Every break receives swell through a certain window, and no two breaks have exactly the same window. That is the whole answer, and the rest of this piece is about learning to see it.

To get there, we will borrow the standard physicist’s trick for problems too messy to solve outright: solve a cartoon of the problem first, then correct the cartoon in layers. The first pass is deliberately too simple. Each pass after that adds back one piece of ignored reality, and you stop when the answer is good enough for the decision in front of you. The jargon for this is orders of approximation: zeroth order for the cartoon, first order for the first serious correction, second order for the fine print, and higher orders beyond that. When a physicist calls a number “right to first order,” they mean it is the napkin answer: not exact, but close enough to act on, and honest about what it ignored.

That is where this series gets its name. First Order is a set of interactive tools, each built around one question surfers argue about, each answered in layers you can see and drag. The articles walk up the ladder; the tools let you play with it. This first one takes on the question in the title, because swell direction is the corner of forecasting where the napkin answer does the most work. To make it visible I built an interactive map of the Southern California Bight, the long bend of coast between Point Conception and San Diego, over its real seafloor, for ten of its well-known breaks. Every number below comes from it, and you can check each one yourself: open the explorer →

Zeroth order: swell goes straight

A storm is a wave generator parked over deep water. For a day or two it drags on the sea surface, piling chop into bigger and bigger waves, and when those waves finally outrun the wind that made them they become swell: organized, long-crested, and pointed wherever they were heading when they escaped. That direction is fixed at the moment of escape, like a bullet out of a barrel. Deep water barely touches a swell afterward. A 14 second swell covers about 940 kilometers of open ocean a day, and left to its own devices, does not turn. Whatever heading it left the storm with is the heading it arrives with, days and thousands of kilometers later.

So the simplest model of surf, the zeroth order cartoon, is a ruled sheet: parallel crest lines sweeping across the map from wherever the storm sat. Most of a forecast lives inside this model. When the chart says 285 degrees at 15 seconds, it is describing the simplest picture. The first number is where the lines come from. The second is their period, the time between one crest and the next.

First order: draw the sight lines

From the lineup, let’s imagine drawing a line straight out to the open ocean and swinging it across the horizon. The arc where that line reaches deep water without running into anything is your swell window.

Everything you ever surf (I know, I know: rivers, cruise ship waves, weird quirks aside) comes through it. Whatever sits inside that arc, an island, a headland, the bend of the coast itself, blocks the swell that would have arrived from behind it. The window is a property of the place, not the weather. It doesn’t change with what the buoy is reporting, and on any timescale that matters to a surfer, it never changes. A swell either comes from inside your arc or, to first order, it does not exist for you.

To a wave, an island is a wall. Swell cannot pass through rock, so a body of land drops a shadow on the water behind it, stretching downswell the way a building blocks the sun, and the Channel Islands are walls that run tens of kilometers end to end. A break parked in one of their shadows misses out on swells from a huge patch of ocean offshore. Not weakened, gone, at least in the straight-line version of the story.

The explorer draws this for ten breaks, using the islands’ real outlines traced from the bathymetry, with every sight line starting at the sand, and the spread is hard to ignore. Oceanside and Lower Trestles tie for the widest window of the ten, 88 degrees each. Long Beach sees 42, and most of what it lost sits in plain sight: Palos Verdes, the breakwater, and Catalina stacked across its horizon. If you have ever wondered why Long Beach is the flattest town on this coast, its window is the answer.

The blocking wall that matters is often the one down the beach. Point Dume covers a wedge of Malibu’s west. Dana Point takes Salt Creek’s south. La Jolla eats swell south of Blacks. None of these headlands is large by island standards, but blocking is about the angle a wall covers, not its size, and a point a few kilometers away covers more horizon than an island forty kilometers out, the same way your thumb at arm’s length covers the moon.

Baja California runs about 1,200 kilometers past the southeast corner of the frame in our tool, but it still closes up to 11 degrees of window here, Malibu most of all. Putting every wall together, no single direction reaches all ten breaks at once. At most, a particular swell hits seven out of ten. At 177 degrees, the best the south can do, the three missing breaks are El Porto, Salt Creek, and Blacks, every one of them shut out by its neighboring headland rather than an island. The wave model W. C. O’Reilly and R. T. Guza built for this bight, the one CDIP still runs, treats the islands as near-solid blocks, because for the directions that matter here that is what they are.

The ten breaks are just samples. Computing the window every two kilometers of shoreline helps us tell the story for the whole coast at once. Reading the figure below left to right is like you are walking the coast from northwest to southeast. Above Point Conception the coast stares at about 180 degrees of open Pacific, the half of the compass not blocked by the state of California. Around the corner into the bight, the window collapses, bottoming out at 41 degrees along the Santa Barbara waterfront, the most sheltered stretch of open coast in Southern California, pinched between Conception wrapping off one shoulder and the four northern islands fencing the other. The water inside the Long Beach breakwater rounds all the way to zero, though that one is by design for ships. From there the window rebuilds slowly, reaching about 107 degrees at Point Loma. Santa Barbara’s summer flat spells are pretty easy to conceptualize with this one figure.

The straight-line model also makes a claim that should bother you. From 300 degrees, the classic winter west-northwest, it reaches zero of the ten breaks. Every one of them hides behind Conception, a headland, or an island from that angle. Yet west-northwest swell is the backbone of a Southern California winter. A model that declares the season’s busiest direction impossible is missing three things. The first is about what actually arrives: a storm is not a point on the map but a weather system hundreds of kilometers wide blowing for days, so the swell it ships is a fan of directions 20 to 40 degrees across, not a single line. The 300 on the chart is the middle of the fan, and the fan’s southern flank reaches down toward whatever slivers the windows leave open. The other two are physics the straight lines ignore: shadows are not hard-edged, and the seafloor steers. Those are the higher-order corrections, and they are the rest of this piece.

The first correction: shadows are soft

Waves do not travel in perfectly straight lines. They bend into the space behind an obstacle, a process called diffraction, and you already know it intuitively from sound: you can hear someone around a corner you cannot see them through. Sound waves are meters long and bend easily around obstacles their own size; light waves are a millionth of that and barely bend at all, which is why you can’t see around those same walls. Ocean swells, with wavelengths in the hundreds of meters, live on the sound side of that divide.

Penney and Price worked out the math for water waves in 1952, borrowing the optics of light slipping past a straight edge, and their results are very useful to us. Right at the edge of the ruler-drawn shadow, a wave stands at about half the height it carried in the open, and it keeps dropping the deeper into the shadow you go. Real seas do a little better than the math, because a real swell arrives as that fan of directions rather than a single line, and parts of the fan slip past the island’s edge at friendlier angles; accounting for the fan and the shadow’s edge pushes it closer to seventy percent of open height. Either way, a shadow is not a clean wedge of dead water. It is darkest straight behind the island and feathers out toward the edges. I did some work on a project looking at the diffraction of internal waves around an island, and I can tell you what was told to me when I got stuck: diffraction is difficult. We’ll leave the math here for this one because it’s gnarly.

