Know Your Product: Wave Period
A new series in which Swellnet will explain surf forecasting information, from the basics to the loftiest of concepts, using simple language and real examples. First up, wave period.
Wave period is one of the Big Three variables used to measure breaking waves - the other two being swell direction and swell size. There are other elements to consider, including bathymetry, local winds, and mix of swells, but the Big Three have the most bearing on breaking waves. Know them and you'll have a pretty fair idea what's happening at your local.
Wave period is a measurement of time and it's expressed in seconds - unless it's a tsunami which can be measured in minutes or even hours! But from here on in we'll only be talking about wind-borne waves.
The definition of wave period is: the time it takes two consecutive wave crests (or consecutive wave troughs) to pass a fixed point. See diagram below.
To understand wave period we need to step back and observe how waves are formed. At their very beginning, waves are the product of wind blowing over water. All waves start as capillary waves; very small wavelets, the type you see on a lake during a windy day.
Once capillary waves have formed the wind is no longer blowing on a flat surface, it now has the back of the wave to efficiently transfer its energy. If the wind continues to blow from the same direction the waves will build into bigger waves and a few things will happen:
- The waves will gradually begin to space out from one another.
- By degrees they'll create some semblance of order.
- And increasingly more of the waves' energy will begin to travel underwater.
All three of those elements are associated with wave period. If the wind continues to blow from the same direction those tiny wavelets will eventually turn into the waves that we surf.
Hawaii regularly receives waves that measure 15 – 20 second period. Swells of this magnitude have often travelled great distances away from their original source. The swell has become mature. The technical term is a groundswell (as opposed to a windswell which has a shorter period and hasn't moved beyond the wind that created it).
Often the surf media talks up wave period. This is because large wave periods are usually (though not always) associated with large wave size. However not everyone lives on a coastline that handles Hawaiian-size waves, therefore we shouldn't all be hoping for Hawaiian size wave periods.
On the swell's journey the space between waves has drawn out (period), as has the time between sets (consistency), while an important aspect for surfers to understand is that more of the wave's energy has begun to travel underwater. That final element requires further examination because it determines what wave period works best for your coastline.
The energy on a short period swell may only reach a few metres underwater, while on very large period swells the energy can be felt up to 200 metres underwater. When a wave begins to 'feel' the bottom it stands up and it begins to break. Therefore large period waves generally break in deeper water and shorter period in shallower water.
If you live on a straight section of coastline, or somewhere dominated by beachbreaks, long period swells will often overpower the coast breaking beyond the usual wave zone causing closeouts. In Australia, long period swells prefer bomboras, deep water reefs, and pointbreaks surrounded by deeper water.
There are two more things to consider: refraction and consistency.
Refraction: Because long period swells carry more energy, they're more able to refract – or bend - around headlands or points into less exposed parts of the coastline. A 15 second swell will bend much further around that pointbreak than a 7 second swell will. It can be the difference between a wave breaking or being flat (or barrelling and fat).
Swell lines wrap around Lohis in the Maldives
Consistency: As mentioned earlier, the longer the wave period the further apart the individual waves and the sets are. So while long period groundswells are well-formed they're also often inconsistent. Keep this in mind of you're chasing a long range groundswell on a crowded stretch of coast.
Using wave period to your advantage
Knowing what period your local works best in is the first step. Does it require a longer period for the swell lines to wrap? Or, like many East Coast beachbreaks, does it require shorter period swells? In the car park, surfers really only quantify waves in terms of size and direction, yet a jump in wave period can turn that A-frame bank into a closeout and that fat pointbreak into a grinding wall. Of course a reduction in period can do the reverse.
Keeping tabs on the swell period is the most obvious way and you’ll find that information under the 'Surf Forecast' tab on Swellnet. Simply toggle your mouse over a particular day and the data will appear.
Another factor to be wary of is that there’s often more than one swell in the water, however most swell graphs can only display one reading. This is deceptive as secondary (or even tertiary) swells can display significant period yet they’re ignored due to the shortcomings of the graph format.
