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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Instruments go where ships go, and ships go where commerce sends them. The Northern Hemisphere has the bulk of commercial shipping routes, research institutions, and national ocean monitoring programs. NOAA’s buoy network alone stretches from the Bering Sea to Hawaii and from the Gulf of Mexico to the North Atlantic, with over 100 moored buoys operating at any given time. The Southern Hemisphere has no equivalent, and ship-based measurements of temperature, salinity, and carbon dioxide have always reflected the routes ships actually travel. The global south has been measured mostly in passing.

The South Atlantic, the South Pacific, and the Indian Ocean are all part of this story, not just the waters near Antarctica. The southern portions of every major basin have consistently thin coverage, with measurements collected opportunistically rather than systematically. The majority of ocean carbon observations come from routes that favor the Northern Hemisphere. The subtropics and tropics of the Southern Hemisphere are nearly as sparse as the subpolar regions.

The Southern Ocean represents the extreme case. For most of the 20th century, sustained in-situ observations there amounted to two sub-Antarctic moorings, one of which has since gone offline. Winter data were rarest of all, because research cruises don’t run in the Southern Ocean in winter. Nobody wants to be on a ship at 50°S in July. The consequence is that the entire pre-2000 Southern Ocean winter profile database contained less data than autonomous floats now collect there in a single year.

Being a connected ocean system and all, this undersampling biased what scientists thought they knew. Ocean heat content estimates for the Southern Hemisphere were running roughly 40% too low through much of the mid-20th century record, a finding that only became apparent once better observations existed to compare against. Models built on that incomplete historical record inherited the bias. Reanalysis products, which are ocean model simulations constrained by observations, are only as good as the data assimilated into them, and in the Southern Hemisphere, the data were thin in space and time and almost nonexistent in winter.

For wave forecasting, this creates a specific problem. The storm belts of the Southern Hemisphere, the roaring forties and furious fifties, are among the most energetic sea-states on the planet and generate swell that propagates across entire ocean basins. These are also the regions where numerical wave models have historically had the least in-situ validation data. A model’s error in representing a storm’s size, track, or intensity at 50°S doesn’t stay at 50°S. It propagates outward with the swell. Forecast uncertainty at the source becomes forecast uncertainty everywhere downstream.

The Argo program, which began deploying profiling floats in the early 2000s, was the first instrument system to observe the Southern Hemisphere systematically and year-round. Argo floats don’t follow shipping lanes. They drift with ocean currents, park at depth for 10 days, and transmit data back at the surface regardless of season or sea state. By 2007 the array had achieved nominal global coverage. Southern Hemisphere ocean science effectively has a before-Argo and after-Argo era.

But, Argo floats can’t sample under sea ice or in shallow coastal waters. The deep ocean below 2,000 meters is still largely unobserved in the south. Biogeochemical measurements of carbon, oxygen, and nutrients require specialized sensors that only a fraction of floats carry. And the observing infrastructure that fills in around Argo’s gaps, the mooring arrays and coastal buoy networks that densify coverage in the North Atlantic and North Pacific, has no Southern Hemisphere counterpart of comparable scale. The baseline is better than it was. It’s still not close to symmetric.

Further Reading:

Southern Hemisphere

ARGO Floats

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In the Northern Hemisphere, trade winds blow westward across the tropics and westerlies blow eastward across the mid-latitudes. When these two wind belts push on the ocean surface in opposite directions at different latitudes, the water ends up spinning in a large clockwise loop called a gyre. The North Atlantic and North Pacific both have one, and both spin in the same direction. The difference is what happens on each side.

The Coriolis effect, which deflects moving objects to the right in the Northern Hemisphere, causes water to pile up in the center of each gyre. That creates a slight pressure gradient, and the whole system reaches a balance called geostrophic flow: water circling around the central mound rather than draining out of it. You’d expect the return flow to redistribute itself evenly around the gyre. But the Coriolis effect isn’t constant. It weakens toward the equator and strengthens toward the poles, and that variation breaks the symmetry. The return flow can’t spread itself evenly east to west, so it piles up on the western side of the basin instead. The result is a fast, narrow, deep, warm western boundary current.

