Two forecasts showing the same wave height can describe utterly different days at the coast. The figure that separates them is the period, and it is the number experienced water users read first.
Period measures the gap between waves
Swell period is the time in seconds between successive wave crests passing a fixed point, and it reflects how the waves were generated.
Short periods indicate local wind chop, produced nearby and still being driven by the wind that made it. The water is disorganised and the waves are close together.
Long periods indicate swell that has travelled from a distant storm, sorting itself into orderly lines as it crosses open ocean and arriving as distinct, well-spaced waves.
Energy scales with period, not just height
A long-period wave extends much deeper below the surface than a short-period wave of the same height, so it carries substantially more energy.
When that deeper wave reaches shallow water it feels the bottom sooner and further out, standing up more abruptly and breaking with far more force.
This is why a modest long-period swell can produce heavy surf on a beach where a larger short-period sea produces only messy chop.
The bottom decides what arrives at the shore
Identical swell produces different waves on different beaches, because the shape of the seabed determines where and how the energy is released.
A steep shore causes waves to break suddenly and close in, while a gently shelving bottom spreads the break over a wide zone.
Reefs, sandbars and channels focus energy into specific spots, which is why one end of a bay can be calm while the other is unworkable.
Direction determines whether it arrives at all
Swell travels in straight lines until it refracts around obstacles, so a coastline facing away from the swell direction may receive almost nothing.
Headlands, islands and offshore banks create shadows, and local knowledge of which beach works on which direction is worth more than any general forecast.
Wind direction interacts with this separately, since an offshore breeze grooms incoming waves while an onshore one destroys their shape regardless of the swell.
How to use this before entering the water
Reading period alongside height gives a far better sense of the day than height alone, and a sharp rise in period is a warning that conditions are about to change.
Long-period swell also arrives in sets with quiet gaps between them, so a shoreline can look manageable for several minutes and then not be.
Watching the water for a full set cycle before committing is the standard practice among people who work these coasts, and it costs nothing but patience.