The forecast numbers
<a id="wave-height"></a>Wave height How big the waves are, usually the significant wave height (see below). It's the number everyone looks at and the least useful one on its own — the same height at a different period is a completely different wave. Reading a surf forecast
<a id="significant-wave-height"></a>Significant wave height The standard measure used by forecast models: roughly the average height of the largest third of waves. It is not the biggest wave you'll see — individual waves can be considerably larger, which is why a forecast that reads 2m still has bigger sets in it.
<a id="period"></a>Swell period The time in seconds between passing waves, and the most informative number on any forecast. It tells you how far the swell has travelled and how much energy it's carrying. Under 8s is weak local windswell; 12–16s is real groundswell; 16s+ is serious. Reading a surf forecast
<a id="swell-direction"></a>Swell direction The compass direction the swell is travelling from. Only meaningful relative to which way your spot faces and what's in the way — a swell can be huge offshore and absent at your beach because a headland blocks it.
<a id="groundswell"></a>Groundswell Swell generated by a distant storm that has travelled far enough to organise itself into clean, evenly spaced lines. Long period, powerful, and what everyone is waiting for.
<a id="windswell"></a>Windswell Swell generated by local wind, close by and recently. Short period, choppy, disorganised, closes out easily. Common on the Irish Sea and in summer.
<a id="primary-secondary-swell"></a>Primary and secondary swell Forecasts often show more than one swell at once, arriving from different directions with different periods. The primary is the largest; the secondary can matter more if it's longer-period or better aligned with your spot.
<a id="fetch"></a>Fetch The stretch of open ocean a storm's wind blows across. Bigger fetch, longer duration, stronger wind — bigger swell. It's where waves come from. European swell seasons
<a id="refraction"></a>Refraction The bending of swell as it moves into shallow water, which lets it wrap around headlands and into bays. Longer-period swell bends more, which is why a sheltered spot can be flat on a 9-second swell and working on a 15-second one of the same height.
Wind
<a id="offshore"></a>Offshore Wind blowing from the land out to sea. Holds the wave face up and grooms it clean. What you want — though a very strong offshore makes it hard to get into waves, and hard to get back in. Reading a surf forecast
<a id="onshore"></a>Onshore Wind blowing from the sea onto the land. Pushes waves over early and leaves them crumbly and disorganised. The most common wind, and the reason most days are average.
<a id="cross-shore"></a>Cross-shore Wind blowing along the beach. Bumpy and uneven. Whether it matters depends on the spot — some coves are sheltered from it.
<a id="glassy"></a>Glassy No wind at all. The wave face is smooth as glass. Most common around dawn, which is why dawn patrols exist.
<a id="blown-out"></a>Blown out Too much onshore wind for the waves to be worth surfing. There's swell; it's just been wrecked.
<a id="knot"></a>Knot One nautical mile per hour, about 1.15 mph. The standard unit for wind at sea. 5 knots offshore is perfect; 30 knots offshore is a problem.
Tides
<a id="high-low-water"></a>High water / low water The highest and lowest points of the tide. Most Irish, UK, Portuguese and Moroccan coasts get two of each per day, roughly 12 hours 25 minutes apart — and each day's tide runs about 50 minutes later than yesterday's. Tides explained
<a id="tidal-range"></a>Tidal range The vertical difference between high and low water. Depends far more on your coastline than on the moon — it varies from a few centimetres to over 15 metres around the world.
<a id="spring-tide"></a>Spring tide The biggest tidal range, occurring around new and full moon when the sun's and moon's pulls line up. Nothing to do with the season. A spring tide is also a faster tide — more water moving in the same six hours — so a spot with a narrow tide window comes good and goes off again quickly.
<a id="neap-tide"></a>Neap tide The smallest range, around the quarter moons, when the sun's and moon's pulls partly cancel. A lazier, more forgiving tide.
<a id="rule-of-twelfths"></a>Rule of twelfths A rough way to estimate how fast the tide is moving: of the total range, it moves 1/12 in the first hour, then 2/12, 3/12, 3/12, 2/12, 1/12. The practical point: the tide barely moves around high and low water, and races in the middle two hours.
The wave and the break
<a id="peak"></a>Peak The part of the wave that breaks first. Whoever is closest to it has right of way. Surf etiquette
<a id="a-frame"></a>A-frame A wave that peaks in the middle and breaks both left and right. Two surfers can take it, one each way — call your direction.
<a id="left-right"></a>Left / right Named from the surfer's point of view, looking at the beach. A wave that peels to the surfer's left is a left, even though it looks like a right from the sand.
