger a swell can become.

5. Movement – Storms and Fetch are not stationary…they are in constant movement. Direction of movement is very important. The more that a swell moves “toward” a spot the more energy that it will impart to the swell. If the fetch is moving obliquely or away from a location that hampers the swell strength. Speed of movement is also a factor…if the fetch area moves to fast or to slow it allows the swell to move out of the area and away from its energy source.

Once an area of fetch has been established, (hopefully aimed at and moving toward your surf spot), and the winds are blowing over the surface of the water they start to impart energy into the ocean as the air molecules run up against the water molecules (I just really wanted to write molecules in this post somewhere). The friction between the air and the water is the process that moves energy from the atmosphere into the ocean.

At first, if the sea is calm, you will start to see small ripples forming as the friction from the air gains purchase. These are called capillary waves.

Capillary waves act like grip-tap on a skateboard…once they have formed they add a more exposed surface area to the wind allowing it to gain more purchase and add more energy to the waves (I bet you can see where this is going).

If the wind is strong enough these capillary waves continue to grow along with the space between the crests of the waves. Once the larger waves are formed this is sometimes called a sea-state (or at least sea-state refers to the conditions of the seas within the target area).

As the winds become more intense, or the duration of the wind becomes longer. These waves grow.

And grow…

Eventually if the winds are super intense you get huge waves at the storm core. (The boat in this picture is bummed).

Now an area of fetch doesn’t have to be world-ending intense to create a swell. You can have a swell with almost any amount of wind, but the more wind, the bigger the seas, and the larger the fetch. The larger the fetch means that more energy will make it into the swell, which in turn means that the swell will have a better chance of making it across a longer distance (eventually hitting your break).

Part 1: Overview – Types of Waves
Part 3: Characteristics of swells
Part 4: Breaking waves

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