What Exactly Is Wave Period?

2 Jan 2018 3 Share

Ben Macartney

Chief Surf Forecaster

SURF FORECASTING BASICS

At Coastalwatch we often receive emails with questions about our surf forecasts. Here, Chief Swell Forecaster Ben Macartney explains how Swell Period affects surf quality and answers a reader question: Which is better, long period or short period swells?

When it comes reading forecast graphs, swell period is definitely the magic number. Indeed, for a forecaster, explaining the implications of swell period is a bit like the birds and the bees... of surfing. The basic definition is as follows: Swell period is a measure of the time, in seconds, between successive wave crests (or troughs) passing through a stationary point. 

A good way to get a handle on it is to manually count seconds or time set waves reaching a stationary point. For example, if you were watching a fishing boat anchored beyond the breaking zone, you could count the seconds between one wave going under the boat and the next. Or you could measure time between one wave hitting a rock ledge and the next – and there you would have an approximate wave period.

However, wave period gives you a lot more information about a swell than just how long you’ll have to wait before the next wave lands on your head. Primarily, it provides a deep insight into the underlying energy contained in a swell. The shorter the period, the weaker and slower the swell, and the closer to the surface it travels. The longer the period, the more wave energy is contained within the swell and the faster and deeper beneath the sea surface it travels.

Indeed, it’s the mathematical relationship between wave period and wave speed and how far the wave energy penetrates beneath the sea surface that allows us to calculate, with great accuracy, things like swell arrival times as well as breaking wave heights. Big interval groundswells of 15 to 20 seconds carry much more energy, penetrating over 1000 feet below the sea surface (for 18 second intervals). This means groundswells feel the sea floor in much deeper water than windswells (a windswell is loosely any wave exhibiting periods of 6 seconds to 12 seconds) and hence will refract and react the sea floor further offshore.

So, getting back to the question; when asking what's better; long periods or shorter ones, it depends on what kind of waves you are looking for. More often than not, big groundswells react to a sand bank in deeper water, often beyond the rips and gutters that shape good beach break surf – and hence tend to close out. Conversely, windswells are often more broken up and only begin to "feel" sand banks in much shallower water, and are therefore more likely to produce good surf as they peel off along gutters and rips.

If you're looking for reefs and point breaks, then longer period groundswells generally produce better surf – but this of course is heavily dependent on the characteristics of an individual break – or, more specifically, the bathymetry (the shape and depth of the seafloor that the approaching swell interacts with). Most world class point breaks love long interval groundswells that wrap in from oblique angles producing long, high quality waves.

Note: This feature was originally published on Coastalwatch in 2012 and titled Forecasting Tutorial: Wave Period Explained.


Tags: topnews , (create Alert from these tags)

blog comments powered by Disqus
More From Latest News
THE O'RIGINALS – Connor Lee

THE O'RIGINALS – Connor Lee

Surfing's in the blood

22 Jan 2020
Good Times On The Goldy With Brent Dorrington and Crew

Good Times On The Goldy With Brent Dorrington and Crew

There's been a few banks about...

18 Jan 2020
Two Electric Sessions on the Tweed Coast With the Future of Australian Surfing

Two Electric Sessions on the Tweed Coast With the Future of Australian Surfing

Ethan Ewing and Liam O'Brien are on fire

4 16 Jan 2020
Recent

THE O'RIGINALS – Connor Lee

Surfing's in the blood

22 Jan 2020
Good Times On The Goldy With Brent Dorrington and Crew

Good Times On The Goldy With Brent Dorrington and Crew

18 Jan 2020
Two Electric Sessions on the Tweed Coast With the Future of Australian Surfing

Two Electric Sessions on the Tweed Coast With the Future of Australian Surfing

4 16 Jan 2020
Sean Doherty: Davey Cathels and the High Country Fire

Sean Doherty: Davey Cathels and the High Country Fire

3 14 Jan 2020
Nick Carroll: "The Only Thing I Could Compare It to Is War"

Nick Carroll: "The Only Thing I Could Compare It to Is War"

15 10 Jan 2020
Latest News

THE O'RIGINALS – Connor Lee

Surfing's in the blood

Sean Doherty: Davey Cathels and the High Country Fire

“It's been a pretty strange start to the year, that’s for sure.”

Nick Carroll: "The Only Thing I Could Compare It to Is War"

A southerly saved Phil Macca’s town. But what now?

Popular This Week

Lennox Head's Nyxie Ryan Rips

Get to know her powerful rail game!

Long Weekend Surf Forecast 24 - 27 January, 2020

Line up your Australia day long weekend.

The Best Film Recap of the Pipe Masters and World Title Just Dropped, Isabella Nichols Ripping Indo, & Asher Pacey Glides Mex

This Week In Surfing: Ten Things from Surfing & the Internet on the Week That Was January 24, 2020

The WSL Has a New CEO, the Volcom Pipe Pro Gets Upgraded, and Russell Bierke Is a South Coast Tube Star

This Week In Surfing: Ten Things from Surfing & the Internet on the Week That Was January 17, 2020

Go to Top