In the Atlantic, that’s the Gulf Stream. It moves roughly 30 sverdrups of water — the equivalent of over 12 thousand Niagara Falls — in a current sometimes under 100 kilometers wide. It carries warm tropical water northward along the East Coast before peeling northeast toward Europe, and it keeps the water warm well offshore. Standing on the beach in North Carolina and dipping your toes in, you’re feeling sometimes-disturbingly-warm water that started near the Caribbean.

The California Current is the eastern boundary current of the North Pacific gyre. Eastern boundary currents are slow, broad, shallow, and cold. Rather than a concentrated ribbon of warm tropical water, the California Current is a wide drift of subarctic water moving southward down the West Coast. Broad and slow means less heat, more time for mixing, and a lot of exposure to persistent northwesterly winds that push surface water offshore. When that surface water gets pushed out, cold water from depth wells up to replace it. This upwelling is why the California Current is one of the most biologically productive systems on the planet and also why 4/3s are awesome.

This asymmetry is a feature of every major ocean basin. The warm Kuroshio Current runs up the eastern coast of Japan the same way the Gulf Stream runs up ours. In the Southern Hemisphere the gyres spin counterclockwise, but the same Coriolis asymmetry applies — western boundary currents still pile up on the western side of the basin, meaning the eastern coasts of continents still get the warm side. Brazil gets the Brazil Current; Chile gets the cold Humboldt.

Further Reading:

Ocean Gyres

Boundary Currents

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Salt gets into the ocean through river runoff and seafloor vents, and then it just stays. Evaporation pulls water out of the ocean and into the atmosphere, but salt doesn’t evaporate. It’s left behind. Rain adds freshwater back to the surface, diluting what’s there. So salinity is a ledger of how much freshwater a basin has gained or lost relative to its salt content. More evaporation than rainfall means saltier water. More rainfall than evaporation means the opposite.

The Atlantic loses more water to the atmosphere than it gets back from rain. The Pacific is wetter — precipitation outpaces evaporation across most of the basin. That alone would explain the difference. But the more interesting part is where the Atlantic’s evaporated water actually goes. Trade winds blow westward across the tropics, and the Central American isthmus is narrow enough (about 80 kilometers at its narrowest) that significant amounts of water vapor cross it in the atmosphere. The Atlantic is effectively exporting its freshwater to the Pacific through the air. Researchers estimate this cross-isthmus moisture flux moves on the order of 0.1 to 0.2 sverdrups of freshwater equivalent annually (100k-200k cubic meters per second - an absurd number), enough to matter over ocean timescales. The Atlantic loses salt-free water and gets saltier. The Pacific receives it and gets fresher.

The Atlantic connects to the Mediterranean Sea through the Strait of Gibraltar, and the Mediterranean is hypersaline. Intense evaporation and limited inflow from rivers make it one of the saltiest major bodies of water on earth, averaging around 38 parts per thousand. The dense, salty Mediterranean water sinks and spills westward into the Atlantic at depth through the strait, a slow but persistent input of high-salinity water. The Pacific has no equivalent. Nothing comparable is pouring extra salt into it from an adjacent basin.

The Indian Ocean sits in between, averaging around 34.5-35 ppt, but masks a wild internal range. The Arabian Sea runs closer to 36-37 parts per thousand because evaporation is high and few rivers drain into it. The Bay of Bengal, on the other side of the subcontinent, drops to 32-33 ppt because the Ganges, Brahmaputra, and monsoon rainfall pour freshwater in faster than the ocean can salt it back up. The Red Sea and Persian Gulf, both hypersaline, spill dense water into the Indian Ocean at depth the same way the Mediterranean does for the Atlantic, but the Bay of Bengal offsets it. Each basin ends up where its geography, rainfall, and neighbors leave it.

Saltier water is denser, so you float marginally higher in the Atlantic than the Pacific, but you’d never notice it. Your wetsuit has more effect on your position in the water than a 1.5 ppt salinity difference does. Two connected oceans ended up with different salt content because of where they sit, what sits next to them, and which way the wind blows.

Further Reading:

Salinity

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Quartz makes up most of mainland beaches because it’s extremely hard and chemically stubborn. Softer minerals like feldspar dissolve or crumble along the way. By the time sediment reaches the coast, quartz is often the last man standing, which is why sand from San Diego to Santa Cruz looks more or less the same despite coming from very different source rocks.

