Book Science is a podcast dedicated to celebrating science books and their authors. Through in-depth discussions and author interviews, we explore the stories, insights, and craftsmanship behind books that make science accessible and engaging for everyone. Our mission is to champion long form science communication, inspire readers, and support aspiring authors in sharing their passion for science with the world.
In this special standalone episode, host Tripp Collins steps out of the standard author interview format to share a story from his own field: ocean wave science. Learn how a 1913 storm in Casablanca, WWII Higgins landing craft, and two Scripps oceanographers led to the mathematical breakthroughs in surf forecasting that made the D-Day landings possible.
Key Highlights:
The origin of the world's first operational swell forecast in Casablanca and the Azores in 1922.
Why flat-bottomed Higgins landing boats were dangerous liabilities in waves over five feet.
How Walter Munk and Harald Sverdrup defined Significant Wave Height and quantified wave growth, decay, and surf transformation under pressure.
General Eisenhower's 24-hour D-Day delay decision and the unpredicted severe storm that hit two weeks later.
00:00:00 - Introduction to this special episode on wave science history 00:00:49 - Context on YouTube visuals and inspiration from the movie Pressure 00:02:37 - The practical power of ocean wave prediction for ports, warfare, and surfing 00:03:29 - The 1913 Casablanca swell and early pressure chart analysis 00:05:16 - The Azores field office in 1922 and the first operational swell forecast 00:06:41 - Why early local forecasting methods failed when applied elsewhere 00:07:55 - WWII amphibious landings and the need for universal wave physics 00:09:08 - Early British empirical rules developed by Claude Suthens 00:10:20 - Walter Munk's journey from Austrian skier to Scripps oceanographer 00:12:51 - Army service and Munk's return to Scripps under Harald Sverdrup 00:14:17 - Operation Torch and Higgins boats: Floating shoeboxes in heavy seas 00:16:44 - Spotting the wave height discrepancy for the North Africa invasion 00:18:22 - Building a mathematical model for wind wave growth from weather maps 00:20:12 - Standardizing Significant Wave Height with landing craft coxswains 00:21:49 - Operation Torch in November 1942: Forecasting Casablanca surf 00:23:49 - Training weather officers at Scripps for Operation Neptune 00:24:46 - Balancing tides, moon phases, and surf limits for D-Day 00:26:41 - Maureen Flavin Sweeney's barometric warning and Eisenhower's 24-hour delay 00:28:25 - June 6, 1944: 133,000 Allied troops land on Normandy beaches 00:30:22 - The unpredicted storm 13 days after D-Day and the timing of the invasion window
Hey everyone, welcome back to Book Science. This is a special episode maybe not even an episode, uh, proper. It's something different. So usually we have on the author of a science book, and we do an interview asking about the content of the book and how it was written. This is gonna be something completely different. Um, in my day job, I'm a scientist, and I study ocean waves, and this is, a story from the history of the science of ocean waves. So if you're looking for our regular content, you can feel free to skip this and carry on. But if you're interested in a science story and, um, wanna hear me tell it, then hang on for the ride
Future Tired Tripp here. I, wanted to bring in some additional context,, before I get started on this. And one is that, um, this is based on a long piece I had already written, so this is sort of just like an edited down, version, more suitable for podcast or YouTube video. Two, if you're listening to this as a podcast, I encourage you actually to watch the YouTube video where I went to great pains to add a bunch of visuals, and it should be pretty fun to watch. And, three is... if there's a third one. One of the things that motivated this was the, movie Pressure, which I haven't seen, but it sort of details the role that weather played, leading up to the D-Day invasion, which is sort of a pa- parallel story to this one. Anyways, really hope you enjoy it. Um, and if you wanna hear more stuff like this from me, please let me know.
