Why period matters more than height
Should I care more about the height or the period?
5 min read · every figure below is solved, not drawn
Two swells arrive at your break. One is 3 ft with a 6-second period; the other is 2 ft at 17 seconds. The forecast lists the first as bigger. The second will almost certainly give you the better wave, and the larger one.
The reason is that a wave's energy is not its height. Period governs how deep the wave's motion reaches, how much seafloor it can feel, and — this is the part that settles it — how much energy it is carrying in the first place.
Height is squared, period is not
The energy a swell delivers per metre of crest has a closed form, and it is not complicated: it goes as the height squared multiplied by the period. Height squared sounds like it should dominate, and over a big enough height difference it does — but the period range in surf forecasting is much wider than the height range. Periods run from 5 to 20 seconds routinely. Heights rarely vary by four times inside the same forecast.
3.0 ft
2.4 kW per metre of crest
2.0 ft
3.0 kW per metre of crest
3 ft at 6 seconds is the taller swell. 2 ft at 17 seconds carries 26% more energy into the break.
Long-period waves reach deeper
A wave's orbital motion extends downward to roughly half its wavelength, and wavelength grows with the square of period. A 17-second swell has a wavelength around 1,500 feet and is already interacting with the bottom in water 650 feet deep. A 6-second windswell has a wavelength near 180 feet and feels nothing until it is almost on the bar.
That head start matters. The long-period swell spends miles refracting, bending toward the shallows, organising itself into long straight lines. The short-period one arrives as it left: disorganised, with crests that are short and don't line up with the beach.
3 ft at 6 seconds
Short-period windswell
2 ft at 17 seconds
Long-period groundswell
Reading a forecast with this in mind
When you see two swells listed, look at the period column before the height column. Anything at 14 seconds or more has crossed an ocean and will arrive organised. Anything under about 9 seconds was made by local wind in the last day or two and will be weaker, shorter-crested, and more affected by the wind that is still blowing.
This also matters for which swell your forecast should be quoting. When several trains arrive at once, the one that organises the break is the one carrying the most energy into it — not the one with the tallest number offshore. Getting that backwards is a real failure mode, and it is one we have had to fix in our own pipeline.
See it at a real break
- Ocean Beach3-4 ft right now
- Steamer Lane2-3 ft right now
- Mavericks~2 ft swell right now
Read next
- What makes a wave powerful, and what makes it mushy
Same height on the forecast, and one of them is a wall while the other is soup.
- Why two breaks a mile apart get different waves
One swell arrives. The ground underneath decides what you get.
The physics behind these figures, its sources and its limits are written up in the methodology.