Swell period: the number beginners overlook

Stand at the end of a jetty and count the seconds between two crests passing in front of you. That's the period. A "12-second swell" means twelve seconds of open water between each wave.

Those seconds translate into distance, and the gap grows fast. Out at sea, two crests of a 6-second swell sit about 55 metres apart; at 12 seconds, roughly 220 metres. Twice the seconds, four times the spacing.

What matters even more happens below the surface. Under a wave the water turns in circles, and that motion fades out at around half of that spacing: a short-period swell stirs the top few metres, while a long-period swell sets more than a hundred metres of water column moving. That hidden mass of water is what stands up all at once when it meets a sandbank or a reef.

Same height on paper. Two different oceans in the water.

• 1 m at 6s: waves packed close together, arriving nonstop, lifting lazily and collapsing all at once. Little push, a shoulder that dies quickly, usually some chop on top. This is learning water — great on a soft-top or a longboard for drilling take-offs.
• 1 m at 12s: spaced-out lines arriving in sets, with real lulls in between. The wave stands up sharply over the shallows, breaks with force, holds a shoulder — and the face is often well over a metre.

Then there's the classic trap: the height in a forecast is the offshore swell height, and it's an average — the average of the highest third of the waves. The biggest waves of a set comfortably exceed it by half. It is not the height of the face you'll be dropping into.

Every wave starts with wind. The question is where, and how long ago.

• Wind swell is made by wind blowing nearby, sometimes right over your head. It's young: short periods (roughly 4 to 9 seconds), crests close together, mixed directions, a choppy surface. It builds fast when the wind picks up and falls apart as soon as the wind drops.
• Groundswell was born in a storm thousands of kilometres away. On the journey the waves sort themselves out: the longest ones travel fastest and arrive first, clean and lined up. Most surfers call it groundswell from around 12 seconds; 9 to 12 is an in-between zone that is often very surfable. A 15-second swell train covers roughly 1,000 km a day — the storm behind Saturday's session was blowing somewhere near Iceland at the start of the week.

Two very practical consequences. First, a long-period swell feels the seabed much further offshore: it bends, wraps around headlands and pushes into sheltered bays that short-period swell never reaches. Second, forecasts often list several swells at once (a primary plus one or two secondary ones): when different periods cross, the sea turns lumpy and the sets go irregular. Always check which swell is coming from a direction your spot actually accepts.

The habit to build: read the period right after the height, never the height on its own.

• Just starting out: a small short-period day is your best playground — plenty of waves, water that pushes gently, little punishment. A long-period swell, even a "small" one on paper, breaks harder and forgives less.
• Progressing: long period is what you're after — waves that stand up, a shoulder that runs, time to set up and turn.
• A big long-period day: head for sheltered spots and the back of bays rather than the most exposed beach.

On safety, without the drama: the longer the swell, the more water it pushes towards the beach — and that water has to get back out. Rip currents (the baïnes on the French Atlantic coast) are far stronger on those days, and most treacherous around low tide, two hours either side. The shore break lands harder, and there are always more people in the water exactly when conditions get serious. Spend ten minutes on the sand: count the sets, spot where the water is running back out, watch where everyone else sits. If a spot looks above your level that day, it is.

That is exactly what Yosurf's per-level score translates for you: the same metre of swell doesn't earn the same number depending on whether it arrives every 6 seconds or every 12 — or on whether it's your fifth session or your five-hundredth.