Tripp Collins
Accurate prediction is powerful. In the case of ocean waves, being able to predict their wave height and the wave period and the direction at some future time and location, well, if you're running a major port, that could be an operational advantage. And if you're a general and it's wartime, knowing the wave heights in some future place could be a tactical advantage. And if you're a surfer, knowing about what the waves are gonna do could get you the cover shot of Surfer Magazine when you score a major swell. But building up that requisite knowledge, that intimacy with the physics to achieve that kind of predictive power, well, that was a long and twisty road, a road that started just after the turn of the 20th century. I wanna take you back to 1913. The port of Casablanca, it was rocked by a massive swell. The city, which was then occupied by France, it was growing rapidly from a population of about twelve thousand in nineteen oh seven to a hundred and ten thousand fourteen years later in nineteen twenty-one, and was one of the most important ports in the region. This damaging swell was not an isolated event. It was the culmination of a series of swells that wrecked small crafts and rendered the harbor inoperable for months. Now, at the time, a calamity like this was usually chalked up to a natural act of God. But the French government decided to invest in studying these events to try to reduce future harm. They assigned a man named Gain to the duty. Now, Gain started examining weather charts, and these were synoptic scale maps of the atmospheric pressure at sea level over the North Atlantic. The chart showed broad areas of high pressure and low pressure. Think of it like this. Wind rushes from areas of high pressure into areas of low pressure, just like, air rushing out of a pressurized balloon. And the deeper the low pressure, the faster the wind. Now, Gain was able to link the appearance of these low-pressure systems in the North Atlantic directly to swell events. He was perhaps the first to show that swell arriving in the Northwestern African coast actually originated way out in the North Atlantic. Furthermore, he showed that different pressure patterns resulted in swells of different character. This information was the key to unlocking a warning system. The swell formed offshore, far to the west, and it propagated towards the coast. But to get to Casablanca, it would first encounter the Azores. It's a small island chain about fifteen hundred kilometers west of Morocco. Since the swell tended to arrive from the west, the idea was that the swell observed in the Azores would be the exact same swell that later impacted Casablanca. And indeed, this was the case. In nineteen twenty-two, they set up a field office to make visual wave observations in the Azores. Swell observed there would take one to two days to reach the coast of North Africa, giving the office time to warn Casablanca via telegraph of an impending event. Thus began the world's very first operational swell forecast, and the year was nineteen twenty-two. By 1943, a researcher summarized the state of the art of wave prediction by saying, With some experience, one may become proficient in forecasting the phenomenon, at least at Casablanca." And that was the catch. It was a kind of empiricism that couldn't be applied anywhere else. For example the island of Bermuda is the similar 1,500 kilometers to the east but of the port of Charleston, South Carolina. But observing swell in Bermuda is completely unhelpful for predicting swell in Charleston. And why is this? Because low pressure systems tend to form off the US eastern seaboard and then move eastward. So a storm in Charleston might later impact Bermuda, but not the other way around. The Azores to Casablanca forecast worked purely because of the typical atmospheric patterns in the North Atlantic. So in the mid-latitudes, the prevalent flow is to the east. Storms tend to follow that flow, creating swell that also travels eastward. So this method developed in the Azores and Casablanca couldn't really be applied elsewhere, but a universal method of predicting surf, predicting wave heights, and especially heights of the surf was what was needed as the world plunged into the Second Great War. So amphibious landings were becoming an increasingly important tactical approach for the Allied forces. To accomplish this, scientific understanding needed to make a massive leap forward in three specific areas. So number one how did waves develop under the influence of wind? Number two, how did waves propagate? How did they get from one place to another? And number three, how did they transform once they encounter the coast? What was happening when waves encountered bathymetry and the sea floor? So there were some hints about how waves developed. For example, the, the Beaufort scale was in wide use, and it roughly gives a wave height as a function of wind speed. And by the turn of the 20th century, scholars knew that, space and time were also important factors. But regarding propagation and transformation, so I found this paper by Captain Charles Bates. He was a meteorological officer during the war. He later recalled and I quote, "Contemporary research did not adequately describe how a wave train decayed within distance from its generating area and then in shallow water changed into a plunging breaker." In essence, at the