Site Info :: General Information on Surf Forecasting

Wave Generation

The oceans of the Earth are in a constant state of interaction with the atmosphere. The atmosphere itself is in a constant state of fluctuation as well, both vertically and horizontally. This causes the atmosphere to arrange itself into a series of high and low pressure. Air tends to pile up in certain places creating large domes of high pressure, while air is rising in others creating areas of low pressure.

Air will travel from high pressure to low pressure. This mass transport of air is how wind is created. There is a lot more to it than a mass of air moving from one location straight over to another. We have to take into account other forces as well such as the role the Earth’s rotation plays. The force the Earth’s rotation is called the Coriolis force. The Coriolis force is what is known as a fictitious force. It is not a real force in the sense of an actual pushing or pulling on a mass of air, but more of an imaginary force or result created by the rotating coordinate system of the planet. To keep things basic and make long story short the Coriolis force is one of the key elements that cause wind to travel clockwise around areas of high pressure and counter clockwise around areas of low pressure. Of course, in the southern hemisphere, this is just the opposite. A good way to think about it would be to picture yourself sitting in the center of a rotating, circular platform with a tennis ball in your hand. If you were to release the ball it would be taking a curved path with respect to the platform. This is basically the same effect the Coriolis force has on the wind.

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So what does this have to do with waves? Quite a bit. Waves are created by wind, which bring us to the three major ingredients that go into creating waves. Wind creates waves by dragging across the water. This frictional interaction is how the atmosphere uses wind to transfer its energy to the ocean. Wave size will be dependent on wind speed, duration and the fetch area. Wind speed ispretty self- explanatory. That is simply how fast the wind is blowing at the surface. The duration is how long the wind blows and fetch is the area these winds of common direction are covering. All three of these elements are directly proportional to the wave sizes. The stronger the winds, the longer they blow and the greater the ocean surface they cover, the larger the waves they will generate. These waves travel outward from the system tangentially. This simply means that even though the system is rotating, swells travel outward in straight lines from the direction of the winds that created them.

Swells travel outward from a system in straight lines. On a map, the curvature of the Earth must be taken into account. The great circle path is basically the shortest distance between two points on the surface of a sphere. A good way to get an idea of common great circle routes is to take a string and connect it to different locations on an actual globe. You’ll be surprised at some of the swell routes that show up. Here in Southern California we pull in swell from many directions. Theoretically some swells can make it from the Indian Ocean all the way to our beaches here in Southern California. Swells too, like the wind, are called according to the direction they are coming from, not going toward.

Swell Periods

The swell period is measured by the time it takes for two successive swell or wave crests to pass a stationary point. Swell periods are very important when it comes to surfing. Periods let us know quite a bit. The more intense the storm, the more energy will be transferred to the ocean, creating larger waves. The larger the waves become the longer the periods will become as well. Generally, the stronger the wind blows and larger the seas, the longer the periods will become once the swell leaves the generation area.

Once waves leave the generation they like to cooperate. Its almost if they know they have a long journey to your beach where they want to break with maximum energy and size. So, waves organize in to what are called wave trains. These waves band together based on qualities such as speed and momentum. Wave trains allow the swells to conserve energy as they travel great distances across the ocean. Picture a wave train consisting of 6 waves. The first wave of the train will move into the last spot as the wave following it moves into the first spot. This is a repetitive process for the duration of the wave train’s life.

This brings us to the subject of wave speeds. The speed of a given wave train and individual wave within that wave train are related directly to the period. What it boils down to is, the longer the period, the faster the speed of the wave train. The speed of a swell is approximately one and half times the period. So, waves with a 10 second period moves about 15 knots and waves with a 20 second period moves at 30 knots. And, on top of that, don’t forget that individual waves within a wave train must move even faster. Individual waves will be moving at twice the speed of the entire wave train or 3 times the period. So a wave train moving at 15 knots will have individual wave speeds at 30 knots and the wave train moving at 30 knots will have individual wave speeds up to 60 knots!

Now that you know more about wave periods and the role they play in wave speed, I’d like to tell you about forerunners. And to all you smart asses out there no…it’s not the SUV made by Toyota either. You may have head this term before. Or you may have heard people refer to them as “the leading edge of the swell”, “that early longer period stuff” or maybe it was difficult to hear any term as the people around curse the surf forecasters for claiming a swell is going to arrive and it looks like it didn’t. That’s not exactly the most unreasonable response, because what forerunners are is the fastest moving swell with periods exceeding 18 or 19 seconds. This swell will be first to arrive, and unless the bottom is just right it can be almost undetectable with the actual breaking wave at a half a foot or less sometimes. Now I just mentioned something about how the bottom affects an incoming swell. This will take us on to the next subject.

Wave Decay

There are probably just as many factors, or maybe even more, that go into wave decay as wave generation. Just as strong winds create waves, they can lead to its decay as well. Waves with shorter periods are much more vulnerable to the damping effects of opposing winds. This is mainly because the energy associated with wave periods less than 12 seconds does not run as deep. Wind waves are much steeper which basically gives wind a greater amount of surface to act upon. (Conversely, let it also be noted that the opposite can happen if strong winds develop aimed in the same direction of the pre-existing waves. Those winds will be able to increase the current waves more quickly than if it had to start from scratch with a flat ocean). Longer period swell is more protected from the wind. The bulk of the energy is located beneath surface and the actual swells are not nearly as steep. This protects them from the adverse effects of the wind. Waves with periods of 12 seconds or less are generally referred to as short period wind waves. Longer period swell of 12 or 13 seconds or greater, which consists of the more organized, cleaned up energy, is called groundswell.