How far the swell bleeds into the shadow depends on its wavelength. Deep-water wavelength grows with the square of the period, from about 156 meters at 10 seconds to 625 at 20, and a wave bends more when its wavelength is large compared to whatever is blocking it. An island tens of kilometers across always throws a real shadow, but a 20 second groundswell wraps into a lee that a 10 second windswell runs past. Long-period south fills corners that short-period south never reaches.

The second correction: the seafloor steers

A wave in deep water does not know the bottom exists, because a wave is not just the moving hump you see. Under every crest the water is circling, and the circles shrink with depth until, about half a wavelength down, the motion dies out. If the seafloor sits below that depth, the swell passes over it untouched. If the seafloor pokes up into the circles, the wave feels the drag and slows. This is the basis of refraction.

A 6 second swell, wavelength 56 meters, wakes up to the bottom in about 28 meters of water, 90-odd feet. In the grand scheme of the ocean, this is practically at the beach. An 18 second swell feels bottom in 250 meters, which in this bight means it is already steering as it threads between the islands. And where one end of a crest slows before the other, the whole crest pivots toward the shallow side, the way a shopping cart with a janky wheel pulls towards the jammed side. Long swells begin turning tens of kilometers out. Short swells run straight until the final moments.

To show this in the explorer, I’ve actually coded this process backwards. Wave paths are reversible, the route from deep ocean to the sand is the same route walked in either direction, so the explorer traces rays outward from each break across the real seafloor and keeps every path that reaches open water. This allows you to pick a break and see the whole family at once, every route a swell of that period can take to get in.

Try it: drag the direction and the swell’s route goes bold. Adjust the period and the fan reshapes a bit: some routes detach at long period, bending off grazing approaches before they arrive, and new ones appear where the seafloor wraps swell around a corner. At 18 seconds, Long Beach picks up west swell bent around Palos Verdes from directions no 6 second swell can deliver.

Below, I show the same physics run forward, one swell at two periods: every ray pair starts together offshore, and by the time they cross the island shelves the 18 second rays have peeled away from their 6 second twins, hooking into the Santa Barbara Channel while the short-period rays run straight past it.

Putting the whole coastline on one line, the first order picture and the second order corrections snap together. Let’s trace a realistic 18 second swell from the SSW, a fan spread across 190 to 200 degrees rather than a single bearing, and log the energy the rays deliver along the shore, scored against an open-coast baseline: 1.0 is what the same swell would hand a straight beach with nothing in the way. Where the windows are open, from Trestles through Oceanside, the fan lands at essentially full strength. Where the islands stand in the way, the model delivers almost nothing: the Santa Barbara and Rincon stretch reads near zero, and Catalina drops a second hole over Santa Monica Bay. Those zeros are the point. The real ocean is never that dark, and the reason is the leak from the previous section: diffraction backfills the shadows the ray model leaves empty. The gap between this curve and what the buoys record on a real south swell is diffraction.

The winter mystery resolves the same way, piece by piece. The fan does the first part of the work: a west-northwest storm’s southern flank reaches the slivers the windows leave open near 263 to 285 degrees. Diffraction picks up the second part, feathering energy past the island edges into the lees. Refraction does the last, turning what survives toward the beach as it crosses the shelf. None of these paths exist in the straight-line picture but are important to consider for realistic modeling.

As fun as this project is, I need to share a word on what the explorer is not. It is geometry, not a forecast: it does not tell you what will be working today, tomorrow, or any specific time. The ten breaks are measured from the sand while the coast-wide curve is measured from a standard three kilometers out, the traced ray paths start just offshore so the last stretch to the beach is not modeled, diffraction is not drawn (the windows are not as stark as they seem here), and the energy numbers are relative to an open-coast baseline, not wave heights. We are also completely ignoring currents, which do affect waves. When it disagrees with the buoy, trust the buoy: that is the current wave state. Note: I am not saying the models derived from the buoys, I am saying the buoys themselves. The windows, though, are just geography, and geography does not update: explore your break’s window →

To end this on a concrete note, the sharpest version of all of this is two south swells in the same stretch of summer. One born off New Zealand and one born off South America both showing up out of roughly the south, but the few degrees between their approach angles decide your surf. One threads the gaps and lights up the western channel while the other lands almost wholly in the islands’ shadow, shut out from the west end before it reaches the coast. What gets through is set by where the swell was born.

This is the first post of our First Order series. Since tools will accompany each of these, they will be sporadic and only written when complete. All tools are free and open-source with the goal of providing resources for the curious surfer. I hope you have explored and enjoyed the tools. Check it against your favorite forecasting platform next time and let me know how it does.

Further Reading:

Penney and Price paper

O’Reilly and Guza paper

Buoy observations

Bathymetry data

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The arsenal itself has warmed the ocean by nothing you could measure, which is the good news. The heat came from the sun. In the instrumental record, 2024 was the warmest the global sea surface has ever been.

In 2025, La Niña spread across the Pacific and pulled the surface back down, about a tenth of a degree below 2024, taking it to the third-warmest year instead of first. Underneath, the heat content of the upper two kilometers kept climbing to a new record.

In February 2026, NOAA changed how it defines El Niño. The old index compared a patch of the tropical Pacific to a fixed thirty-year average, but the whole tropical ocean has warmed so far that the fixed average kept flagging ordinary heat as an event. The new one subtracts the background warming first and measures only what stands above it. When you have to redefine the baseline to keep a word meaningful, that tells you something about the baseline.

Warm water is bigger water, and a warmer ocean is a taller one. In 2024 the global sea surface rose 0.59 centimeters against an expected 0.43, and for once the reason was mostly expansion rather than melt. In a normal year about two-thirds of the rise is meltwater off the ice sheets; in 2024 about two-thirds was the ocean swelling as it warmed and became less dense.

Interestingly, 2025 sea level barely budged, a mere 0.08 centimeters. The same La Niña that cooled the surface dumped its rain over the Amazon and a few other basins, and for a few months the land held onto water that would otherwise have been in the sea.

Cumulatively over the two years, the ocean grew by about 2,400 cubic kilometers. Poured over California, that is a layer almost six meters deep, coast to state line.

Down at the bottom of the planet, the ocean did the opposite of what was generally forecasted. The projections all had the Southern Ocean surface freshening as ice melted into it. Instead, since about 2015, the surface south of 50 degrees has been getting saltier. Alessandro Silvano and colleagues laid this out in PNAS this year, pairing satellite salinity with float measurements, and over that same stretch the Southern Ocean has shed sea ice on the order of the area of Greenland.