Rivers are the main arteries in this system. Weathered sediment washes downstream, bumper-carting against riverbeds and other particles the whole way, getting smaller and rounder with every mile. A grain of sand on your beach might have started as part of a granite cliff face a few hundred miles inland. California has over 1,500 dams, and most of them are trapping sediment that would otherwise end up on the coast. With waves that keep pulling sand offshore, and rivers not restocking it the way they used to, the beaches run a deficit. Cliff erosion chips in locally, waves eating into coastal bluffs and dropping material directly into the surf zone and oblivious sunbathers, but it doesn’t come close to replacing the river supply.

In tropical environments, however, a huge portion of beach sand comes from biology. Parrotfish scrape algae off coral with their beak-like teeth, chewing up chunks of coral skeleton in the process. What comes out the other end is white sand. A single parrotfish can produce several hundred pounds of it per year. Hawaii’s green sand beach at Papakolea is olivine, a mineral in the local basalt that weathers out faster than the surrounding rock and concentrates on the shoreline. Those white Caribbean beaches are largely crushed shells, urchin spines, coral fragments, and literally-rock-hard fish poop.

Once any of this sediment reaches the coast, waves decide what stays. High-energy beaches exposed to open ocean swell lose their fine grains offshore, currents carrying the small stuff away and leaving coarser, heavier material behind. Sheltered beaches don’t generate enough force to move the fines, so they keep them. Push this far enough and sand disappears entirely. The rockier stretches around San Diego are hard local geology meeting persistent swell with no major river nearby to resupply. Whatever fines existed got stripped out long ago, leaving cobble that clacks together in the backwash, clipping your ankles as you time your paddle.

Further Reading:

Sand

Parrotfish - not kidding about the pooping out sand part

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The same growths have turned up in Homo erectus skulls and in Pre-Hispanic remains from coastal Gran Canaria, where archaeologists used their presence to figure out which individuals were the community’s designated swimmers. Surfer’s ear predates surfing by a species-wide margin.

What Boule found in that Neanderthal skull in 1911 is the same thing an ENT would find in yours after a couple decades of dawn patrols: new bone growing on the walls of the ear canal. Your skeleton slowly and permanently walling off the eardrum from the cold.

The tissue lining the ear canal sits almost directly on bone. When cold water floods in, blood vessels in that lining dilate, and the repeated cycle of cooling and inflammation triggers bone-producing cells to start laying down new layers. Over time, the body is narrowing the canal to protect the eardrum, which would be thoughtful if it didn’t also trap water and wax in there. Water below 19°C is the primary culprit, but wind-driven evaporative cooling allows warmer waters to lead to the same results. A study of surfers in Queensland, where the water barely dips below 20°C, still found exostoses in 70% of participants.

Most surfers don’t notice for a decade or more, which is why most diagnoses land in your mid-30s to late 40s. Water sits in your ear a little longer after sessions, then infections start showing up, then hearing goes. One ear is often worse than the other, depending on prevailing wind or which side of your head meets the wave first. All the more reason to surf lefts and rights, day and night.

The prevalence among surfers is pretty grim. A systematic review of roughly 3,000 surfers worldwide averaged around 68%. A smaller study of elite WSL competitors found it in every single one of them, and not one had ever worn ear protection.

Once bone grows, it doesn’t recede. Surgery is the only way to remove it, and it can grow right back with continued exposure. Earplugs and hoods keep cold water out of the canal and are the simplest way to slow the whole process down. If you’re like me, you should probably actually use the never-seen-water pair sitting in the bottom left corner of your trunk. So easy a caveman can do it.

Further Reading:

Tinkaus paper

Surfer’s ear

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A relaxed hand with fingers slightly apart outperforms a tight, squeezed hand. Multiple studies from research groups across Europe and Australia land on the same finding: a small spread of 3-8mm produces 5-10% more propulsive force than fingers pressed together. Marinho and colleagues at the University of Porto used CT scans of an Olympic swimmer’s hand to build three computational fluid dynamics models: fingers closed, small spread (3.2mm), and large spread (6.4mm). Across every angle of attack they tested, from flat to nearly vertical, the small spread consistently produced the highest drag coefficients. Not the closed hand. Not the wide spread. The relaxed middle ground.

