Surf and swell glossary

Every word the guides use: what it means, and where the thing comes from.

Beach break: A wave that breaks on sand. The banks move with the storms and the tides: the peak changes place from one week to the next. It is where people learn.
Where it comes from
From sand that the currents lay down in banks. The swell breaks on the bank, then the wave rolls to the shore.
Buoy: A float anchored out at sea that measures the waves lifting it, and sends them every half hour. It is the real sea, not a forecast.
Where it comes from
From the measuring networks of states and institutes: every buoy Tonnr reads is named on the sources page, with its licence.
Chop: Small, short, messy waves raised by the wind of the moment, under 8 s of period. It cannot be surfed, or only badly.
Where it comes from
From local wind, which has blown neither long nor far: the waves had no time to grow or to sort themselves.
Cross-shore: A wind that runs along the beach. It gets in the wave's way without ruining everything, and pushes the surfer along the shore. In between, Tonnr says cross-offshore and cross-onshore.
Where it comes from
From the angle between the wind and the coast: the same wind is cross-shore here and offshore on the next beach, which faces another way.
Energy: What a wave carries, in one figure made of its height and its period, which the week's table writes in kilojoules per metre of crest. Two waves of the same height do not have the same energy if their periods differ.
Where it comes from
From physics: the energy of a wave grows with the square of its height and the square of its period. Double the period, and you quadruple the energy.
Exposure: How much swell a spot receives, depending on whether it faces the open sea or is sheltered: an exposed spot gets big as soon as a swell rises, a sheltered one waits for a big swell.
Where it comes from
From the shape of the coast and the bottom, which filter the swell. Two spots a few kilometres apart do not have the same exposure.
Falling tide: When the sea falls, from high tide towards low tide. It empties the spot, and the current flowing back out to sea can flatten the wave.
Where it comes from
The bulge is leaving. The current that flows out to sea works against the wave coming in.
Fetch: The stretch of ocean over which a wind blows in the same direction. The longer it is and the longer the wind lasts, the bigger and longer the swell.
Where it comes from
From the winter storms of the North Atlantic and the Pacific, which blow for days over thousands of kilometres: that is where the great swells come from.
High tide: The moment the sea is at its highest, before it falls again. The app writes it "high water", with its time.
Where it comes from
The bulge of water the Moon pulls passes your coast. On many spots, too much water over the bottom: the wave breaks softly.
Knot: The unit of speed of the sea and the air: one nautical mile an hour, 1.85 km/h. Ten knots is a breeze; twenty-five, a wind that ruins the session.
Where it comes from
From sailors, who measured the speed of the ship by counting the knots of a rope paid out behind it. Tonnr writes the wind in the unit you choose.
Lineup: The place, out from the shore, where surfers wait for the waves, where they start to break.
Where it comes from
From the surfers lined up facing the open sea.
Low tide: The moment the sea is at its lowest, before it rises again. The app writes it "low water", with its time.
Where it comes from
Between two bulges. The bottom is close: the wave breaks fast and hollow, sometimes too much, and rocks come out of the water.
Mid-tide: Halfway between low and high tide, when the water rises or falls fastest. It is the moment of most beaches.
Where it comes from
From the shape of the tide, which moves faster in the middle of its course than at either end: the sailors' rule of twelfths.
Model: A program that computes tomorrow's sea from the forecast wind, on a grid of a few kilometres, several times a day. A forecast is the result of a model.
Where it comes from
From the equations of the physics of water and air, solved over the whole planet by the weather services. Each computation, or run, starts again from the latest measurements.
Offshore: A wind that comes from the land and blows out to sea. It holds up the lip of the wave, which stands taller and hollower before breaking. It is the wind everyone hopes for.
Where it comes from
Often from the night. The land cools faster than the sea, and the air flows back down towards it: the land breeze of dawn, which drops as the sun warms things up.
Onshore: A wind that comes from the sea. It pushes the wave in the back, flattens it, and makes it break too early, soft and messy.
Where it comes from
Often from the afternoon. The sun warms the land, the air rises there, and air from the sea comes to take its place: the sea breeze, which picks up around noon in fine weather.
Period: The time between two crests, in seconds. It is the speed and the energy of the swell in one figure: a 14 s swell moves at over 40 km/h in deep water and feels the bottom much sooner than an 8 s one.