A hole in the ice near Maud Rise reopened around the same time, the first time it has done that since the 1970s, which is tied to the same saltier, less layered surface. This is the part that got mangled in the press as an ocean current reversing, essentially the plot of the movie “The Day After Tomorrow.” No current reversed. A decades-long freshening trend reversed, which is a different and much less impactful sentence. The saltier surface mixes more easily, letting heat from below reach the ice from underneath.

The Antarctic Circumpolar Current runs about 173 Sverdrups through Drake Passage, a Sverdrup being a million cubic meters a second, which works out to something like 10 million cubic kilometers of water shoved past a single point every two years. Putting the two numbers side by side: the entire two-year rise in global sea level, all 2,400 cubic kilometers of it, would slide through Drake Passage in under four hours.

The coolest thing the ocean did in these two years, in my opinion, it did on December 21, 2024. A storm in the northwest Pacific threw up a sea state so large that the SWOT satellite, passing overhead, measured a significant wave height of 19.7 meters. That is the largest wave height ever measured from orbit in 34 years of satellite altimetry, past the previous record set in 2011. Fabrice Ardhuin and colleagues reported it in PNAS, and from those numbers the biggest individual waves in the storm were likely over 35 meters, taller than a ten-story building.

The storm didn’t have a name, because storms in that corner of the Pacific don’t get them, so the scientists named it themselves. They called it Eddie, because the swell it made ran the Eddie Aikau at Waimea Bay the next day. The day after that, the same swell lit up Mavericks for what people are still calling one of the best days that wave has ever produced.

SWOT and the modelers tracked it radiating out for 24,000 kilometers, running south down the eastern Pacific, slipping through the gap at Drake Passage, and curling up into the tropical Atlantic more than two weeks after the storm that made it had already died. This wave jumped oceans.

On June 11, 2026, NOAA named a new El Niño and gave it better than even odds of turning strong by fall. El Niño winters tend to drag the North Pacific storm track south toward California, which usually means more swell, and more sand pulled off the beaches by spring. This one might do all of that. With all of the surprises these past years, who knows what’s in store for us in the next two.

Thank you for two years.

Further Reading:

Sea ice paper

Large wave paper

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Sunlight arrives at Earth in parallel lines. Near the equator it lands close to head-on, and a beam of a given width warms a patch of ocean about that same width. Toward the poles, an identical beam comes in at a slant, spread thin across a much larger patch. Same energy and more area, so less heat per square meter of water. Averaged over a full year, the equator takes in roughly two and a half times the sunlight the poles do at the top of the atmosphere, and the gap at the sea surface is wider still, because slanted light also has to punch through more atmosphere to get there and tends to glance off the water rather than soak in.

To complicate the explanation, the atmosphere and the ocean spend every hour of every day hauling heat out of the tropics and dumping it toward the poles. Warm air climbs, storms carry moisture and heat north and south, currents drag tropical water up the western edges of ocean basins, and eddies peel off those currents and carry warm water further still. Near 35 degrees latitude, where the transport peaks, the combined system moves something like five and a half petawatts poleward, a frankly unfathomable amount. Every power plant, engine, and furnace humans operate, added together, runs a couple hundred times short of that. If this were the only fact you knew about the ocean, you’d expect the whole thing to have evened out a long time ago.

All of that circulation makes the poles warmer than they would otherwise be, not colder. Shutting it off would cook the tropics while the poles froze harder than they already do. The temperature difference between the water off Baja and the water off Alaska is a push and pull of the heat imbalance and the restoring currents. The system never finishes, because the sun refills the imbalance every morning. Weather and currents moderate the temperature difference between poles and equator.

This is the general trend of the system, but there are exceptions around the world. Northern Europe is mild for how far north it sits because warm water is delivered to its coastal waters. California is cold for how far south it sits because northerly winds push surface water offshore and cold water rises from below to take its place, a process called upwelling. There is a tongue of genuinely cold water sitting directly on the equator in the eastern Pacific, which sounds like a mistake. None of it changes the trend. Average around the globe at a fixed latitude and the temperature lands where it always has (global warming waters aside). The same holds over time. Weather moves the water temperature day to day, eddies week to week, and El Niño year to year, but averaging enough of them together and the leaves the pattern unchanged.

Looking at the ocean as 3D helps close the story. A thousand meters below the surface anywhere in the open ocean, the water sits within a few degrees of freezing, including directly beneath the equator, underneath the warmest surface water there is. That deep water was made at high latitudes. It got cold, got heavy, sank, and crept out along the bottom of every basin until it filled them. The warm water we surf in is a thin skin stretched across the top of a fundamentally cold ocean, and the poles are what put the cold there.

So the water gets colder toward the poles because the planet is round and the sun is far away, and no amount of churning changes the angle at which the light shows up. The currents, the storms, the eddies, the heatwaves, all of it is the ocean mitigating this imbalance.

Further reading:

Solar radiation

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Broken-wave water is full of air. Void fraction, the share of a volume that’s air rather than water, can reach roughly 65% in the collapsing cavity just after the lip lands. It holds above 20% through the mixed region below, and it lingers, because bubbles take time to rise up and out. For a stretch of seconds, you are immersed in a seawater-foam mixture.

This mixed fluid has a different density than straight seawater, significantly lower. In still water, a wetsuited surfer with full lungs runs a little less dense than seawater, which is the reason we float. But if the density of the surrounding water drops by a fifth, the margin can shrink or even flip. Water/foam beats surfer.

But less float doesn’t really explain the feeling of being stuck near the bottom, waiting for permission to leave. A hard plunging wave does drive some air down with it, enough to dent your buoyancy at depth, but the foam thins the deeper it goes and can’t account for all of it. So what is actually holding you down there? In shallow water, where the column is thin enough for the wave to reach the bottom, it comes down to where the wave dumps its energy.

When a wave pitches out and lands, it throws a jet of water down into the column ahead of it. In a strong plunging wave, the vortices that the jet spins up penetrate nearly the whole depth as soon as the wave breaks. The vertical velocities inside these bursts run larger than the wave’s own orbital motion, an order of magnitude above the turbulence ordinary bottom friction makes, somewhere around 1 m/s in surf-zone measurements. This speed is in the same range as the speed your buoyancy lifts you, so it is possible that it actively pushes you down.

This is likely why two waves of the same height treat you so differently. A wave that stands up and throws its lip forward, the way it does over a shallow reef or a slab, drives that jet deep and stirs the whole water column at once. A wave that slumps and slides down its own face on a gentle sandbar spreads its turbulence slowly and weakly, and leaves the water near the bottom mostly alone. Face height is the factor that many would assume to be the biggest driver, but whether you get parked on the bottom also depends on how the wave breaks, which is mostly a question of what the seafloor looks like where it does. 3ft shore breaks have hit me way harder than 8ft crumblers.

In summary, the answer is two-fold. Waves plunge water and air down into the water column, both actively pushing you down and taking away your buoyancy at the same time. Just remember to count in your head, it’s never as long as you think down there.

