In short, swell travels across the ocean, reaches shallow water near the coast causing it to rise up out of the water - producing surf (the waves we see breaking at the beach).
What is swell?
"Swell" is a group of waves that travel across the ocean, created & energized by wind blowing over the surface, pushing 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).
Check out Surfline's charts to watch swells travelling around the world. Use the Wave Height and Wave Period map layers to identify groups of waves. Use the Wind layer to see areas in which waves will be generated or energised under strong winds.
|1. Two swells in the Southern Pacific are visible using the Wave Period map layer. The blue and pink boxes show 2 different swells that have been created in different places, with unique directions of travel.|
|2. Two swells in the Northern Atlantic are visible using the Wave Height map layer. Both "blobs" have similar height and directions of travel, but have formed 2 distinct and separate "blobs".|
|3. On the Wind map layer, the 2 swells above look to have been created by the same wind system which explains the similar height/direction of travel, but 2 distinct groups/trains will arrive one after the other in Western Europe.|
What is surf?
As swell arrives at the beach, shallow water forces waves to slow down and rise above the surface, morphing through a process known as "wave shoaling". Check the diagram below.
- The leading trough (bottom) of the wave starts to drag against the sea floor
- The crest (top) of the wave continues at a faster pace
- The top of the wave moving faster than the bottom eventually causes the wave to "trip over", which is what surfers are looking for.
Relationship between swell and surf
Surfline gives you a surf height which has been calculated based on 3 things:
- The orientation of the surf spot. A huge swell travelling parallel to the surf spot won't produce huge surf. A moderate swell travelling directly towards the surf spot will produce significant surf.
- The height, period and direction of the swell. Surf height is calculated based on these 3 metrics.
- The combination of swell predicted to be arriving/passing the surf spot. Swells interact with one another. A range of wave height is given based on how these 2 swells could interact with one another.
The swells contributing to surf height show as you hover your mouse over the surf height graph. Many prefer to figure out surf size
If you scroll down the page, you'll find Surfline's swell graph, where individual swells are plotted by height over time. Arrow size represents period. In the example below, we see a significant bump in surf height over the weekend at Supertubos, Portugal.
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.
- Swell period peaks at the very beginning of the swell's arrival. 00:00 am Saturday, the swell at 03:00 am on Saturday is 0.3ft @ 24 seconds. This small height long period characteristic is typical of the arrival of any swell. See how the arrows on the line get smaller through the weekend and peak period gradually drops.
- Swell height peaks later, at midday on Saturday.
How Swell Influences Surf
No two swells are identical. Even if numbers look the same, the conditions under which they were generated will be unique. To visualise the unique nature of a swell, use the Swell Spectra.
- Identify a bump in surf height at your local surf spot
- Check the swell graph to see how the swell impacts your spot
- Check the charts to identify the swell as it travels towards your local surf spot, tracking its characteristics and approach
Following on from the example above (Supertubos November 4th/5th), I open the charts looking at the North Atlantic in the lead up to the arrival of this swell.
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
Small height long period( e.g. 2ft @ 20 seconds)
That height and length we mentioned give us an idea of the "volume" of water the wave is moving.
Sitting on a boat out at sea, you would barely notice a wave with a long wavelength but small height (e.g. 2ft @ 20 seconds) passing underneath. The up/down movement sitting on a boat would be minimal.
But the volume of water in that passing wave is quite large (due to the high period of 20 seconds). When this swell meets shallow water, it will likely produce large surf.
Significant height but short period (e.g. 5ft @ 6 seconds)
A swell with decent height but short period 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.
Longer-period waves travel faster than short-period waves. This means that the first waves to arrive at our shore from a storm out at sea will have a longer period.
Longer-period swells have a deeper influence than short-period waves. They "feel" the bottom deeper which starts the shoaling process, causing waves to come up out of the sea.