Two swells can carry the same height reading and be entirely different events in the water. The difference usually comes down to whether you're looking at a groundswell or a windswell.
Groundswell: energy from far away
A groundswell is generated by a storm system far out at sea — often thousands of miles from the coast that eventually receives it. By the time it arrives, it's had time to organize into clean, well-spaced, powerful lines. Long period is the signature of a groundswell, and it's the type of swell responsible for the biggest, most organized surf almost anywhere in the world.
Windswell: energy from nearby
A windswell is generated by wind blowing much closer to shore — sometimes just miles away. Because it hasn't had time or distance to organize, it tends to be choppier, shorter-period, and less powerful, even when the height reading looks similar to a groundswell day. It also tends to fade faster, since local wind is far less consistent than a distant storm system.
How to tell them apart on a forecast
Period is the tell. A windswell rarely carries a period much past nine or ten seconds, while a genuine groundswell typically starts around twelve seconds and can run well past twenty on the biggest, most distant storms. If a forecast shows solid height paired with a low period number, that's usually windswell — decent for some spots, weak for others, and never the same category of wave as a long-period groundswell.
For the full breakdown of what period numbers actually mean for wave quality, see What Swell Period Actually Means for Wave Quality. For how a groundswell actually forms in the first place, see How Waves Are Actually Formed.
We check the type, not just the height.
Period, direction, and source all factor into whether we text you — not just whether there's a number on a chart.
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