Further Reading:

Void fraction

Velocity paper

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A fin moves through water at a Reynolds number of a few hundred thousand to half a million. In most basic terms, the Reynolds number is a number that allows us to predict how a fluid will flow at certain speeds, viscosities, densities, etc. That number fixes the thickness of the boundary layer, the thin film of water that clings to the fin and gets hauled along with it. For fins on a surfboard, that layer is about a millimeter thick.

Anything sharper than that millimeter is buried inside the boundary layer. Water flowing past cannot tell a honed edge from one rounded off with a few passes of sandpaper, because both sit beneath that film, down in the slow viscous layer the free stream never directly touches. So you can polish the edge to a razor and the water keeps seeing the same blunt millimeter of sluggish fluid wrapped around it.

There is one real catch, and it sits at the trailing edge. A thick, blunt back end drags, so thinning that edge genuinely helps. But the gain is about thickness, not sharpness. Once the trailing edge is down to roughly a millimeter, it is as fast as it will ever be. Grinding it past that into a blade does nothing the water can feel, and only leaves you an edge that chips on the first rock.

On the other side of the fin, a sharp nose makes the water whip around a tight corner, and the flow peels off the fin too soon. The fin loses its grip and the tail slides out. Rounding the nose off causes the water to bend around it smoothly, so the fin holds longer and lets go gently when it finally does, instead of all at once.

The rails are no different from the trailing edge. They sit under the same millimeter of clinging water, so sharp or rounded, the flow cannot tell. The tip is the only place a point even looks useful, because that is where water spills around the end and spins off into a little vortex. But how strong that vortex gets depends on the size of the tip and the angle it sweeps back at, not on whether the last few millimeters come to a needle.

So, let’s stop making knife blades on the bottom of boards, shall we? There is little benefit and boy can it hurt you or others.

Further Reading:

Reynolds number

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A big set rolls through, and for about a minute after it passes, the surface where it went is glassy. The wind chop is just gone. Then the wind rebuilds it and we’re back to the chop. The wind never let up the entire time, so it isn’t that causing it. It really does look like the big wave annihilates it.

My first thought is surface tension. Think of pulling a wrinkle out of a sheet by stretching both ends. But, it just can’t be the answer. Surface tension only governs the smallest ripples, the capillary waves shorter than about 1.7 centimeters, where the water really does behave like it’s straining against itself. The chop getting wiped out is bigger than that.

Next thought, Doppler shifts. A big set shoves a mass of water toward the beach. A current running with the chop will stretch the wind waves out longer and lower, which is a real effect. The problem is two-fold. The surge, and the backwash that follows it, relax in seconds, ten or twenty at most, while the glass hangs around for a minute or two. And stretching the chop doesn’t erase it. You’d be left with longer, mellower waves, not the flat water we actually see.

Now, we aren’t totally in the dark, because we know how to make glassy water in other situations. Convergence zones pile up the thin organic film on the surface and damp out the ripples, the same effect that draws those slick lines off internal waves. But it doesn’t give an obvious reason for a one-minute patch that shows up the instant a set breaks. There’s nothing about a passing set that should concentrate and hold a film against a wind that’s actively working the surface.

As of now, my thinking is focused on bubbles. When the set breaks, it drives a plume of air down under the surface and leaves a raft of foam behind. Bubbles near the surface can damp short waves, and this is the guess whose timescale actually fits, since a bubble raft lingers for tens of seconds. These trapped air bubbles attenuate small surface waves by dissipating their energy, knocking down the wind chop.

There’s even precedent for this idea, just in a different context. One proposed explanation for how rain calms the sea is exactly this, the drops forcing air bubbles under the surface to soak up the chop. But, nobody has measured it in the surf, and if bubbles were the whole story the glass should sit only on the bubbly water, which isn’t always where you see it. As far as I can tell at least.

Each of these potential ideas has a hole in it, and it seems to be an open question. So keep an eye out for this smooth lull and let me know if you have any ideas as to what is causing it.

Thank you all for reading 100 of these. I’m excited to continue on with more surf science and welcome any questions you’d like answered.

Further Reading:

Rain paper

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Emerita analoga, aka sand crabs or sand fleas, live buried in the swash zone, dug in tail-first and facing the water, with only its eyes and a pair of antennae poking above the surface. Each time a wave drains back over it, the crab uncoils a second set of feathery antennae and combs the backwash for plankton. They migrate up and down the beach with the tide to stay in that band, and on the right fine-sand beach they pack in by the thousands per square meter, so one bed can stipple a whole patch of sand with V’s.

Backwash thins and speeds up as it runs down the slope, and by the time it’s a sheet a few millimeters deep it’s moving fast enough that anything sticking up throws a standing wake, the same way a rock in a quick creek kicks up a fixed little peak. It holds in place because the water is draining downhill faster than the wake can climb back up against it, so the disturbance gets pinned right where the obstacle is. The flow scours a shallow horseshoe around the crab and trails that wake behind it, which is why the point of the V sits at the animal and the arms open seaward. Finding bait is pretty easy when they’re around.

But most of the V’s on the sand aren’t crabs. Walk ten feet up the face to where the sand is tiled edge to edge with the same diamond pattern and start checking the points and you’ll find nothing there. No crab, no shell, no pebble, no bump at the apex of any of them. These are rhomboid marks, and they don’t need an animal, or an obstacle of any kind. The water makes them on its own.

As the thin sheet of backwash drains down the slope, it organizes itself into a repeating lattice, the way fast shallow water patterns up into a regular grid instead of running smooth. The draining flow sets up standing waves, with the angle of the diamonds tied to how fast the sheet moves against the slope. The whole field freezes the direction of the last sheet to leave the beach.

In order to figure out what caused the shape, check the apex. A V with a little lump at its landward point, especially a few of them clustered together, is something solid sitting in the flow, usually a crab, sometimes a shell or the tube of a buried worm. A clean grid of V’s across open sand with nothing at any of the points is just the backwash itself.

Further reading:

Sand crabs

Backwash

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Turbines pull momentum out of the air, and downwind of the array sits a wake of slower wind that can run for tens of kilometers. Less wind over the water means less energy feeding the waves growing under it. The modeling puts the result at a few percent off the wave height near the farm, fading to almost nothing within twenty kilometers or so. It lands mostly on the local chop being generated right there, not on the long-period groundswell that was generated far away.

A bottom-mounted turbine sits on a single pile maybe ten meters across. Setting that against a swell whose crests run two or three hundred meters apart, the pile is a pencil standing in a doorway to the shelf. It scatters a sliver of energy, nudges the direction a touch, and the effect on wave height comes in under a few percent.

A few hundred piles in tidy rows looks like it should reflect waves back out to sea, the way evenly spaced bars on the seabed bounce a swell (Bragg reflection). The usual dismissal is that the spacing is wrong, that piles a kilometer apart would need a two-kilometer wave to resonate and surf is far shorter than that. The dismissal itself is wrong, however. The reflection also happens at higher multiples of the spacing, so a coarse grid does not fail to resonate, it just resonates with the swell at a higher order.