Swellnet can show up to three concurrent swells. They can be found under the ‘Surf Forecast Train Analysis’ on the ‘Surf Forecast’ tab:
Note the tertiary swell, the third swell in the water, is registering a period of almost 20 seconds at 0.7 metres. Though the size is far smaller than the primary swell, the large period means otherwise protected stretches of coast will be receiving mid-range waves.
This is important when swells are coming from different directions. A normal plot graph will only output the larger swell and surfers will begin to think which breaks will be working. However, a large period secondary or tertiary swell can light up unexpected waves and catch many surfers by surprise.
Comments
Why do people say that the swell was moving fast when the periods short say 13 s ? Is it possible theres some merit to this ? We know when the swell periods over 17 it's truly moving fast eg 20s . thinking about it after ppl said the 13s swell was moving fast & I wonder if theres something im failing to notice or is it possible ? Would like to hear from anyone else that has noticed how fast a mid period swell could be somehow moving fast when scientifically it isn't really so fast. Im pretty sure I understand the reason. for some time thought it could be a worthy strange topic to know what other surfers thinks
I think this is just because the waves are more frequent with a 13s swell and there's more action to watch, it appears faster but isn't at all.
Then if you watch a drawn out 18s swell, it seems slower because there are bigger gaps/waits between wave peaks and it appears slower.
Hi Stu, how do you define wind swell from ground swell? i've always considered any wave period of 12 seconds or above as a ground swell. The goal posts seem to have moved a bit and it now seems to be anything over about 10 seconds? When you guys call ground swell, what's your cutoff?
Your refraction explanation didn't mention wave speed slowing down as swell starts to feel the bottom, which is the actual cause of refraction.
For me I split it in threes.
Windswell up to say 10s, mid-period swell up to about 14/15s then groundswell higher than that.
But this is also relative because a 14s swell on the East Coast is quite strong and I'd call it a groundswell, but in the southern states I'd class it as mid-period energy.
Hey BD,
At Swellnet, there are surprisingly few times when groundswell has to be distinguished from windswell. It's more an academic exercise - you know, 10 or 12 seconds you lot are gonna go surfing anyway!
In terms of period, I've read the cutoff as anywhere between 10 and 13 seconds. And then I've also read that the difference between groundswell and windswell is that the former has moved beyond the wind field that created it, meaning the difference isn't the period at all.
We tend to define groundswell as 'somewhere' above 10 seconds, though like I said it kind of floats about. It's more important to determine size and quality when the swell does break.
Also, refraction, I didn't mention wave speed because I had to limit information so as not to stray from the topic. A lot of people reading this will be coming across the concepts for the first time so I'm trying to give them the info that most helps. Long period swells will refract into protected nooks, that's important in terms of chasing waves. If they want to know why they refract, well, that's another lesson for another time.
Surfing on the QLD / NNSW coast, I've noticed a definite step-up in wave quality once you go from 9s to 10s swells. In particular the beach breaks around here change from peaky to straighter lines once you get over 10s. Also the few reefs we have here tend to improve remarkably in shape over 10s. Also the intervals between sets seem a considerable step longer from 9-10s than say from 8-9s.
For this reason I am used to calling a 'groundswell' 10+ seconds, but if you were living in WA or even SA/VIC, maybe you would up the classification because the average beachie or reef might behave differently due to the higher prevalence of longer period swell in the region. All trivial I guess.
I find theres a big jump in energy from 13 seconds up to 14 seconds, but not as pronounced from 10, 11 or 12 seconds to 13 seconds.
14 seconds seems to be the next step up. 15 seconds again seems exponentially larger.