<a id="shoulder"></a>Shoulder The unbroken part of the wave ahead of the breaking section. Where you ride to. Taking off on the shoulder in front of someone with priority is dropping in.
<a id="closeout"></a>Closeout A wave that breaks along its whole length at once, with nowhere to ride to. Common on short-period swell and at dead low tide on some beaches.
<a id="set"></a>Set / lull Waves arrive in groups — sets — with quieter gaps between them, the lulls. Watch a few sets before paddling out; the lull is when you go.
<a id="lineup"></a>Lineup Where surfers sit waiting for waves, just beyond where they break.
<a id="impact-zone"></a>Impact zone Where the waves are breaking, and the place you least want to be caught paddling out.
<a id="beach-break"></a>Beach break Waves breaking over sand. The most variable kind, because sandbanks move — a beach break's character can change within a season.
<a id="reef-break"></a>Reef break Waves breaking over rock or coral. Consistent in shape because the bottom doesn't move, but usually with a narrow tide window and a hard, shallow bottom. Tides explained
<a id="point-break"></a>Point break Waves wrapping along a headland or point, peeling in one direction, often for a long way. Usually the most forgiving across the tide.
<a id="slab"></a>Slab A wave breaking over a sudden, very shallow ledge — thick, heavy and dangerous. Not a beginner's wave in any conditions.
<a id="sandbar"></a>Sandbar A raised bank of sand where waves break. The gaps between sandbars are where the water drains back out — which is where rips form. Rip currents
Currents and safety
<a id="rip-current"></a>Rip current A narrow, fast channel of water flowing away from the beach — the drain for all the water the waves push ashore. Don't fight it. Stay with your board, float if you don't have one, signal, then move sideways out of it. Rip currents: how to spot one and get out
<a id="channel"></a>Channel A deeper stretch of water where waves don't break. Surfers use channels to paddle out — and at a beach break, the channel is often a rip.
<a id="cold-water-shock"></a>Cold water shock The involuntary gasp and loss of breathing control on sudden immersion in water below 15°C — which is most Irish water, most of the year. It affects fit, competent people; it's a reflex, not a fitness problem. Surf safety
<a id="float-to-live"></a>Float to Live The RNLI's instruction if you get into difficulty: tilt your head back with your ears submerged, relax and control your breathing, move your hands and legs gently, accept that your legs may sink, and hold it until the initial effects pass — under a minute. Then swim or signal.
<a id="beach-flags"></a>Beach flags Red and yellow = the lifeguarded swimming area. Black and white chequered = the zone for boards and hard craft. Red = don't go in. No flags usually means no lifeguard — not that it's safe.
<a id="leash"></a>Leash The cord attaching your board to your ankle. Safety equipment, not a convenience — it keeps your flotation attached to you. Check it before every session and replace it before it fails.
Gear and technique
<a id="duck-dive"></a>Duck dive / turtle roll Two ways of getting under a breaking wave while paddling out — duck-diving by pushing the board under, turtle-rolling by flipping it upside down and holding on. Which one you use depends mostly on the board's volume.
<a id="volume"></a>Volume A board's size in litres — the single most useful number when choosing one. More volume means easier paddling and easier wave-catching; less means more manoeuvrability.
<a id="ding"></a>Ding Damage to a board's glass. An open ding lets water into the foam and should be fixed before you surf again. Ding repair
<a id="flush"></a>Flush When cold water cycles through a badly fitting wetsuit instead of being held and warmed. Fit beats thickness — a loose 5/4 is colder than a well-fitting 4/3. Wetsuit care and repair
Forecasting and data
<a id="buoy"></a>Buoy A moored instrument measuring what the sea is actually doing right now. We show M6 and M3 readings on Irish spot pages. A buoy is a measurement; a forecast is a prediction — when they disagree, the buoy is right.
<a id="model"></a>Model The numerical weather and wave model that produces a forecast. Ours is Open-Meteo's marine model. It's physics, not opinion — but it can't see your sandbank, and it doesn't resolve local wind effects around cliffs and headlands. How SurfCheck works
<a id="nao"></a>NAO — North Atlantic Oscillation The pressure see-saw between Iceland and the Azores that steers the Atlantic storm track. Positive: storms aim at Ireland and the UK. Negative: they drop south towards Portugal and Morocco. The most useful medium-range signal European surfing has. European swell seasons
<a id="bathing-water"></a>Designated bathing water A beach formally monitored for water quality during the bathing season. Status and restrictions are published at beaches.ie. "Not designated" means not monitored — it doesn't mean poor quality. Sea swimming