The piles block maybe a percent of each row, the rows are never perfectly spaced, and real swell shows up smeared across a band of periods and angles. Sharp reflection needs a dense, precise lattice and one clean wavelength, and a wind farm is none of those. A little energy comes back at a few specific frequencies, but most slips through.

So for the farms standing on the seabed today, the swell crosses the array and arrives about the way it left. If anything the water is slightly cleaner, since the wind the farm skims off would otherwise be stacking up chop.

Floating turbines are a bit of a different story. They do not stand on a pole, they ride on a platform of fat, half-submerged columns, and a column that wide compared to the wave does not diffract neatly. Idealized modeling has a single floating platform cutting the wave height behind it by more than half under the right conditions, with a shadow trailing about a kilometer downwave.

Whether a shadow a kilometer long still matters several kilometers down the line is genuinely unsettled, tangled up in how fast the wave field heals around the gap. The waves we ride should mostly survive the steel bolted to the seabed. The ones riding on top are worth watching.

This is an article based on a comment from Dr. Lonneke Goddijn-Murphy of SeaLetters on a previous article. If anyone has a question they want answered, I’d love to hear it.

Further reading:

Wind farms

Wind farms affecting surface waves

Floating wind farms

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At Scripps, my research focused on internal waves on the inner shelf, the region of the coastal ocean between roughly 10-100m depth and home to lots of physical and biological processes. I studied two specific types of internal waves: superinertial coastal trapped waves (CTWs) and shoaling non-linear internal waves (NLIWs).

The first chapter is a rewrite of my first peer-reviewed paper, where I describe how these CTWs drive stronger currents than the tides do. The paper answers a question going back nearly 60 years in this region: why is the sea surface elevation at the semidiurnal frequency not correlated with the currents on the inner shelf?

To follow up this paper, we ran a nonlinear model that showed these waves are coherent over long distances, as in 100s of kilometers. We also reanalyzed the data to show that the inner shelf currents can actually be predictable, finding that more than half of the current 90 days out is explainable by this CTW signal. This paper is currently in review and one that I feel very proud of.

And to close out the dissertation, the final paper pivoted to detection of the NLIWs on a fiber optic instrument. Using machine learning algorithms from self-driving cars, I built an algorithm that automatically detects these waves on the instrument in real time and used it to show how these shoaling waves lead to a significant portion of the mixing on the inner shelf. This one will be submitted soon as I finish up a postdoc at Scripps.

All together, these three chapters add a bit of understanding to the inner shelf dynamics of the Southern California Bight, with applications to other regions where similar signatures have been observed: Tasmania, Hawaii, etc. for the CTWs and the Oregon coast, offshore of Japan, etc. for the NLIWs.

It has been a process of ups and downs, wave jokes a necessary evil in the field. This is one of the ups and it is nice to share some of the work that I have done with you; I hope you enjoyed this one.

Further Reading:

When my dissertation is published, I will send it to anyone interested.

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The gray was never really about cold water by itself. It requires cold water at the surface and a band of warmer air parked on top of it like a lid. The bigger the temperature jump between the two, the stronger the lid, and the better it traps the damp air underneath that cools and condenses into our morning cloud. The cold water matters mostly because it widens the gap, which is why our coast, fed by chilly northern water, gets so much more of this than the warm-watered East Coast ever does.

Warming the surface closes the temperature gap some. A smaller jump is a weaker lid, and a weaker lid holds less gray. So yes, warmer water should mean less marine layer. But the honest version of this argument is that the water is not dimming the clouds directly. It is loosening the lid that lets them pool near the coast. Warming waters look like the cloud base creeping higher, until some days it never really commits at all. The sun normally has to chew through a thick gray ceiling but meets a thinner one, and clears it earlier in the day.

You can watch this dial move across a single summer without any heatwave involved. June is the grayest month along the Southern California coast, then the gloom starts surrendering through July. A good chunk of that is the local water warming as the season runs on, and warming faster here than it does up north. Up in Northern California, where the water stays cold longer, the grayest stretch doesn’t show up until July, a full month behind us. The effect happens every year on its own without a heatwave, so a heatwave just turns the same knob harder.

So should the warmest ocean on record hand us a clear June this spring? The 20.6°C headline is mostly a northern, open-water number, with the core of the heat sitting up toward the Gulf of Alaska across something like eight million square kilometers of sea. May Gray answers to a much smaller patch, a thin band of water right off our coast. The California Current still drags cold water down from the north, and upwelling keeps pulling more up from the deep, so the coastal strip can hold its own temperature while the rest of the basin cooks.

It is plausible we get a shorter June Gloom this year, if that warm anomaly works its way down the coast and settles into the water off the Bight. But it leans on a lot of moving parts. Where the warm water actually pools, how hard the coastal winds blow, whether upwelling keeps the nearshore cold, and whether the El Niño now building on top of the heatwave nudges things one way or the other. Whether our particular stretch of coast gets to tan this spring is a toss-up.

Further Reading:

Marine layer

Warmer waters

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A 17-second North Pacific groundswell can hold waves at the coast for a week, the dominant period stepping down a second or two each day. A 9-second windswell can show up and be flat within a day or two. The difference between the two has to do with the generation offshore rather than anything at the break itself.

Wind dragging on the ocean surface starts by making short, choppy waves at the top of the spectrum. That wind blowing over a big enough patch of ocean works the surface into longer, bigger waves. A 20-second peak only emerges after days of sustained generation. Shorter storms only output shorter periods.

Back to the big storm: while the long end of the spectrum fills in slowly, the wind is still making 14-second, 12-second, 10-second, and shorter waves at the same time. We talk about “an 18-second swell” because that’s the dominant period at the buoy, but the whole mess is leaving the storm together.

Walter Munk’s 1963 work tracking Southern Ocean swells to Alaska showed how dramatically period controls travel speed in deep water. A 20-second wave moves at about 35 mph, but a 10-second wave moves at just half that. The further the waves travel, the more they spread out in time. Because of this, a storm 4,000 miles offshore sends its 18-second leading edge to the buoy more than a day before the 12-second component arrives.

But when the buoy reads 8 to 10-second primary with no long-period leading edge, we can expect a shorter run of swell. The storm didn’t blow long enough or over enough fetch to develop the longer period swell, so there is a smaller window of time to surf it. Short-period waves also dissipate much faster as they travel. The longer end of the wave spectrum crosses a basin without much energy loss, but the short stuff burns itself out within a few hundred miles. Most short-period swells at the coast are generated locally in the past day or two, not days to weeks before like the long period.

The one exception is a productive storm that sits close to the coast. North Pacific lows in winter sometimes track right past California, throwing 15 to 17-second swell from a few hundred miles offshore. The storm still had to last long enough to make those periods, so the spectrum is still there. But without much ocean distance to do the dispersion sorting, the longer and shorter components arrive bunched together. The event fires for a day or two and disappears. There are plenty of waves, just no slow tapering tail. Gotta get ‘em while they’re hot.