Now let’s start moving our way into the subject of how the ocean floor affects different types of swells. Like we stated earlier, longer period swells have quite a bit more energy beneath the surface. This makes those longer period swells are more susceptible to the frictional influences of the ocean floor.

Swells with periods of 20 seconds or more, will begin to be influenced by the bottom at a depth well over 1000 feet. Swells with periods of 18 second start feeling the sea floor around 830 feet, 15 seconds at 576 feet, 14 seconds around 500 feet and 12 seconds at approximately 370 feet or so. Swells with periods less than 10 or 11 seconds don’t really feel the bottom until they get into water shallower than 250 feet. As swells approach a coastline they begin to slow down. This process is referred to commonly as shoaling. As swell continues on into shallower and shallower water, it begins to slow down and get steeper as the underlying energy begins to get pushed upwards. Theirs is a common misconception that was started back in the 80’s by the movie North Shore. I’m sure many of you have seen it. Remember? Rick Cane, a teenage kid from Arizona goes from barney to Pipe master in something like 2 months. There is one point where Chandler tells Rick that a wave will start to break in water that is half as deep as the wave is tall. Of course this has some accuracy to it, especially for very hollow waves, but a wave will actually start to break in water deeper than the wave is tall…approximately ~1.3 times the wave height. A 10 footer will break in about 13 feet, a 15 foot wave in about 19 to 20 feet of water and so on. A gradual sloping beach will cause a wave to break softly and crumble, while a more abrupt change in depth like a reef or shorebreak will cause a wave to throw. This happens for a pretty simple reason. A waves forward speed is directly proportional to the water depth. Once a wave reaches the beach or reef, the bottom of the wave slows while the top of the wave maintains a faster speed causing it to break. It might be a crude analogy, but think what would happen if you were sitting on the roof of a car doing 50mph and somebody put the breaks on…you’d go flying. That is essentially what happens to a wave. As you can probably imagine, there are a countless number shapes to a breaking wave.

As surfers, we tend to look for long period swell. Don’t get me wrong, there are countless days of great short period windswell. Shorter period windswell will be a lot closer in proximity to its generation area, so, most of the time you will have some wind to go along with it. That broken up shorter period windswell tends to like sand bottom beach breaks where it turns into a fun, peaky, skate-park style playground of surf if conditions are favorable. It is still those long period swells that turn reefs, point breaks and select beaches into mechanical, barreling perfection. The energy of a long period swell allows the topography of the ocean’s bottom, or bathymetry, to have a greater effect on the shape of a breaking wave.

Bathymetry does more than make a wave break. Just a second ago I mentioned how swells wrap into point breaks or bend around reefs. This happens for a reason. As waves travel they tend to focus their energy toward shallower water. This causes part of the wave to slow down and begin to change direction. This is known as refraction. Longer period waves will refract more due to their deeper running energy. Long period waves will also suffer more from island blockage.

The topic of island blockage is an issue some places more than others. Again, using Southern California as an example, we deal with island blockage everyday. The shape, angles and proximity to coast of the islands also come into play, but for we’ll keep things simple. The incoming swells will wrap around these islands. These islands can change the swell direction slightly. Depending on certain other factors that can refract a swell, on average about 5 or 6 degrees. So say we have a swell coming in from 270. Once that swell passes San Clemente Island, it will cause the swell to change direction slightly, reducing the shadowing effect it has on Southern Orange County. And that shadowing effect will be greater for bigger islands that are closer to the shore.

I think it is also important to discuss diffraction. This is a lot like refraction. Actually it applies more to waves passing through an opening. Think of shining a flashlight through a whole in the wall into a dark room. That light spreads out and lights up most of the room, not just a single spot on the opposite wall. Ocean waves behave this way too. Here’s a good example. Some of you may remember a large Category 5 hurricane in the Caribbean Sea back in 1998. It parked itself off the northern coast of Honduras where it killed thousands of people. Well, sustained winds were clocked at over 150mph at the center. Needless to say that generates some huge swell. Just to the north of the hurricane lies the Yucatan Channel. That is the small pass, or opening, between the Yucatan Peninsula and Cuba. There is a point to this, I promise. Well, that hurricane sent a huge swell straight north through that channel where it hit Alabama’s barrier island at double overhead+. But, Alabama was not the only place to get waves. Of course they saw the biggest surf, but there was rideable swell from Florida through most of Texas. That was due to diffraction and how it spread that swell through most of the Gulf of Mexico.

One more thing I’d like to touch on that often helps surf put on a great show is reflection. This is another term in which I have heard people use several other expressions to describe. I’ve heard “bounce”, “wedge effect”, and even “reverb.” Reverb? Wasn’t that a show on M-TV or VH-1 a long time ago? Either way they are all referring to the same thing. This is common at breaks like the Wedge in Newport Beach, CA and Sebastian Inlet in Melbourne, FL. It happens many places, although, these are just some of the more popular locations. As an incoming wave hits a jetty at an angle the wave is reflected back off usually a bit smaller due to energy loss. As the reflected portion of the swell comes back off the jetty combines with the incoming wave forming those classic peaks. Due to a phenomenon called superposition, it can cause the peak to double or triple in size. This is also called constructive interference. Theoretically constructive interference will lead to a doubling of the amplitude (in our case this is the wave height) of the individual waves. So an incoming wave of 6 foot, combined with reflection of 4 feet and the right bathymetry can cause a wave to jump to 10 or 12 feet (or higher) especially at places like The Wedge.