Why the Buoy Says 3ft and the Wave Is Overhead
Buoy wave height is measured in deep water; face height is what the wave stands up to when it breaks. Shoaling converts swell period into height, so breaking waves are routinely far larger than the offshore reading that produced them.
This is the question that makes people distrust forecasts: the buoy reads 3 ft, and the waves are twice that. Both numbers are correct. They are measuring different things in different water depths.
PierMonkey estimates breaking face height explicitly rather than showing you the offshore number and hoping, and the model is worth understanding because it tells you which conditions will over-deliver.
Step 1 — the physics baseline
The starting point is the Komar & Gaughan (1972) breaking-height relation, which predicts breaker height from deep-water height and period:
Hb = 0.39 · g^(1/5) · (T · H0²)^(2/5)
What matters is the exponents. Breaking height scales with offshore height to the power 0.8, but with period to the power 0.4. Period is not a minor correction — it is a substantial multiplier. Doubling the period of the same offshore swell increases breaking height by roughly a third, before any local effect.
Step 2 — the swell window gates it
Every break accepts swell only from a range of directions. Energy arriving from outside that window is blocked or heavily attenuated by headlands, islands and shelf geometry. PierMonkey applies a directional falloff with a soft shoulder — real coastlines wrap some energy in through refraction, so the model never zeroes out entirely.
This is why a huge offshore reading can produce nothing at a break pointed the wrong way, and why direction deserves as much attention as size.
Step 3 — the break's own bathymetry
Finally, each break gets a period-band multiplier reflecting how its bottom focuses energy — what the literature calls a Caldwell coefficient, after Caldwell & Aucan (2007), who fitted exactly this against Hawaiian observations and found focusing reefs reaching around 2.1.
A steep focusing reef like Teahupo'o sits near 1.9 on long-period swell. A wrapping point break, where energy bends around and spreads, sits below 1. Same offshore swell, radically different outcomes.
Honest limits
About ±20% is the physics floor for this kind of estimate, and that is before sandbars move. Per-break factors are climatology, not a live survey. Island-shadowed coasts are the hardest case, because the shadowing itself varies with swell direction in ways a single coefficient cannot capture.
Any forecast presenting face height to the inch is overstating its own precision. PierMonkey's calibration data and every revision to it are published in the changelog.
See it on live data
The fastest way to make any of this concrete is to go and look at the numbers being described.
Common questions
Why do surfers and buoys use different numbers?▾
Can I convert buoy height to face height myself?▾
Why is long-period swell so much bigger at the break?▾
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