Further Reading:

Dispersion

Spectrum

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Behind your face are four pairs of air-filled cavities called the paranasal sinuses: maxillary under each cheekbone, frontal above the eyebrows, ethmoid between the eyes, and sphenoid tucked deep behind the nose. Each is lined with mucus and tiny hairs called cilia that constantly sweep debris toward the throat. They connect to the nasal cavity through small openings called ostia, typically 1 to 4 mm wide, about the width of a wooden pencil tip.

The maxillary sinus is the biggest of the four, roughly walnut-sized, and its drainage ostium sits not at the bottom of the cavity but near the top of the medial wall. The official medical phrasing is “poorly positioned for free drainage.” The frontal sinus is the other troublemaker, with what the literature calls the most complex and variable drainage of any paranasal sinus, routing everything through a twisting frontonasal duct before it can reach the nose. Basically, the drains are on the ceiling for both of these.

Physics is a pesky part of this problem as well. Water in a tube only a few millimeters across forms a concave meniscus, the molecules at the surface clinging to the walls and to each other strongly enough to support the column’s weight. The effect is called capillary action, and it’s the same mechanism that lets paper towels wick up spilled coffee against gravity. In your sinus ostia, it creates an air lock, preventing gravity from clearing the last close out.

At roughly 3.5% salinity, seawater is nearly four times saltier than the fluid in your tissues, hypertonic enough to pull water out of mucosal cells and stress the lining on contact. Cold makes this even worse, triggering vasodilation and a rush of mucus in the sinuses. The lining swells, narrowing the ostia further, sometimes closing them completely. The already small exit gets clamped down, and the water sits awaiting an awkward moment to emerge.

Eventually something tips the balance, often the first or second inversion of the day. Maybe you bend over to towel off your feet and the head angle finally beats the meniscus. Maybe the swelling subsides after an hour or two. Or maybe a sneeze creates enough pressure to break the seal on an unsuspecting guest. Around 35% of swimmers report nasal congestion after a session, and the timing of release is mostly unpredictable. The phenomenon is recognized enough that ENTs have a name for the chronic version, “swimmer’s sinusitis.”

Our sinuses evolved for breathing air, warming and humidifying it on the way to the lungs, with the mucus-and-cilia system handling the occasional bit of dust. Repeated head-first dunks in cold salt water came along later, and the design hasn’t caught up.

Further Reading:

Sinuses

Meniscus

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California Department of Fish and Wildlife confirmed in late March that starvation, not avian flu, is the primary cause. Brown pelicans and common murres are dying alongside the cormorants, almost all of them young birds in their first year. 2025 was an unusually strong breeding year for these species, which means a larger-than-usual cohort of juveniles is now competing for whatever food is available.

On September 9, 2025, the northeast Pacific reached the highest average temperature ever recorded in the basin: 20.6°C, nearly half a degree above any previous record. The marine heatwave responsible, designated NEP25A by NOAA, first emerged in the Gulf of Alaska that May and expanded through the summer to roughly 8 million square kilometers (about half a Russia), larger by area than any prior northeast Pacific heatwave on record. It shrank through October and November, then swelled again along the coast in December. As of early 2026 it persists, with an El Niño developing on top of it.

Warm surface water pushes anchovy and sardine schools deeper into cooler layers, away from some seabirds’ reach. A brown pelican can only dive about six feet, so a small change is significant. Common murres can dive much further, hundreds of feet, but each dive is intensive and burns lots of calories. A juvenile that hasn’t dialed in its hunting yet runs its energy budget into the red trying.

Warm surface water also resists mixing with the colder water below it. That matters because the deep water carries the nutrients (nitrate, phosphate, silicate) that phytoplankton need to grow. Less mixing means less fertilizer reaching the sunlight, and phytoplankton, the base of the food web, runs short. The community shifts from large, fat-rich diatoms toward smaller dinoflagellates. Their predators, zooplankton, follow: warm-water copepods are smaller and carry less fat than the subarctic species they replace, so a fish eating the same number of copepods gets fewer calories. Krill hit an 18-year low in southern California during the 2013-2016 Blob. Pyrosomes, gelatinous tube-shaped colonies that are essentially low-calorie filler, proliferate and absorb energy that would otherwise have moved up the chain.

Each link in the chain loses calories. A fish eating smaller, leaner copepods gets less energy from a meal, and a cormorant eating that fish gets less still. The bird has to catch more fish to compensate, but the fish are deeper than they used to be.

The 2013-2016 Blob, the previous benchmark for this kind of event, killed roughly a million common murres and 400,000 Cassin’s auklets along the same coastline. NEP25A is bigger by area and now in its second year. The juveniles washing up this spring are the cohort that would have started breeding around 2029.

Further Reading:

Marine heatwave tracker

Effect of heatwave on phytoplankton

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Most channel boards have four or six grooves running roughly parallel toward the tail, starting somewhere around the front fins and exiting at the back. The pitch is just short of a miracle cure: they generate lift by compressing water flow, push water toward the tail for “drive,” add bite on rail, and cut through tail rocker to flatten the back of the board. Like most cure-alls, these qualities don’t all hold up equally well.

The first issue is that parallel walls don’t compress flow along the length of the board. The compression-creates-lift story gets borrowed from single concaves, where a curved, narrowing surface really does squeeze water toward the tail, drop the pressure, and push the board up. That mechanism needs the curve to work. Channels run flat-walled and parallel, which is the geometry that doesn’t compress flow in the direction it matters. The Bernoulli explanation that fits a concave bottom gets applied to channels too, even though the shape isn’t doing the same thing.

The rocker effect is the part that holds up. Cutting half-inch grooves through a curved tail leaves a series of flatter planing surfaces in the back of the board, while the rail rocker stays kicked up where it needs to be for turns. Less curve under the planing surface means earlier planing and less drag at speed. It might be the most defensible thing channels actually do. Some shapers describe channels as a way to run extreme tail rocker without the board bogging under the back foot, which is the same effect in “feel” language.

When we search the peer-reviewed literature for surfboard hydrodynamics, we mostly find fins. Multiple CFD studies on three-fin and four-fin configurations, grooved fin designs, fin position, even pressure sensors embedded in fins on a river wave. But bottom contours barely show up. The closest thing to a direct test on channels is Riccardo Rossi’s CFD work for Firewire, where the channel-bottomed Sci-Fi showed lower drag and friction than the rounded-tail Omni, with the tail channels credited as a key factor. The boards also differed in tail shape and rocker, so the channels weren’t cleanly isolated. The same testing found that half an inch of tail rocker shifted dynamic lift by about 50%, which makes the rocker-straightening case stronger than the flow-compression one.

