Before we go any further, some definitions

Like pot holes, the first “perturbation” is needed to set the above scenario up.  In this case, it is a little eddy that cause the wind to push down on the water surface.  One started, the horizontal wind takes over and pulls the water up into a wave.

As the wind speed increases, the force of the wind on the water increases.  The increase of force imparts more energy to the water increasing the wave height and speed.

The waves will keep growing larger if the wind keeps blowing at a constant speed.  As the waves grow, their speed increases.  Eventually, the wave speed matches the wind speed and the waves can grow no further.  This is called a fully arisen sea.

Looking at it from an aerial view, … as the wind continues to flow over the water, the wave grows larger and larger.  But there are also new small waves being formed next the the larger ones.  Some of these smaller waves  radiate out of the path of the wind and grow no more. This is what causes the water to look chaotic during windy days.  The thick crescent on the right represents a wave that has continued to grow throughout the windy region. 

What happens when the wind stops?  Well, just as the waves created by a tossed pebble continue to travel from their origin, so do the waves generated by wind.   Once they leave the fetch, they are called swell.

These waves were generated far away, traveled to this coast, and now end as crashing surf

Let’s focus on wind waves…

If wind speed doubles, the wave height quadruples.  Fortunately, there are other limiting factors such that this does not occur without limit.

Fetch is an area of roughly similar winds (direction and speed) in which wind waves are generated.  We are mainly concerned with length but width can also impact the size of waves generated because of wave-to-wave energy exchange along their crests.

Duration is also important and applies to a particular fetch of similar winds. 

With fully developed seas, the wind and waves are traveling at the same speed.

This complicates our simple view: The wind may not stop completely at the end of the fetch, it may only weaken or change direction. As we end the section on wind waves, therefore, we see that we can have waves that seem like wind waves because they exist in a windy region, but they are actually swell that are traveling faster than the wind and therefore are not really affected by the wind.  They may, if fact,  already be taking on swell characteristics.

Conversely, the wind can be completely dead immediately adjacent to a very windy region.  The swells that exist in this region can be very steep, like wind waves, but they are technically swell because they are not under the influence of wind.   They are called “fresh swell”

If the wind can’t keep up with the wave, it can’t exert force on it!   This is also true if the wind is not blowing in the same direction that the wave is traveling.

And now for Swell.

The first three bullets basically say that the wave wants to flatten out in all directions

Gravity is pulling the wave down, and pushing the front forward, and the back towards the back.

As the swell matures, the height decreases which reduces the further tendency to flatten.  Is this valuable to know?  Yes – see the next image.

The first wave is very steep and was generated just beyond the buoy.  It has much flattening to do and will likely hit the shore as a 6 foot by 11 second wave. The second wave has already done all its flattening out and was likely generated may miles to the west of the buoy.  It will hit the coast with very little change to its height and period.

Not very intuitive.  You would expect that when the force on the wave (the wind) ends, that it would slow down. Consider the analogy on the next slide.

Just for fun.  Hot surf spot, long period swell.

We’re not going to explore further this idea of individual versus group wave speed.  Please except it on faith, or next time you can look at a wake, notice that waves seem to appear behind the wake, move through it and disappear in front of it.  You are seeing the individual waves moving at their wave speed but the group is moving half that speed.  Weird indeed.

Just for fun…

When at sea, you can feel the effect of several groups of waves from different directions, and of different height/periods.  This is the real, complex world, but it can still be viewed through the simple concepts we have developed so far – with a few more that we’ll address under “describing sea state”

The top plot corresponds to the previous image – lots of different waves of different periods.

When talking about wave height, you can’t describe every single wave.  Instead, statistics are used.  You can talk about the significant wave height of all waves, or just waves, or just swells. In fact, whenever a number is give for a wave height, it is likely a significant wave height.

Steepness not only capsizes boats, but can knock crew in the water because of the sharp motion.

The square rule only applies to a small range of wave heights, up to about 11feet‘ by 11 seconds.  A 20 foot by 20 second wave is not steep, for example.  The problem is that wave length is equal to the period squared.  As the period increases, the wavelength increases much faster.

The National Data Buoy Center’s webpage has plots of energy versus period from each buoy.  This shows pretty clearly how many different wave trains are present at the buoy.  The example on the left will be a rather confused sea due to the interaction of the two dominant waves while the example on the right will be quite regular.

Strong wind indicates there may be a wind wave that is not showing up because its energy is slightly lower than that of the swell.