Advanced Swell - Swell Spectra

  • Updated

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
  • This feature is only available to our Premium and Forecast members
  • iOS app - Available on app version 10.11.0 or above
  • Android app - Available on app version 7.1.0 or above

Where can I find the Swell Spectra graphs?

On the website, head to any spot page, and scroll down the Reports & Forecast tab to view the Swell Spectra graph:

Screenshot 2024-04-24 at 12.35.52 PM.png

In the app, head to the Live or Forecast tab from any spot page. Then scroll down the page and select the Advanced Forecast link:


* Note: If you are looking for the Advanced Swell link while on the Forecast tab, you will need to have your forecast view set to the traditional forecast view, and not the table view.


3. After selecting the Advanced Forecast link, click on the Swell Spectra tab at the top of the proceeding page:



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)


Screenshot 2024-04-04 at 16.42.52.png

This simplified graph shows how much energy is distributed across different periods in a swell at a given moment. We will consider the directional element later. 

This is an example of a "12-second swell". See that the peak of the curve is over 12 second period, but note that there's energy in this swell from 5 up to 18 seconds of period (most energy is in the higher period). 

Sitting on your surfboard, most waves passing under you will be 12 seconds apart. These will be the most powerful waves. 

You'll see some waves 7 seconds apart and occasionally some waves 17 seconds apart. You won't see any waves at all 20 seconds apart.


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

Screenshot 2024-04-04 at 16.55.22.png

Now we can see two peaks in the graph. One at about 7 seconds, and another at around 12 seconds, but there is energy distributed all the way from 4 to 16 seconds. 

Sitting on your surfboard the 12 second period waves passing under you could potentially be great small wave longboard conditions, but an equal mix of energy in the lower periods could deteriorate surf quality as both swells will combine as they come into shallow water making potentially messy conditions. 


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/concentrated shape suggests energy concentrated across a small range of period & direction
  • Surf likely from a faraway source
  • Long wait time between sets
  • More waves per set 
  • Organized 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 described 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 disorganized 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 energy 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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