Despite the lack of literature, people still chase down channel-bottomed boards for heavy hollow waves in Western Australia and Indonesia, and the design hasn’t gone away despite half a century of competing alternatives. That kind of long anecdotal record carries real signal, just not the kind a controlled experiment would.

A study of two identical boards differing only in whether they have channels, tested side by side, has never been run. This would help close the gap between how confidently channels get explained and how little has actually been measured.

Further Reading:

CFD Comparison

Bernoulli Equation

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Inside a Sofar Spotter, a common surface wave buoy for shorter deployments, the GPS records where the hull sits in three dimensions 2.5 times per second. Older designs like the Datawell buoys use accelerometers and tilt sensors to get the same information a different way. Either way, the whole measurement comes from one point in the water, three axes of motion, and about half an hour of recording.

Water particles in a passing wave don’t just go up and down. They trace circles in deep water, moving forward on the crest and back in the trough, and the buoy rides those orbits. The diameter of each orbit at the surface is roughly equal to the wave height, so big waves make the buoy swing through big circles and small waves through small ones. Vertical motion tells us how tall the wave is, while horizontal motion at the same instant tells us which way it’s going. In deep water, the vertical and horizontal motions are locked a quarter-cycle apart, which is what makes the math work cleanly. The direction the wave is traveling shows up in whether that horizontal motion is mostly east-west, mostly north-south, or some mix of the two.

Thirty minutes of motion data gets fed through something called a Fast Fourier Transform, which splits a signal over time into its constituent frequencies. A 15-second swell shows up as a peak near 0.067 Hz (1 divided by 15). A 6-second wind chop shows up higher (1 divided by 6). The frequency spectrum is what a wave buoy actually produces, and everything else makes use of it. Significant wave height is a statistic computed from the area under the curve. Peak period is the frequency where the curve is tallest. A messy sea with several things happening at once turns into a clean plot of energy against frequency, where separate swells become separate peaks instead of overlapping confusion.

At each frequency, direction is encoded in how vertical motion correlates with horizontal motion. A wave from the west pulls the buoy up and east-west together. A wave from the south pulls up and north-south. Sorting those correlations by frequency gives us a bearing for each component of the seastate. More simply: if there are two swells, one from the south and one from the west, we can pick that out of the data. A second pair of correlations tells us how tightly the waves at a given frequency stack around that direction, which is where directional spread comes from. A clean groundswell shows up as a narrow peak with a sharp direction. Local wind chop smears across the compass as the buoy is tossed around.

While the measurement of offshore swells is relatively stable and straightforward, what arrives at the break takes a few more steps that introduce a lot of error in prediction. But we all know that about the forecasts.

Further Reading:

Sofar Ocean

Fourier Transform

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The biggest fossil fuel user is foam. A shaped 6’2” shortboard holds roughly a kilogram of foam. Both ingredients that make polyurethane foam, the polyol and the isocyanate, come from crude oil or natural gas. About a quarter to a third of the original blank never even makes it into the board, and even though it gets planed and sanded off during shaping, it still had to be produced to make the board.

The resin is the other heavyweight. A standard comp-glass shortboard soaks up about 1.2 kg of polyester resin across the lamination coats and the sanding coat. Also petroleum-based, styrene and polyester are both refined from oil. The fiberglass cloth, the stuff most people assume is the environmental villain due to its pervasive itchiness in insulation form, is made from melted sand and minerals. The stringer running down the middle is wood. Those two are about the only parts of your board that didn’t come out of an oil well.

Then there’s everything you bolt or stick onto the board. Fin boxes are plastic. A thruster set, made from about 200 grams of fiberglass-reinforced nylon, is roughly 70% petroleum by weight. The leash cord is urethane and the cuff is neoprene and nylon. Surf wax is paraffin, which is a direct byproduct of crude oil refining. If you run a traction pad, it’s likely EVA foam. The accessories don’t weigh much individually, but they add a few hundred grams of petroleum on top of the board.

Add it all up and a shortboard contains roughly 2.7 kg of petroleum-derived material. To produce those 2.7 kg of finished plastics and resins, you need an estimated 1.3 to 1.6 gallons of crude oil as feedstock. But raw materials are only part of the story. According to a lifecycle study by Sustainable Surf using data from Channel Islands and Firewire, a standard poly shortboard produces around 48 kg of CO2 from blank to surf shop. Manufacturing energy, running the shaping bay, heating resin, ventilation, is actually the single biggest contributor to that footprint.

For context, a petroleum-based neoprene wetsuit sits in roughly the same ballpark, somewhere between 55 and 77 kg of CO2 per suit depending on the study. Similar footprint to a board on a per-unit basis, but wetsuits wear out much faster unless you’re snapping your board yearly. Boards can last to double digit years if you take care of them. Over a five-year window, a surfer who replaces a wetsuit every 18 months racks up far more petroleum consumption than someone riding the same board the whole time.

The 48 kg of CO2 per board sounds like a number worth caring about until you compare it to getting yourself to the surf. A one-way flight from LAX to Honolulu puts roughly 280 kg of CO2 into the atmosphere per passenger. One surf trip to Hawaii wipes out the carbon math of about six surfboards.

The only scientifically valid reason for Locals Only.

Further Reading:

Lifecycle Study

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Seawater comes in at about 1,025 kg/m³. Surf wax, mostly paraffin with some beeswax and petroleum jelly mixed in, lands around 900 to 930 kg/m³. And the foam core of your board, whether PU or EPS, ranges from about 25 to 40 kg/m³. That makes wax roughly 90% as dense as water, while foam is about 3% as dense. The gap between foam and water is the reason your board floats: every liter of foam displaces seawater that outweighs it by roughly 995 grams, generating nearly a kilogram of buoyancy per liter. Wax displaces water too, but a liter of it only generates about 105 grams of net buoyancy. Same volume but only a tenth of the float.

A standard bar of Sex Wax weighs about 75 grams, with most brands falling between 75 and 85 grams per bar. Waxing up a fresh board, say a 40-liter midlength, takes roughly one bar. Archimedes told us the board will sink until the weight of displaced seawater equals the total weight it’s supporting - board, rider, and wax included. The extra 80 grams works out to about 0.08 liters of additional submersion, or 0.2% of your board’s volume.

How about if you surf a few times a week, reapply every couple sessions, and haven’t scraped in months? Maybe you’ve stacked three to five bars’ worth, somewhere between 240 and 400 grams of accumulated wax. The board sits an extra 0.25 to 0.4 liters deeper in the water. On a 40-liter board carrying an 80 kg surfer, that’s only about 1% of the board’s total displacement. Even the worst case, the board that is now gray with 500 to 800 grams of crusty wax buildup, loses maybe 0.5 to 0.8 liters of effective displacement, only around 1 to 2% of a 40-liter board.

Your lungs hold about 6 liters of air, and a single deep breath shifts a few liters of buoyancy, far more than months of wax accumulation. Your wetsuit absorbs a few hundred grams of water during a session too, riding on your body the whole time, and nobody loses sleep over that.

