What makes a swell period “good” depends mainly on the topography of the ocean floor (i.e. bathymetry).
So the question you really want to be asking is: What is a good swell period for surfing at my local break(s)?
That’s a question you can actually answer. But finding that answer will take time. You’ll have to experiment by physically observing different conditions and/or talking to the locals.
If the locals aren’t forthcoming and your only option is observation, understanding the basics of swell period and the variables that affect it will accelerate your learning curve.
Understanding the basics of swell period
Swell period is the average time between the crests or troughs of a wave. It is measured in seconds. In marine forecasts, you’ll see swell periods displayed next to swell direction and swell heights. Here’s a forecast from the NOAA:
In conversation, you’ll often hear people say a swell is long, short, or mid-period.
In my experience, a swell is generally called short-period when it’s 13 seconds or less and long-period when it’s 14 seconds or more. But I’m from California—surfers on the East Coast might consider a 12-second period long.
The most important takeaway: the larger the swell period, the faster the swell moves, the deeper it reaches, and the longer and more energetic the wave is.
How a swell becomes a wave
For a wave to be surfable, something needs to slow it down. And for a wave to slow down, it has to run into something, like a point, sandbar, reef, or rocks.
Since longer-period swells reach deeper, a long-period swell will “feel” the seafloor sooner than a short-period swell.
When the leading edge of a swell starts to feel the seafloor, it slows down. The trailing edge of the swell catches up, which causes a buildup.
Since the swell’s energy can’t go out, it goes up and (sometimes) becomes a surfable wave. As the wave travels upward, it becomes increasingly unstable and eventually “breaks”.
You can see a basic representation of how wavelengths shorten and wave heights increase as swell lines reach shallow water in the gif below.
As Surfline explains, “The longer the swell period, the more water gets pushed upward. A 3-foot wave with a 10-second swell period may only grow to be a 4-foot breaking wave, while a 3-foot wave with a 20-second swell period can jack up to be a 15-foot breaking wave.”
Swell period’s impact on surf
So we’ve established that for a wave to be surfable, part of it needs to slow down. But the wave also needs to bend.
If a wave comes in perfectly parallel to the coast, it will close out, meaning it will break all at once. There will be nothing but whitewater to ride. The wave in the photo below is about to close out.
Swell period’s impact on surf is so different from break to break because how the swell bends (or not) depends on the seafloor.
Pointbreaks, reef breaks, and beach breaks with established sandbars are more likely to deliver good surfing on long-period swells. But beaches with no sandbar become overwhelmed by long-period swells because there’s no structure to slow down the swell and cause it to bend.
On a beach with uniform bathymetry, the leading edge of a swell coming in parallel to the beach will start slowing down and collapsing at about the same time which is not good for surfing.
If you want a primer on how waves bend i.e. refract, check out this 5-minute video:
What’s a good swell period for my local break?
The only way to determine what a good swell period is for your local surf break is to experiment by surfing in and observing different conditions and/or talking to the locals.
If you can remember that longer period wave energy “feels the bottom” sooner in deeper water, you’re a step ahead.
If you’d like to start logging your observations, you can use my Swell Log template by copying this Google Doc. Take a look at the tab with instructions on how I like to use the Swell Log or feel free to make it your own.
Also, Swell Info’s forum post on swell logging has some great discussion on how different people approach logging their surf sessions.
A final note on swell angle, and a word of caution
There is one more variable I’ve neglected to address: swell angle. And when it comes to creating good surf, swell angle is at least as important swell period.
Even a swell with the perfect period for your local break could result in no waves if it’s not angled the correct way. But that’s a post I’ll write on another day. In the meantime, this post from 7Till8 might help bridge the gap.
Word of Caution
Long-period swell can result in much larger waves. And that can be dangerous.
A rookie might see a forecast for a 1.5-foot swell at 20 seconds and mistakenly believe it’s a good day to surf. But with a 20-second period, even a seemingly small 1.5-foot swell can jack up and cause some pretty large waves.
So if the swell period is especially long ( > 15 seconds ), take a few minutes to watch the waves before you paddle out. Wait for a proper set to come through to see how it’s breaking and how big the waves are.
If you’re not sure you can handle what’s on offer, don’t paddle out. It’ll be dangerous to you, your board, and your fellow surfers.