time, not a lot was known. In Great Britain, Instructor Commander Claude Suthens was assigned to the task of transforming sparse data, wave data, into a sort of rule of thumb. And he was working by himself. He developed some relationships between the effective wave height data and wind speed. And he produced these graphs linking fetch, which is like the distance that waves are developing, and duration, which is the time over which waves are developing, to wave height and wave period. And he estimated a decay rate over space. So he was sort of the first person that I could tell that was really trying to dive into the data and give empirical answers to these questions. So it was an empirical, like engineering-style solution. He characterized the system using available observations and basically hoped that nothing happened outside the scope of the data that he was working with. Meanwhile, the American effort to predict surf was led by two European immigrants. And this is when Walter Munk enters the story. As a young man, Walter Munk's primary interest was outdoor sports, skiing in particular, which he had plenty of opportunity to do in his native Austria. His well-to-do family eventually sent him to a prep school in New York, to distance him from the sport, hoping he would fall in line and join the family banking business. Munk tried and failed as a banker in New York. He eventually made his way west to Caltech in Pasadena, California, and he pursued a degree in physics. In the summer of 1939, the 21-year-old Munk followed a girlfriend to the serene coastal village of La Jolla, just north of San Diego. To afford his stay, he picked up a summer job at the Scripps Institute of Oceanography. He worked under, the director, Harald Sverdrup, who was a 51-year-old Norwegian explorer and pioneering oceanographer. The inciting romance faded, but Munk became completely smitten with the ocean. The following summer, Munk asked Sverdrup if he could become a PhD student under him at Scripps after graduating. Sverdrup Initially hesitated. He told Munk he didn't know of a single position in oceanography that would open up within the next 12 years. You see, in the 1940s, oceanographic institutes in America numbered exactly three: Scripps in La Jolla, Woods Hole in Massachusetts, and University of Washington in Seattle. Positions were so incredibly rare that you essentially had to wait for someone to die to create a new job opportunity. And Sverdrup wasn't just trying to dissuade what he perceived as an overenthusiastic youth. He was genuinely concerned about the young man's future prospects. But Munk, for his part, was totally unfazed, and without hesitation, he replied, "I'll take it." These were the early days of oceanography, and Munk liked to say that for some years he made up the entire student body at Scripps. There is perhaps no more idyllic place in America than La Jolla, and Scripps, with its beachfront campus, was a paradise. It is a paradise. Munk had hit the jackpot, except all was not well back home in Europe. Austria had recently been invaded by Nazi Germany. Compelled by the growing conflict, Munk put his oceanographic studies on hold. He volunteered for the US Army, and he applied for American citizenship. While his oceanographic compatriots, they joined the Naval Reserves, Munk joined the ski battalion in Seattle, Washington, for reasons which I hope are altogether obvious. After serving eighteen months and growing frustrated with the lack of action, Munk was actually discharged at the behest of Sverdrup, which allowed him to return to Scripps. German submarines were picking off US merchant ships seemingly at will, and the anti-submarine warfare effort needed Munk's help. He barely unpacked his bags back at Scripps when Japan attacked Pearl Harbor, and just like that, the US was fully entered into the fray. Discharges were no longer possible. Had Munk still been with his ski company, it turns out he would have been deployed to Papua New Guinea, uh, a place where the ability to ski was definitely not an advantage. According to Munk, his former company was virtually wiped out. But as fate would have it, Munk found himself in the company of oceanographers when the war escalated, and this may have made all the difference Under an anti-submarine research detail, Munk was whisked off to the Pentagon. There he learned about plans for Operation Torch operation Torch was to be the first major offensive by the Allied forces, which included amphibious landings in North Africa utilizing a newly built landing craft vehicle personnel boat. You probably know them better as Higgins boats, named after the New Orleans-based designer Andrew Higgins. Imagine a miniature barge. It's a transport vessel. It's built to carry loads, loads of people and equipment from deep water right up to the shore. The bow of a Higgins boat, it articulates, so upon beaching, the front wall drops down like a ramp and troops and equipment spill forth. That is, if the Higgins boat first manages to navigate safely to the beach Beaching the vessel required a shallow draft and relatively flat bottom. And this was helpful also for carrying the heavy loads it needed to carry. But it was a terrible design for navigating waves. Larger landing ships had tanks that could flood to increase their draft and improve their handling in heavy seas. But the Higgins boat had no such mechanism. Captain