A single cup of seawater that seeps through a ding in the board weighs about 240 grams and contributes exactly zero buoyancy, because water doesn’t float in water. A neglected ding is costing you more float than a year of lazy wax habits ever could.

Layer it up, use a comb, whatever works to keep you on the board.

Further Reading:

Archimedes’ Principle

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Every wave that washes up the beach leaves behind a thin film of seawater in the spaces between sand grains. Seawater carries about 35 grams of dissolved salt per liter, mostly sodium chloride. When the sun heats the surface and that water evaporates, the salt has nowhere to go. It stays behind, clinging to the grain surfaces and concentrating in the water that remains.

Water deeper in the sand gets pulled upward through the narrow gaps between grains by capillary action, the same force that draws water up a paper towel. The smaller the spaces, the stronger the pull. Beach sand has pore spaces on the order of tenths of a millimeter, small enough to maintain a steady upward supply of moisture from the water table below. As fast as the surface evaporates, capillary forces are resupplying it with more salt water from underneath. It’s a conveyor belt for dissolved salt, continuously delivering NaCl to the evaporation front at the surface.

A 2016 study published in Scientific Reports by Geng, Boufadel, and Jackson measured pore-water salinity on an estuarine beach and found that evaporation drove near-surface concentrations to roughly double that of normal seawater. The effect was strongest in the upper intertidal zone during low tide on hot days, when the sand surface was exposed to air and the capillary fringe was actively pumping moisture upward. The deeper sand stayed closer to ambient ocean salinity while extra salt was concentrating right at the top.

Once the pore water at the surface reaches supersaturation, sodium chloride begins to crystallize. The crystals precipitate at the contact points between them, forming tiny mineral bridges that lock neighboring grains together. The same process destroys ancient stone buildings and monuments over centuries, salt crystals wedging into pore spaces and generating enough pressure to crack rock from the inside. On a beach, the result is less dramatic but mechanically identical: a thin brittle pancake of sand held together by crystalline NaCl cement, just waiting to be tossed and shattered.

The crust has almost no flexibility. Salt crystals are rigid and the connections between grains are thin. One footprint is enough to collapse the structure back into loose sand. Underneath, where evaporation hasn’t concentrated the salt, the grains never bonded in the first place.

The crust forms best on hot, calm days with low humidity, above the high tide line where the sand hasn’t been recently swashed. Fog or a high tide dissolves the crystals and resets the process and morning dew can weaken it. You’ll rarely feel the crunch on a beach that’s been washed by waves in the last few hours, but give the upper beach a full day of sun and it’ll be there.

Further Reading:

The paper

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The reason is that you’re not standing on the same beach, not really at least. Most sandy beach profiles are concave. Down near the low tide line, the slope might be two or three degrees. Up near the berm, where the high tide waterline sits, it can steepen to ten or fifteen. When the tide rises, waves stop interacting with the flat part and start hitting the steep part.

How a wave breaks depends on the relationship between the slope it encounters and its own steepness. There’s a ratio for this called the Iribarren number. A low Iribarren number, meaning a gentle slope relative to the wave steepness, produces spilling breakers. These are the mushy waves that crumble gradually and spend their energy across a wide surf zone. A high Iribarren number, steep slope relative to the wave, produces surging or collapsing breakers. These waves don’t fully break. They charge up the face and reflect back, sometimes retaining more than half their energy.

At low tide on a gentle slope, a wave might break fifty meters out and spend itself gradually on the way in. At high tide on the steep upper beachface, that same wave surges up a few meters of sand and comes right back. The energy that would have been chewed up by friction and turbulence across a wide, flat surf zone instead bounces off the steep face like sound off a wall. When that reflected water meets the next incoming wave, the two collide and stack. If you happen to be in the right place, you get launched.

That collision point also builds what’s called a beach step, a submerged ledge of coarse sand and shell at the base of the beachface where the backwash vortex dumps the heaviest material it’s carrying. It’s the little shelf you trip over wading out at high tide that doesn’t seem to be there at low tide.

Reef breaks get some backwash as well, but through a different mechanism. Roughly speaking, a reef is a fixed ledge. The slope doesn’t change with the tide, so the Iribarren number stays about the same. Backwash at a reef break has less to do with the reef and more to do with what’s behind it. If the wave breaks on the reef and the remaining energy runs into a cliff, a seawall, or a steep rocky shore, it reflects. At low tide though, this reflector may be out of the water and not reachable at all, so no backwash.

Further Reading:

Backwash

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Three systems usually tell your brain where you are in space. Your inner ear tracks rotation and acceleration using fluid-filled canals and tiny calcium crystals called otoliths. Your eyes relay what’s moving and what isn’t. And receptors in your muscles and joints register pressure and body position. When all three agree, you feel fine. Sitting on a boat (or board in the case of the wretched few) in a rolling swell, your inner ear says movement while your eyes on the horizon say everything’s still.

Of all the possible responses to confused spatial information, throwing up seems unnecessary. The going theory is that when sensory inputs stop making sense, the most likely explanation in the wild isn’t a boat: it’s a neurotoxin. Having eaten bad food, poisonous plants, or anything else that messes with motor control and perception, the body’s move is to purge whatever you ate. The response is so deeply ingrained that even fish can get seasick during aquarium transport.

People who spend time on boats like to say everyone has their frequency. Some particular combination of swell period, wave direction, boat size, and boat speed that’ll eventually get you. And that’s mostly right. Susceptibility varies by a factor of about 10,000 across the population, and roughly a third of people are highly prone under normal conditions. Women get it more than men, about 5:3. Kids peak around age 9 or 10 and gradually adapt. The only people who seem truly immune are those who’ve lost their vestibular system. In the late 1950s, NASA recruited eleven deaf men whose childhood meningitis had wiped out their inner ear function. They sat in rotating rooms for days without a hint of nausea while their hearing colleagues were gripping their chairs for hours.

Ever notice how the driver of the car almost never gets carsick? When you’re controlling the motion, your brain can predict what’s coming and the three signals line up. Passengers reading in the back seat don’t get the visual cue and tend to roll down the window. The same thing applies in surfing. Paddling, popping up, and riding are all activities where your brain is generating those movements and anticipating the feedback. But sitting still in a rolling lineup waiting for a set…

If you are one of the “unlucky” group with strong hearing and sensitive stomach, drugs like Dramamine can make a world of difference. Diphenhydramine, an antihistamine, blocks receptors in the vestibular system, turning the system down enough to not get sick. Unfortunately for the booze-cruisers among us, Dramamine and alcohol interact fiercely.

Both diphenhydramine and alcohol are central nervous system depressants, but they work through different pathways. Together, the sedation multiplies. Two ocean beers tend to hit like six. When taking it, it may actually be better to stay thirsty instead, my friends.

Further Reading:

Seasickness

Vestibular System

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