Agree with alot of comments below, particularly in the tropics. Super long period swells tend to see the ocean going below sealevel and the waves eating themselves, slingshotting the energy to the inside and detonating, and causing strange currents. From my obs alot of the big slabs down this way don't tend to like the super long period stuff either..not sure why, too much bulk in the wave and just seems to overpower those small granite mounds and roll over the top without properly standing up. Also seems some of the swells are steered away from some kind of shoaling effect further out. But a pointbreak or long reefbreak that's sectiony 99percent of the year could turn into a A+++ lined up piece of perfection with a 20second groundswell. And the only beachies that like long period are technically sand over reef anyway. That's my observations anyway, but still so much to learn. Great thread. Love it. Cheers.
interesting reading! i'll talk up the tertiary swell & give the secondary swell some high regard when forming my swell prognosis re what will be working where, & when! Then i'll probs just walk around the crn & take a look!
In Indo everyone always talks about swell period, its never there is a swell coming next week its should be 6ft+, its theres a swell coming next week 6ft+ at 18 seconds.....18 seconds dude.
I use to be like everyone else and when in Indo be amped on a swell with a high period.
But I'm starting to change my attitude a little, I've found some waves actually seem to work better on a lower period swell and many hollow waves are actually a lot easier to ride and get barrelled with a little lower swell period. (14-16 seconds)
Or maybe I'm just getting old.
I'm hearing ya ID. I know of a few waves in Indo that love that 13-15 sec swell period band. Any bigger swell period and they bottom drag and wash through or hit outer bombies and bend into the reef at different angles, thus sectioning etc.
I actually find the most important factor after direction is how uncorrupted the swell is ie as little background swell or windswell as possible.
Just give me ONE swell on a quality reef from the right direction and I'm happiest.
Wedges and cross waves for beachies thanks, not reefs.
I'd agree with that ID.
I was at Ulu's for a swell in june which was at the time forecast to be the biggest of the season so far, it was very high period swell with forecasts for 12-15ft.
It was 8-10ft at its peak and It ended up being a bit of a disappointment, only because i was keen to see it step up to that other level.
And i remember Craig saying at the time this could of been due to the high period.
(8-10ft ulus is still pretty fucking amazing, not disappointing at all really!)
Hopefully one day we will get a really long period (above 16 second) swell. It was amazing how that massive north east swell was no good for Noosa points. (compared to the beach breaks) On that note what would be the period expected for next weeks swell? Period really does come into it's own on the point at Noosa, could only imagine how epic a 19 sec period would be for that place.
nah, direction is far more important than period for the Noosa points......and for most waves in SEQLD to be honest.
Those last two paragraphs where was a beaut tip of infomation. In regards to period and consistency, in Vic we got a abundance of mid period/ground swells. How would you distinguish say a consistent 14s swell opposed to a inconsistent. Just being out amongst it, if the swell is peaking around 4-5ft @ 14s, there will plenty of lulls. 6-8 at the same period ill find a bit more activity. I understand tides and the fact a bigger wave will break easier than a small one has a role but its pretty easy too tell sets apart from the rest.
Great read. Excellent that you guys are putting this out there. Hi 5s all round.
All depends where the 14s swell was generated. If generated closer to Victoria it will be more consistent, less waits between sets and also likely a lot more filler waves in between.
The further away the 14s is generated, the less consistent and with no other swells in the water, the less filler waves in between.
Good stuff Stu.
Surfline provide the maths of bathymetry here:
http://www.surfline.com/surf-news/the-importance-of-wave-period_125149/
"We see swell on the surface of the sea but its kinetic energy may extend downward to far greater depths. In that regard, swells are like icebergs, with only a small fraction of their true dimensions visible on the surface while the rest is hidden underwater. Just how far underwater a swell’s motion goes depends on swell period. That depth in feet is calculated by squaring the swell period then multiplying the product by 2.56. In the case of a swell with 20 second period, the math is 20 x 20 = 400, 400 x 2.56 = 1,024 feet. So that swell would begin to drag on the seafloor once it moved into waters of that depth. As the sea became shallower, the drag would increase. In contrast, a swell of six second period would not start to feel the resistance of the seabed until it entered seas that were 92 feet deep. Interference with the seafloor can change a swell’s direction, in some instances up to 180 degrees. That wrap is much more pronounced in long-period swells than in short-period ones. So knowing a swell’s period can be critical in forecasting if the swell will head straight into the coast or whether it will take a detour along the way."
