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Why period matters more than height

Q: Should I care more about the height or the period?

A smaller long-period swell can break bigger than a larger short-period one. See the energy each one actually carries, and both solved over the same seafloor.

Should I care more about the height or the period?

Example break

Mon Oct 19 · 6am

At Ocean Beach today 6am, the period of the main swell is 15 s, so its 4.3 ft carries 2.9× the energy of the 3.5 ft at 8 s under it. Height alone would not tell you how far apart they are.

4.3 ft at 15 s · WGroundswell · feels the bottom from ~600 ft12.9 kW/m75% of the energy
3.5 ft at 8 s · WNWWindswell · feels the bottom from ~150 ft4.4 kW/m25% of the energy

See the swell on the map5-6 ft · Fair

Forecast built Today 3:52am. Energy per metre of crest = 0.49 × height² × period, for each swell train offshore.

Your break?Every number above redraws for it.

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.

Height offshore, against the energy it carries

3 ft at 6 secondsLocal windswell, made in the last day or two

3.0 ft

2.4 kW per metre

2 ft at 17 secondsGroundswell, made by a storm days away

2.0 ft

3.0 kW per metre

Example break · Ocean Beach · today 6am

4.3 ft at 15 secondsGroundswell

4.3 ft

12.9 kW per metre

3 ft at 6 seconds is the taller swell. 2 ft at 17 seconds carries 26% more energy into the break.

Energy flux for the two swells, computed as ρg²H²T/64π. The left-hand bars are what a forecast page shows you; the right-hand bars are what arrives at the beach. This is before refraction and shoaling, which widen the gap further in the groundswell's favour.

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.

Solving the wave field…

Model · Ocean Beach · 3 ft tide

3 ft at 6 secondsShort-period windswell

Solving the wave field…

Model · Ocean Beach · 3 ft tide

2 ft at 17 secondsLong-period groundswell

Open Ocean Beach on the map

Ocean Beach, same direction, same tide, both solved on real bathymetry. The left panel is a taller swell with a short period; the right is smaller offshore but far longer. Notice which one organises into clean lines and which one starts working the bottom further out.

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.

Solved at real breaks: Ocean Beach, Steamer Lane, Mavericks. The physics behind these figures, its sources and its limits are written up in the methodology.