Each finger drags a thin layer of slow-moving water along with it as it moves, called a boundary layer. When fingers are spaced just right, these boundary layers overlap between the gaps without merging completely. Water trying to pass through encounters this zone of slower-moving fluid and gets deflected around instead. Your hand effectively acts like it has more surface area than it actually does. Spread your fingers too far apart (more than about 15mm) and water passes through freely, losing the benefit. Keep them pressed together and you never create the overlapped mitten in the first place.

Why obsess over a few millimeters of finger spacing? Because the hand does most of the work while paddling. Your forearm has more surface area, but it’s cylindrical, so water flows around it relatively easily. The hand is flat, and flat surfaces generate far more drag when pushed through fluid. This is why swimming research focuses so heavily on hand position, and why those small percentage gains from optimal finger spread actually matter.

The optimal spacing, that 3-8mm range or roughly 10-12 degrees between fingers, matches almost exactly what your hand does when relaxed. The natural position of an unstressed hand falls right in the performance sweet spot.

There’s no need to think about millimeters mid-paddle. Just stop squeezing.

Further Reading:

The Paper

Drag on a cylinder

Boundary layers

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On land, gravity pulls blood toward your legs. Roughly 500 milliliters, about a Coke bottle’s worth, pool in your lower extremities just from standing upright. Submerge yourself in water and hydrostatic pressure eliminates this effect. The external pressure from the water equalizes blood distribution, pushing that pooled blood back toward your heart and central vessels. Your cardiovascular system registers what it interprets as excess fluid volume, even though total blood volume hasn’t changed at all.

When your skin cools, blood vessels in your arms and legs clamp down to keep heat from escaping. This vasoconstriction shunts even more blood toward your core. Research from the U.S. Army Research Institute of Environmental Medicine found that cold and pressure effects stack: cold water immersion produces significantly more pee than sitting in a warm bath. The threshold isn’t extreme either, with anything below about 35°C (95°F) triggering vasoconstriction.

Tricked by a refreshing duck dive, your body now thinks it’s overloaded with fluid. Pressure sensors in the heart and major blood vessels measure how much blood is passing through. They can’t tell the difference between “you gained extra fluid” and “your existing fluid moved from your legs to your chest.” All they register is more volume than usual, so they send a signal: dump it.

That signal tells the brain to stop releasing antidiuretic hormone (ADH). Normally, ADH instructs the kidneys to reabsorb water and return it to the bloodstream. Without ADH, the kidneys let water pass straight through to your bladder instead. Problem solved, from your body’s perspective.

Except you never had excess fluid to begin with. The water leaving your body as urine is water you actually needed. Studies on prolonged cold water immersion show urine output jumping 200-300% above normal, with blood plasma volume dropping around 17%. That post-session thirst isn’t just from swallowing seawater.

So let it rip knowing everyone’s kidneys are running the same confused program. Also, drink some extra fluids.

Further Reading:

Immersion Diuresis

Dehydration

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Waves still under active wind generation are called “sea” or “wind waves.” These are the choppy, disorganized conditions during a storm or a windy afternoon session. Multiple wavelengths, periods, and directions stack on top of each other with no discernible pattern. The wind keeps dumping energy into the water, pushing existing peaks higher while new ripples form constantly. There’s no rhythm, just overlapping chaos.

Once waves leave the generating area, they become swell. The storm might be a thousand miles away, but the energy it transferred keeps propagating across the ocean. Without wind input, the waves can’t grow anymore, they can only travel. And as they travel, they organize.

We’re adding dispersion to our list of friends: longer period waves move faster than shorter ones in deep water. A 20-second wave crosses open ocean at roughly 35 mph while a 12-second wave manages about 21 mph. Long-period waves pull ahead, short-period waves lag behind, and what started as a soupy sea separates into distinct swells over subsequent days.

