The one thing most people get wrong
Wave height is the number everyone looks at, and it's the least useful one on its own.
Two forecasts both reading 4ft can be completely different sessions. One is a weak, gutless wind chop that closes out. The other is a powerful, well-organised swell that breaks properly and holds a shape. The difference isn't the height — it's the period, and it's the number to learn first.
Swell period: the power number
Period is the time in seconds between passing waves. It tells you how far the swell has travelled and how much energy it carries.
- Under 8 seconds — windswell. Generated locally by nearby wind. Short, weak, disorganised, closes out easily. Common on the Irish Sea and in summer.
- 8–12 seconds — mixed. Usable, often the bread-and-butter of a beach break.
- 12–16 seconds — groundswell. Made by a distant storm, travelled a long way, arrived organised. Far more power than the height suggests.
- 16 seconds and up — long-period groundswell. Serious energy. A 4ft reading at 16s can carry several times the energy of a 4ft at 8s, and will feel far heavier — and it will wrap into spots that a short-period swell never reaches.
A practical rule: the same height at a longer period is a heavier, more powerful, better-organised wave. If you learn nothing else from this page, learn that.
Long-period swell also refracts more — it bends around headlands and into bays that shorter swell can't reach. That's why a sheltered spot can be flat on a 9-second swell and working on a 15-second one of the same height.
Swell direction: does it actually reach your spot?
A swell has a compass direction it's travelling from. It only matters relative to which way your spot faces and what's in the way.
A west-facing beach will take a west or northwest swell cleanly. Give it a due south swell and it may be flat, because the headland blocks it. This is why forecast height for a region tells you very little — the spot's orientation and the shelter around it decide whether that swell arrives at all.
Refraction is the part that surprises people. Swell doesn't travel in straight lines once it reaches shallow water — it bends. That's why:
- a bay that looks shut off from the swell direction can still work, fed by wrapping energy;
- the longer the period, the more it bends, so a long-period swell reaches into corners a short-period one never finds;
- and two spots a mile apart, on the same swell, can be completely different sizes.
None of that is guesswork on our side — each spot page lists the swell direction the break actually works on spot pages, because that's the filter that matters before you look at anything else.
Wind: the quality control
Swell decides if there are waves. Wind decides if they're any good.
- Offshore (blowing from the land out to sea) — holds the wave face up, grooms it clean, can hold it open longer. What you want.
- Onshore (sea to land) — flattens and crumbles the wave, makes it mushy and disorganised. Kills an otherwise good swell.
- Cross-shore — blows along the beach. Messy, usually poor.
- Light or nothing — glassy. Often the best of all, and the reason dawn sessions are worth the alarm: wind is usually lightest around sunrise.
Strength matters as much as direction. A 5 knot offshore is perfect. A 30 knot offshore will hold you out of the wave and make it hard to get in — and on a big day, hard to get back to the beach at all surf safety.
Because "offshore" depends entirely on which way the beach faces, the same wind is offshore at one spot and onshore at another a few miles away. That's the single biggest reason to check a spot, not a region.
Tide: the timing
Most breaks have a tide state they prefer, and some are unsurfable outside it.
Shallow reefs often work only in a narrow tide window — and which window is entirely local. Outside it there may be no rideable wave at all, or not enough water over the rock to be safe. Beach breaks vary with the banks and often improve as the tide drops off a high. Points tend to be the most forgiving, because the breaking point simply moves along the point as the water changes.
Tide also moves fast — quickest in the middle two hours, slowest around high and low water. A spot can go from good to unsurfable in 90 minutes, and on a big spring tide that window is shorter still.
That's why it's worth reading the tide alongside the swell rather than as an afterthought tide pages, and why "good today" usually means "good in a window today."
The full picture — why there are two high tides a day, what spring and neap tides actually are, and what tide does to a reef versus a beach versus a point — is in Tides explained.
Putting it together: how to actually make the call
The sequence that works:
- Is there swell reaching this spot? Direction first, then height. If the direction's wrong, stop — it doesn't matter what the height says.
- Does it have power? Check the period. Under 8s, temper expectations.
- What's the wind doing, at this spot? Offshore or light = go. Strong onshore = probably don't.
- Is the tide in the right part of its cycle? And for how long?
- When is the window? The answer is rarely "all day." It's usually a few hours where swell, wind and tide line up.
That sequence is exactly what SurfCheck's score does automatically — it reads swell, period, direction, wind and tide against each individual spot's own characteristics and gives you one number, plus the window how SurfCheck works. The point of this guide isn't to replace that; it's so you can tell when the number is right, and understand why a spot is rated the way it is.
A few honest limits
- Forecasts get less reliable with distance. A three-day forecast is a decent plan. A ten-day forecast is a hint, not a fact — treat long-range swell as "something might be coming," not a booking.
- Wind forecasts degrade faster than swell forecasts, which matters because wind is usually what decides whether a day is good.
- Models don't see everything. Local effects — a sandbank that shifted, a rip, a river mouth — don't show up in any model. Local knowledge still beats data.
- Look out the window. A forecast is a prediction. A cam is the truth webcams. And before you go, know what you're going into surf safety.
And the bigger question underneath all of this — why one winter pumps and the next is flat, and whether it's worth flying somewhere instead — is a different timescale entirely What drives European swell seasons.
The short version
Period over height. Direction decides whether the swell even arrives. Wind decides whether it's worth surfing. Tide decides when. And the answer is almost always a window, not a day.