· Forecasts
Why the swell period matters more than the wave height
A 0.4 metre wave can be a ripple or a shove. The number that tells you which is the period, and it's the one most swimmers skip.
Every forecast leads with wave height, and it’s the number we all look at first. But two mornings with the same 0.4 metres can feel completely different in the water.
What the period is
The swell period is the time, in seconds, between one wave and the next passing the same point. Short periods (four to six seconds) are wind waves: made nearby, choppy, close together. Long periods (ten seconds and up) are swell: made by a storm far away, travelling across the ocean, carrying much more energy than their height suggests.
Why it matters to swimmers
A long-period swell of 0.4 metres steepens as it reaches the beach and can dump on the shingle with real force, right where you’re getting in and out. The same height on a short period is a bit of chop you swim through without noticing.
That’s why Saltglass shows the period next to the height, and why the verdict counts a long, slow swell against a beach that shelves steeply.
A rule of thumb
- Under 7 seconds: chop. Uncomfortable if it’s onshore, harmless if not.
- 7 to 10 seconds: mixed. Watch the entry and exit.
- Over 10 seconds: swell. Watch a few sets from the shore before you go in, and count the gap between them.