Wind waves typically run periods under 10 seconds and arrive from multiple directions simultaneously. Swells clock in at 10 seconds and longer, often much longer for groundswells from distant storms, with tight directional bands. Pull up buoy spectra during a local wind event and you’ll see broad peaks smeared across frequencies and directions. Clean swell shows narrow spikes. NOAA’s wave models separate “wind wave” and “swell” components, each with their own height, period, and direction. A forecast might show 2-foot wind waves from the northwest combined with a 4-foot swell from the west-southwest. Both contribute to total sea state, but I’m going to bet you’ll like the swell more.

Wind waves lose steam quickly once the wind stops. Their shorter wavelengths dissipate through whitecapping and turbulence. Swells barely decay. Walter Munk tracked swells generated near Antarctica arriving in Alaska two weeks later, having crossed the entire Pacific with enough energy left to register on instruments. Wind chop just can’t handle that sort of distance.

A head-high day of pure groundswell hits very different than head-high wind waves, even if the buoy reads identical wave height. The groundswell arrives in defined sets with consistent faces. The wind waves stack unpredictably with variable steepness and no clear peaks. Period and organization matter as much as size. When someone says “there’s swell in the water,” let’s all hope they mean something fun.

Further Reading:

Swells

Swell - one of my favorite books of 2025

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Waves propagating into an opposing current slow down relative to the seafloor. The period stays locked, set by whatever storm generated the swell, but wavelength has some wiggle room to adjust. Wavelength equals speed multiplied by period, so slower propagation means compression. The same wave energy now occupies less horizontal space, and that increased energy density translates directly to larger waves and steeper wave faces.

In 20 feet of water, waves behave as shallow water waves, their speed set by depth rather than period. The phase speed works out to about 7.7 m/s. A 2 m/s ebb current cuts that ground speed to 5.7 m/s, a 26% reduction. Wavelength compresses by the same proportion, and wave height increases as the energy concentrates into a shorter space. A swell that was nowhere near breaking can cross the steepness threshold just by running into an outflow. This follows conservation of wave action, a quantity that accounts for both wave energy and the medium carrying it. Wave energy alone isn’t conserved when currents enter the picture, but wave action flux is.

If we flip the scenario, everything reverses. Waves traveling with a current stretch out, their peaks getting further apart from each other. The following flow adds to ground speed, flattening wave faces. That same bar crossing becomes forgiving on the flood tide. You time your crossing with the tide.

When currents run at an angle to wave direction, things get more complex. Waves refract toward slower water, bending their propagation direction in response to velocity gradients. We usually think of refraction as a depth effect, waves turning as they hit a shallower bottom. But currents are also out there bending waves. An alongshore current means one portion of the wave crest moves faster over ground than another. The crest rotates. This current-induced refraction can steer swells toward or away from the coast independent of what the bathymetry is doing.

Creek and lagoon outflows during ebb tide show this clearly. Breaks near harbor entrances or river mouths change character through the tidal cycle for exactly this reason. The steepening is localized where velocity gradients are sharpest, and those zones shift position as the tide ebbs and floods.

The surface texture reveals what’s happening below. Opposing current creates a rougher, choppier appearance with compressed wave spacing. Following current smooths things out. Where current shear is strong, visible lines form on the surface, foam and debris collecting along the boundary. These cues help whether you’re timing a bar crossing or just trying to understand why the waves look different at different points in the tide.

Further Reading:

Wave-Current Interaction

Refraction

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Storms don’t produce uniform waves. Wind blowing over water generates a spectrum of wavelengths, periods, and directions all at once. The size of that spectrum depends on how hard the wind blows, how long it blows, and how much ocean it has to work with (called fetch). A massive North Pacific low spinning for days across a thousand miles of fetch creates waves spanning periods from 8 seconds to 22 seconds, all radiating outward from the storm center.

Once waves leave the generating area, they begin to separate. Wave speed in deep water depends on period, with longer period waves traveling faster. A 20-second swell crosses the Pacific at roughly 35 mph, while a 10-second swell moves at half that speed. Walter Munk’s 1963 documentary “Waves Across the Pacific” tracked this dispersion across entire ocean basins, proving that swells maintain their energy over thousands of miles while sorting themselves by period along the way. By the time waves reach your beach, they’ve organized from the chaotic spectrum the storm produced into something more orderly. The longest periods arrive first, with shorter periods trickling in over subsequent days.

