Swell Spectra

Important notes

  • The peaks on the graph represent the individually sorted swell partition readouts (e.g. 5ft at 12 seconds 276º)
  • The shape of the graph is the most important tool for a surfer
  • The Y-axis is the "spectral wave density" which represents the distribution of wave energy across the different periods

Why use this graph?

Get your head around this bit and you'll start to avoid the forecasting pitfalls that 99% of surfers get stuck by!


How does it work?

The Swell Spectra graph represents the state of the ocean at a specific point.

It doesn't track individual waves/swells - it records how much energy there is in all the waves present.

To make this useful it breaks down how energy is distributed based on period and direction.


Example (excluding swell direction)


spectra no directino.jpeg

This simplified graph shows how much energy there is in waves at different periods in a swell - without the directional element.

This is an example of a "9-second swell". See that the peak of the curve is over 9 second period, but note that there's energy in this swell from 5 up to 12 seconds of period.

Sitting on your surfboard, most waves passing under you will be 9 seconds apart. These will be the most powerful waves

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.


Example of multiple swells in the water (excluding swell direction)

spectra no direction 2 swells.jpeg

Now we can see two peaks in the graph. One at about 12 seconds and another at around 17 seconds.

Sitting on your surfboard 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.


Including swell direction

The direction noted beneath the period is the mean direction of waves with that period.

The greyed-out number is the mean spread of that direction value. Higher number = waves of this period are coming from a wider range of directions.

Directional spread tells us how uniform or disorganised waves will be.

Screenshot 2023-11-10 at 15.48.08.png


Examples (including swell direction)

Direct your attention to the shape of the graph. This tells us information beyond the average/overview.


1. Concentrated spectrum of 2.9ft at 16s WNW.

Screenshot 2023-11-10 at 15.17.00.png

  • Sharp/concerntrated shape suggests energy concerntrated across a small range of period & direction
  • Surf likely from a faraway source
  • Long wait time between sets
  • More waves per set 
  • Organised waves with uniform shape/size


2. Wider spectrum of 5ft at 16s WNW.

Screenshot 2023-11-10 at 15.16.39.png

  • More distributed energy across period and direction
  • The shape surrounding the peak is still quite sharp - expect a portion of the swell to have the characteristics ddescribed under the Concentrated spectrum above.
  • Either side of the more-concentrated peak still has a fair share of the energy, so low period waves from the W will be impacting surf quality as well as high period waves which will break in different depths from different direction
  • Surf will feel more disorganised and mixed, less uniform
  • If there was a bigger bump around the 7s to 9s period, I'd be looking to surf somewhere sheltered from the W/WSW


3. "Missed swell"


Screenshot 2023-11-10 at 15.17.20.png

  • Now you understand how average/overview readouts are determined, you will start to appreciate the difficulty in individually sorting "swells".
  • Here, the wave model didn't identify the 6-7 second peak as a swell
  • With your non-computer eyes, you can identify there will be a lot of low period waves creating messy conditions from the W. 
  • A small change in the energy at seven second mark would cause a huge change the swell average/overview readout


How can I translate this graph of energy into wave height?

How does this information about energy in m²s translate to surf size/quality?

Using information regarding the available evergy in the water we can calculate the average heght of larger waves.

Information about height of larger waves is more useful to boaters than surfers. It will accurately tell you how much your boat will go up and down out to sea where waves can hit you from all directions.


So how can I use this as a surfer?

When we say "5ft at 9 seconds from the west", we mean that the average height of the largest third of waves is 5ft, and the most energetic waves are around 13 seconds apart. 

The peaks on the spectra graph are the averages/overview we're calling out here.

Swell Spectra shows you not just the averages but where all the energy is distributed in a swell.


Imagine you're surfing a spot on the West coast of an island that works best with south swell.

The average/overview swell readout is "5ft at 9 seconds from the east" so I presume there will be no surf at my spot.

A smaller swell from the south would be missed with the average/overview.

Look at the Spectra graph - if you see some energy coming from the South you know there will be good waves on the beach.



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