Awesome. Good stuff Rusty
Craig yep that's true , have often been fascinated how those big period swells can look slow to the untrained eye
Yep, wind swells seem so busy whereas a long period ground swell often seems lazy and dawdling. I think it is partly because big period swells are often visible for such a long time before they break.
Probably fitness.
For most of the east coast, especially north of Sydney anything with long period is (mostly, with very rare exceptions) highly refracted swell from the southern quadrant generated in the southern ocean.
Thus by the time it reaches us it has dragged so much on the continental shelf and inshore slope and refracted so much it has lost most of it's energy and does feel slow.
It's funny hearing euros and people who learnt to surf in Indo banging on about wave period when it's next to useless as a forecasting tool for the majority of surf around here.
An 8-10 second ENE/E tradewind swell generated inside Grand Terre will be so much better and surf so much punchier than a 20 second S'ly groundswell generated between the ice shelf and 45S in the window of 130 to 150E.
It' why east coast forecasting is so much different to the "classical" forecasting environments of Indo, Hawaii, California, Pacific Islands, Europe etc etc.
Yep totally.
As soon as any larger swell with a long-period is forecast for the Northern Beaches, I'm outa here. No beaches handle it unless there's an outstanding bank and reef options are limited.
Love windswell/mid-period energy, fires up most beaches here, and also as you said, it's more about direction rather than period along most of the better performing breaks along the East Coast.
The other thing about period is that it leads to the phenomenon of "real" and apparent direction.
Swells of different period signatures wrap into the coast at different angles leading to the swell appearing to be from a different direction, even though the origination direction is similar.
This can actually have a pretty big impact on wave quality at certain coastal alignments.
True. Occasionally hear guys saying "there's X direction swell in the water" when the Directional Spectrum shows there isn't a skerrick of swell from that direction, it's just refracted swell from elsewhere.
Six years here fulltime now (SC) and still getting my head around the nuances of the East Coast. You've been a tremendous help SN. Forecasters notes are a goldmine not to mention the discussions in the comments. Watching a sliced onion curve up from the Carribean, detach from the US Eastern seabord and float across the Atlantic towards the UK (praying for a high pressure ridge to push out from mainland Europe), was a piece of cake compared to the dark arts needed here. They spin the other way just for starters!!
Couldn't agree more 'mos', although unlike you moving from the northern hemisphere I just moved from Vic 5 years ago. Love all the computations the SC/east coast throws up and the guidance SN provides. It's almost as much fun as the actual surfing, almost....
I can relate to this as seeing a supa long period swell above 20 secs in Indo.There was 20min lull's with nothing between set's and when they did arrive it was like trying to jump on a moving train. The next day being the same size and direction swell but back down to around the 16- 17 sec mark made for more waves no lull's and slightly easier to get into.
Good stuff Swellnet! (May I share some borrowed locally relevant info?) Be reminded next Big Wednesday that 2 ft lull wave easy send you flying as it high tails it's 10 ft mother .( At what speed is that then?) re; Wave Basics (Stormsurf) data guides approx'- Multiply 'primary swell speed' x 3, now halve that! = inshore breaking wave speed # kts. (convert end math to km/ph) & this(STORMSURF) guide ...( Chop-3-8 secs/Windswell 9-12 secs/Groundswell 13-15 secs/Strong ground'16-25 secs)Thank You! The longer the swell period the higher the ocean rise.Meaning elevated readings similar to 'sea height' scales. Also these larger swells ground deeper tending to break wide of our established protected 'local' banks seldom standing to reveal their whole face height.Don't be fooled by looks though,as their speed will not lessen.Much data is geared toward incoming visiting freight trains. Bodysurfers seek local myth over math.Reel a riptide, creepy causeway cavern or rollout a burley barrel.Sniff out the local sensory trail,feel the difference and come alive. Big swells = Going nowhere fast... m1 to Brisbane(road block ahead) beep!beep! Hooroo!Outta my Way You!