But dispersion alone doesn’t explain the pulse of sets followed by lulls. That pattern emerges from interference between waves of slightly different frequencies traveling together.

When two waves with nearly identical periods overlap, their crests and troughs add together. Where crests align, the combined wave height doubles. Where a crest meets a trough, they cancel. The result is a beat pattern, an envelope of alternating large and small waves that travels as a group across the ocean. Add more frequency components and the pattern gets more complex, but the same idea holds. The set you’re watching roll through represents a region where wave components are reinforcing each other. The lull is where they’re canceling out.

The energy in these groups moves slower than the individual waves themselves. In deep water, group velocity is roughly half the phase velocity, which is the speed of the wave crests you actually see. Watch a set offshore and you’ll notice individual waves appear at the back of the group, travel forward through it, and disappear off the front. The waves are outrunning their own energy. What we perceive as a set is this slower-moving envelope of reinforced wave height, a moving magnifying glass of sorts.

How pronounced the set pattern appears depends on the bandwidth of the swell. A swell that’s traveled far has had time to disperse and narrow its frequency spread. These clean, long-traveled swells produce well-defined sets with obvious lulls between them. Local wind swell, with its broader mix of periods, creates choppier conditions where sets blur together. The narrower the frequency band, the longer the beat period and the more patience required between sets.

Set size and spacing depend entirely on the frequency content of the swell, which varies with each storm system and changes as the swell evolves. Some sets have two waves, others have eight. You can count them all you want, but it’s not going to tell you how many waves are in the set after next.

Further Reading:

Wave Sets

Wave Interference

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Polyurethane foam is the surfing OG. Its density sits between 25 and 40 kg/m³, with shapers selecting blanks based on the weight and durability they’re after. Polyol reacts with isocyanate in a thermosetting process that can’t be reversed. So, once cured, PU foam is PU foam forever, which matters when you consider that 25-30% of each blank ends up as shaping dust headed for a landfill. Clark Foam supplied most of California’s shapers until 2005, when the operation shut down overnight and left the industry hanging.

Expanded polystyrene started to fill the void. EPS runs lighter than PU at equivalent strength, with densities from 14 to 40 kg/m³ and a closed-cell structure that resists water absorption. The shaping scraps can be recycled since the material isn’t cross-linked like PU. But EPS has compatibility issues. Pour polyester resin on it and the foam dissolves, so boards must be glassed with epoxy, which demands precise mixing and costs more. Heat is also a weakness for EPS. If you leave an EPS board in a hot car, the foam can warp permanently.

Running nose to tail through most boards is the stringer, usually a strip of cedar, redwood, or balsa. It adds longitudinal stiffness and helps maintain rocker as the board ages. Carbon fiber and cork have entered the stringer market, each with different flex characteristics. Some EPS boards ditch the stringer entirely since the foam can be stiffer than PU on its own, though removing it changes how the board responds under your feet.

E-glass fiber dominates the outer skin because it’s cheap and available. S-glass costs more but delivers roughly 30% greater strength and 15% higher stiffness: one ply can match two layers of E-glass. Carbon fiber tops both in strength-to-weight, though manufacturing complexity pushes the price up substantially. The fibers do the structural work; the resin bonds everything together and seals the foam from water. Polyester resin cures fast and costs less but tends toward brittleness. Epoxy is stronger, more flexible, better at absorbing impacts. Since EPS requires epoxy anyway, plenty of shapers have switched over entirely regardless of what foam they’re using.

Buried under the glass are the parts you bolt things to. Fin boxes - whether FCS, Futures, or glass-ons - are composite or plastic inserts that need to bond with both foam and skin without creating stress points. Leash plugs are similar: a small cylinder glassed into the tail that takes repeated yanking every time you wipe out. These components don’t get much attention until one strips out or delaminates. But that’s a great opportunity to double check if you’re working with PU or EPS before grabbing a poly resin repair kit.

Further Reading:

Review Paper

PU Foam

EPS Foam

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The jet stream normally arcs northward across the Pacific, steering storms toward the Pacific Northwest. Seattle gets hammered and San Diego watches longingly from behind Point Conception. The Channel Islands create additional shadowing. A swell approaching from 310 degrees might produce head-high surf at Ocean Beach in San Francisco while leaving Rincon barely waist-high.