Where it comes from
From the distance travelled. Near the storm, every period mixes; far from it, only the long ones have arrived, because they travel faster.
Point break: A wave that breaks along a headland or a cape and peels in one direction, sometimes for a very long way.
Where it comes from
From a swell that arrives at an angle on a coast that juts out: it wraps around the point by refraction and breaks bit by bit.
Reef: A bottom of rock or coral. The wave always breaks in the same place there, often hollower than on sand, and the bottom does not forgive.
Where it comes from
A bank of rocks or a shelf the swell meets all at once: it stands up fast, hence its shape. Many famous spots are reefs.
Refraction: The swell bending as it nears a coast: it slows where the bottom rises, and turns to arrive square to the beach, or to wrap around a point.
Where it comes from
From the speed of the wave, which depends on the depth near the shore. The side that touches the bottom first slows down, and the wave pivots, like a car braking on one side.
Resolution: The size of the cells of the grid a model computes on: 25 km for a global model, 8 km for the wave model Tonnr reads, 2 km for a coastal one. A cell sees nothing smaller than itself.
Where it comes from
From the cost of computing: cells half as big mean eight times the computation. Fine models cover one region only.
Rising tide: When the sea rises, from low tide towards high tide. It pushes the swell towards the beach, and often makes things better.
Where it comes from
The bulge is coming. The current that flows towards the land helps the wave stand up.
Run: One computation of the model, started several times a day from the latest measurements. The forecast changes from one run to the next: that is normal.
Where it comes from
From the fixed hours of the weather services, often 0, 6, 12 and 18 universal time. Tonnr says when its forecast was read.
Secondary swell: A second swell, from another direction or of another period, that arrives at the same time as the main one. The week's table shows it apart.
Where it comes from
From two storms at once, or one storm that has turned. The two swells cross, and the sets come from their beat.
Sector: The direction a swell or a wind comes from, said as a point of the compass: a west swell comes from the west. Tonnr writes it with an arrow that points the way it goes, then the name of the point.
Where it comes from
From navigation, which has always named winds by their origin. Surfing kept the habit.
Set: A group of bigger waves that arrives after several minutes of calm, then goes. Surfers wait for them in the lineup.
Where it comes from
Two swells of close periods overlap: when their crests coincide, the waves add up; when they oppose, they cancel out. Sets are that beat.
Shadow: The part of the sea an island, a cape or a peninsula hides from a swell: behind them, it arrives weakened or not at all.
Where it comes from
A swell goes straight. What lies in its path receives it; what is hidden behind an obstacle only gets what refraction brings back.
Significant height: The height buoys and models announce: the mean of the highest third of the waves. One wave in ten is above it, one in a thousand is almost twice it.
Where it comes from
From oceanographers, who found that this figure matches what a sailor estimates by eye. The biggest waves of a set are above it.
Spring tides: The days when the tide rises highest and falls lowest, every fortnight. In France, the coefficient says them in one figure: 95 and above.
Where it comes from
From the Moon and the Sun lined up, at new and full moon, pulling the same way. At quarter moon, the neap tides: the tide barely moves.
Swell: Waves that have left the wind that made them and travel on their own, regular, over hundreds or thousands of kilometres.
Where it comes from
A storm blows for days over a stretch of ocean, the fetch. The wind raises a choppy sea; as they leave it, the waves sort themselves by speed, the longest in front. What reaches your beach, clean and orderly, is that sorting. The further and stronger the storm, the longer the period.
Swell window: The part of the horizon a spot sees from its beach: only the swells that come from it reach it. On a spot's page, it is drawn as a ring.
Where it comes from
From the shape of the coast. A headland, an island or a bay hide a part of the horizon, and the swell that comes from there passes offshore or arrives weakened.
Take-off: The moment you stand up on the board, at the top of the wave as it rises. The wind, the tide and the bottom decide whether it is easy.
Where it comes from
From English, like most of surfing's words.
Tide: The rise and fall of the sea, twice a day. It changes the depth of water over the bottom, and so the wave.
Where it comes from
From the Moon and the Sun, which pull the water of the oceans. The Earth turns under that bulge: the sea rises and falls about every six hours, a little later each day.
Wind sea: The waves raised by the wind blowing right now, where you are: short, messy, of brief period.
Where it comes from
The wind pushes the surface of the water and wrinkles it. As long as it blows, the waves stay under it, choppy. Once it drops or they move away, they become a swell.