This is the most complex and also the most important part of understanding how swell models work. Get your head around this bit and you'll start to avoid the forecasting pitfalls that 99% of surfers get stuck by.
The swell model represents the state of the ocean in a particular grid square. It doesn't track individual waves, or even individual swells instead it simply records how much energy there is in all the waves present. To make this useful it breaks down how the energy is distributed based on period and direction, but just as the model divides the world into a grid to make managing the maths simple the model separates the periods and directions into some simple chunks. For example a model might separate all energy from the North West and break it into periods of 1-5 seconds, 6-10 seconds, 11-15 seconds and 16-20 seconds.
It's much easier to understand this graphically – here's a chart of all the energy in the sea, irrespective of direction:
This graph shows how much energy there is in waves at different periods in a swell. This is a nice simple example – if you trace down from the peak at the top of the curve you'll see that the period here is 9 seconds. So we'll call this a '9 second period swell'. But you can see that the curve shows there's energy in this swell all the way from 5 seconds of period to 12 seconds of period. So now imagine sitting in the line up on your surfboard. Most waves that pass under you will be around 9 seconds apart. These will be the most powerful waves, although with low energy and that low period they'll be weak and unlikely to be great for surfing. You'll see some waves 7 seconds apart and occasionally some waves 12 seconds apart. You won't see any waves at all 20 seconds apart. This graph represents a fairly 'pure' simple 9 second swell. So lets complicate things by looking at another example:
You can see here two peaks in the graph. One at about 12 seconds and another at around 17 seconds. In this instance we can say there are two separate 'swells' running. Sitting on your board you'll have powerful 17 second period waves passing under you creating potential for great surf, but also 12 second period waves and about an equal mix of both and these waves will combine as they come into shallow water making bigger waves still.
So the crucial issue here is how does this information about energy become the forecast you see on a surf website? Most simply some maths is used that looks at all the available energy and works out the likely average height of the larger waves. This is only of so much use to us as surfers. Imagine you're ten miles from the coast sitting in a boat, waves are approaching from all directions and this forecast is reasonably accurately telling you how far the boat is bobbing up and down as these waves combine. In fact if you appreciate that most swell models surfers use are designed originally for navigation for commercial and military shipping you'll appreciate why this might have been a key concern, for us as surfers the need is much more specific and subtle.
Lets imagine you surf a point break that blocks any swells from the west, but really works nicely when the swell is coming from the south. The best you get from this overview of all energy is an average direction. Let's imagine most of the waves are coming from the west, the average direction is west and so you stay home. But in fact there might have been a smaller swell heading from the south and good waves on the beach. You just can't tell.
To provide a more detailed level of information the swell model can output a partitioned version of this information. For example it's mathematically quite simple to ask the model for the significant height of all likely waves from the North west. Or the significant height of all likely waves less than 10 seconds period. It's also quite normal to ask the model to split out anything it can see as a clearly separate swell – so the model might return the significant height of all likely waves from the most powerful swell.
The model we use on MSW offers us four different interpretations – the overall height, period and direction of all swells, the 'primary' (most powerful) swell, the 'secondary' (second most powerful) swell and 'wind waves'. For wind waves the model looks at the total height and period of all waves that are still being blown by the wind that created them. It's absolutely crucial that you appreciate that local factors may mean that all swells in combination might not reach the beach and that the overall significant height might not be relevant for your surf forecast.