But El Niño extends the jet stream eastward and dips it south. The warm water pooled in the central Pacific shifts the region of strongest rising air and storm development towards California’s latitude. Storms that would normally hook north instead track directly toward the state. The average storm position can shift hundreds of miles south during strong events, putting Southern California thankfully in the crosshairs.

Instead of arriving from the northwest, swells approach from the west or west-southwest. West swells are able to slip past Point Conception and thread through the Channel Islands’ gaps. South-facing breaks that sit dormant through typical winters have a fighting chance of waves wrapping into them.

USGS geologist Patrick Barnard has documented that extreme El Niño events increase wave energy along the California coast by roughly 30 percent. During the 2015-16 El Niño, one of the three strongest on record alongside 1982-83 and 1997-98, winter wave energy equaled or exceeded historical maximums across the entire West Coast.

And beyond having some solid swell, that energy reshapes the coastline. Barnard and colleagues surveyed 29 beaches from Washington to San Diego and found winter erosion ran 76 percent above normal during 2015-16. Central California averaged around 150 feet of shoreline retreat. The event was particularly brutal because, despite massive waves, Southern California received 70 percent less rainfall than the previous two major El Niños. Rivers that normally replenish beaches with sediment ran dry. The waves took sand offshore, and nothing came to replace it.

La Niña pushes everything back north. The jet stream retreats, the storm track favors Oregon and Washington, and Southern California returns to its shadowed existence. Northwest swells dominate. The Channel Islands block everything.

The 2015-16 event highlighted the disconnect between waves and rain. Californians hoping El Niño would end the drought watched storms spin up massive swells, only to have high pressure deflect precipitation away from land. The coast got pummeled; the reservoirs stayed low. Sometimes the jet stream is in position and the storms are there, but the rain just doesn’t come. As for what it means for surfing, forecasts are what they are; go check a break and surf if it looks good.

Further Reading:

Barnard Paper

Part 1

Part 2

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The 1982-1983 El Niño caught the scientific community off guard. One of the strongest events of the century, and nobody recognized it until it was already happening. The tropical Pacific simply wasn’t being watched closely enough. Ship-based measurements revealed the warming months after it began, too late for any kind of warning.

The first successful El Niño forecast came in 1986, predicting the 1986-87 event. That was a milestone - proof that these things weren’t completely random. But success has been inconsistent. The 1997-1998 El Niño was the best-observed event in history, tracked by a network of buoys across the Pacific transmitting data in real time. But models still missed how strong it would become.

The problem is that the ocean and atmosphere don’t take turns on the playground. They’re influencing each other constantly, which means you can’t model one and then the other. You have to simulate both simultaneously and get their interaction right. A slight miss in sea surface temperature nudges the winds, which nudge the currents, which feed back into temperature. Run the model forward a few months and small errors have snowballed into entirely different forecasts.

Compounding errors aside, there’s the question of what kicks off an El Niño in the first place. The Pacific can sit in a primed state for months: warm water pooled in the west, thermocline tilted and ready. Nothing happens. Other times, a burst of westerly winds lasting a week or two triggers the whole cascade. These wind bursts are tied to larger atmospheric patterns that remain difficult to predict more than a couple weeks out. Whether a particular burst will matter depends on where the ocean already sits, a combination of timing and luck that models struggle to capture.

NOAA issues probabilistic forecasts, giving percentage chances for El Niño, La Niña, or neutral conditions several months ahead. These outlooks are useful, but they’re probabilities, not certainties. The 2015-2016 El Niño was forecast reasonably well. The strong El Niño predicted for 2014 fizzled before it fully developed. Spring is particularly tricky, with forecasters calling it the “spring predictability barrier” because the system is in transition and small nudges determine which direction it goes.

Despite all this uncertainty, the forecasts aren’t useless. What the models can tell us is when the odds are elevated. A strong El Niño winter doesn’t guarantee epic surf, but it shifts the probabilities. What those shifted probabilities actually mean for waves at your local break is where we’re headed next.

Further Reading:

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