The numbers on a surf forecast are not arbitrary. Here is what each one measures, which ones actually predict waves, and the model PierMonkey uses to turn an offshore buoy reading into the wave you paddle for.
Most surf glossaries stop at a definition. These go further: every page states the honest limits of what the number can tell you, cites the research where there is any, and links straight through to live buoy readings and break guides so you can see it working.
A NOAA buoy report is a set of instrument readings — wave height, wave period, wind, temperature and pressure — taken at a fixed point offshore and published hourly. Reading one means knowing which numbers describe the swell and which describe the weather on top of it.
Significant wave height (Hs, reported as WVHT) is the average height from trough to crest of the highest one-third of waves measured in a sampling window. It is a statistical summary of a messy sea surface, not the height of any particular wave.
Swell period is the time in seconds between successive wave crests passing a fixed point. It is the best single predictor of surf quality, because period determines how deep a wave's energy reaches and therefore how it behaves in shallow water.
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.
Wind waves are generated by local wind and are still being driven by it — short-period, steep and disorganised. Groundswell has left its generating storm behind and travelled, sorting itself into long-period, orderly sets.
Swell direction is the compass bearing swell arrives FROM, in degrees. A swell window is the range of those bearings a particular break can actually receive, given the land, islands and seafloor around it.
Each of the 74 covered breaks has its own four-part guide going deeper than any general explanation can — the bathymetry that shapes the wave, its season, its exact swell window, and how to forecast it days out.
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