"Swell" is a group of waves that travel across the ocean, created & energized by wind blowing on the back of the waves. “A swell” suggests a group of waves with a distinguishable height, period and direction, more than likely coming from a unique source (such as a specific hurricane, storm, or even a local wind system).
All different swells arriving at a spot, having travelled across the ocean, are laid underneath the surf height in the swell analysis graph.
Comparing the surf height graph with the swell graph, you can start to understand the relationship between swell height/period/direction and surf height at your local spots.
As swells arrive at the beach, shallow water forces waves to slow down and rise up above the surface, morphing as it goes through a process known as "wave shoaling".
The leading trough (bottom) of the wave starts to drag against the sea floor, while the crest (top) of the wave continues at a faster pace. This eventually causes the wave to "trip over" itself, which us surfers recognise as a breaking wave.
How swell influences surf (video example)
Using the swell graph on spot pages, and our global charts that are just 1 click away, we can identify and track swells that are influencing surf height at our spot of interest.
In the video, we identify a WNW swell arriving at a west-facing spot. We then use the wave period charts to see the journey this swell is expected to travel, and the area of influence. Keeping an eye on the wider context as well as spot-level data can help you make sense of the changes that happen to the forecast as our model updates.
Surf is swell that has arrived in shallow enough water to rise up above the surface, and break. You can imagine that the lefthand side of the below diagram represents swell, and the righthand side represents surf.
A single wave in a swell has two dimensions: height and length. Period and height provide us with both the X and Y measurements of the wave, respectively. Measuring length in seconds might seem strange, but measuring a wave’s length in meters would be extremely difficult in practice. So, to get an idea of the wavelength of a wave, you simply measure the time it takes for a buoy to get "bobbed" up and down, in seconds — the time it takes for 2 successive crests/troughs to pass a fixed point.
Swells; a deeper dive
That height and length we mentioned will give us an idea of the "volume" of water the wave is moving.
If you imagine a wave with a long wavelength but small size (e.g. 2ft @ 20 seconds), you might find that — sitting on a boat — the up/down movement feels minimal. But, as the volume of water is quite large (due to the high period of 20 seconds); when this swell arrives at shallow water, it morphs in a way to produce large surf.
Likewise, you might have a swell with a height of 5ft and a period of 6 seconds... This would be quite typical of "stormy", or "messy" seas. On a boat, you will feel a lot of up-and-down movement, maybe even to the point of throwing up! The swell size might be large; but due to the small period of the wave, the "volume" of water that will be forced up above the surface as it arrives at the beach won't be so significant, and therefore will not necessarily result in "large" surf.