As for what constitutes a ground swell, I always pick it based on the look of it, ground swells are connected, long lines up and down the beach, hence why they often make beach breaks shutdown nightmares. Period will be the main factor, and be a guide so you can guess, but looking at it as it hits the beach/reef/point will make it clear.
Yes, most of east coast can't handle a ground swell.
Ground swell, ground swell, you want Ground Swell.
Yes, I want Ground swell,
You can't handle the ground swell!
Agreed all the above
Something Stu said and what a few others have touched in or hinted at .
Surf reports , bouy readings , weather models can be great aides . But nothing beats accurate 2-3 day Synoptics .
Swell periods mean alot of different things in different places .
I think timing /period has nothing to do with the term Groundswell . More think that a groundswell can only be deemed once an open ocean swell reaches a longer period than its height . This totally rules out most events on the East coast , but to be honest it's rare to see uniformity and power . The obvious exclusion , ( bar crossing's / Rock fishing statistics don't lie ) that's with Any swell within 30 degs of East and over 12 sec's ' that is what would catch most rookies unawares .
But back to real groundswell magnets , uniformity starts to occur once the swell moves out of the literal contamination of the very system that formed it . AND before it travels into another system that will disguise it or ruin it .
This point being that surface conditions within 500kms of where your surfing will govern how clean and orderly things are . So sometimes a 16 second swell is not what I would call a groundswell even here .
My back of napkin calcs usually work out that a swells peak height will be around/after it's lost 15% of its initial outliers period . See if you can absorb that Stu , and then attempt to eloquently translate/ explain that to all the punters , taking notes . Cheers
Camel its fitness...so hard taking on a beach at 14 sec ..Penn is looking
Yeah pen , u know your limits thats great . Stu I see that re swell direction so much , often older guys not privvy to modern info . But you don't argue when they say
I think Kirra can handle a ground swell, if it is from the right direction.
Got us there Wally. No doubt there are plenty of east coast point breaks around that can handle ground swell. Actually, if it's a point break or bombie it will tend to, by definition.
Of course if you don't want to surf with a 150 best buddies, then the options drop right off.
Southey, I'm with you there, I think. For me ground swell largely means it's come from a fair distance and had time to organise itself. You're also right in saying that it isn't 'polluted' by other swell noise. Theoretically you could get long period and be close to it, and therefore not organised and not ground swell, and some posit that the long period is already there in the midst of the storm and you just can't detect it because it is hidden in the chaos, and only the long period waves survive the travel distance and subsequently are organised when it hits the beach.
I don't subscribe to the idea that long period is already in the maelstrom of a storm, but that's a whole other debate that I can't win.
I feel this is the right place to ask. Any links too bathymetry maps around?
Correct me on this but isn't a groundswell developed when strong off shores blow for at least 24 hours. By 'strong', I suggest at least 20knots plus. So the swell is within the system and starts with a long period. Hence catches rock fishies very easily.
Have never heard it defined that way TB. As above, the usual definition is tied to the period - measured in seconds - though I've also heard it said that groundswells are swells that have moved beyond the wind field that created it.
what do you mean ?
strong offshores? where?
And actually, long period groundswells are not the most dangerous swells for rockfishing around here. They tend to be predictable and relatively safe, assuming, of course they are of fishable size.
It's mixed swell trains that are far more dangerous, because you get unpredictable phase enhancement, backwash effects and conjoining swell angles at the point of impact on the rock ledge.
HEY FREE, Does longer period swell at Cloudbreak make for more heavy surf out there?
Groundswell is another nonsense word who cares when period is a far superior measure of swell quality
what is the longest swell period that wind can create?