Bates later described them as such: "Blunt, pop-open bows made them extremely sensitive to wave action. To a large degree, they looked like and behaved like floating shoeboxes." Walter Munk flew down to the Carolinas to watch the military field test these Higgins boats. He quickly learned some terrifying performance heuristics. The boats operated well when the seas were three to five feet, but if the waves exceeded five feet, the Higgins boats had a tendency to broach, unintentionally veering ninety degrees. The broad side of the boat would become exposed to the surf, and once perpendicular to the oncoming waves, waves would just simply break over the side rail and flood the interior. With wave after wave breaking over the side, it was only a matter of time until the vessel foundered and sank, spelling disaster for the men and equipment on board. In short, Higgins boats were an asset on calm days and a deadly liability in rough seas. Because of this, the military simply did not run training exercises in seas greater than five feet. A question formed in Munk's mind. How does the wave climate at the test site in the Carolinas compare to the wave climate in Northwest Africa, where, Operation Torch was set to proceed? He gathered what information he could. It turned out that while five-foot waves were rare in Carolina, six feet was the average winter wave height on the northwest coast of Africa, and it frequently exceeded that. Munk saw a catastrophe in the making. What would happen to the soldiers in these high seas? When the alarmed young scientist brought this deadly discrepancy to the attention of his superiors, he was casually dismissed. They told him, "They've probably thought of that." Meaning, mind your business. Do what you're told. Don't ask questions. But they probably thought of that. Well, who is they? It's possible they referred to the British prediction method, but Munk came to believe that there simply was no they. Nobody was looking at this. Munk, at the time, was just a student with low rank, nonexistent reputation. He was in no position to challenge authority, but he couldn't let it go. The military had wave information in advance of an operation. They could make go or no-go decisions that would save lives. He must have believed that he could figure out the problem of predicting waves because he set about doing it without explicit permission. In today's world, a 10-day surf forecast for any beach on Earth is a click away on your phone. But Walter Munk had next to nothing to go on. Little was known about the wind-wave development. Mariners knew local signs of bad weather. People on the coast simply looked at the surf height right out in front of them. A- and with no method for predicting waves, Munk had to invent the implements out of whole cloth. Just like the Casablanca office, he started with synoptic weather charts. It was the only accessible large-scale data to define where winds were blowing. He knew there had to be a mathematical relationship between the growth of waves and the low-pressure areas on these maps. After months of careful study, frustrated by slow progress and unable to influence ranking officers, Walter Munk reached out for help. He called his mentor, Harald Sverdrup. Sverdrup dropped everything, took the next flight from San Diego to DC, and joined Munk at the Pentagon for three months of intense work They didn't have a lot of wave data to work with, but enough to tune their method. However, a problem remained. Some data were visual records from trained officers giving wave height and period, and other data were novel pen records for machines, which tracked every single wave height and trough. How do you reconcile visual human estimates with highly detailed machine records? The answer was given by a coxswain. So each Higgins boat had a coxswain who was sort of a captain and navigator, and they were trained in estimating wave heights visually. Munk consulted with the coxswains training for Operation Torch. From their reports, he realized that their visual estimates matched a very specific type of average from the machine charts. The coxswains weren't estimating the average of all waves. They were visually estimating the average of the highest one-third of waves, and Munk called this metric the significant wave height. Finally, they had a single standardized number that could be associated with the sea state. Sverdrup and Munk had their mathematical relationship. Previous scales only looked at wind speed. Sverdrup and Munk's innovation quantified relationships that accounted for the size, intensity, and duration of a storm. So the significant wave height and the average wavelength grew in storms, according to Sverdrup and Munk's data. After a storm died down, the waves would continue to propagate over the ocean as swell. And the speed of the propagation was related to the wavelength of the swell, which had been worked out analytically since the 19th century. Sverdrup and Munk used data from the French swell forecasters, which informed them how the swell decayed as it propagated over space. Finally, swell turned toward the beach. It slowed down, it steepened until it broke in shallow water. Wind sea, swell, surf Sverdrup and Munk Had cemented the concepts of wind sea and swell and surf into the canon of wave science. Now that they had a workable method, they had to overcome the military bureaucracy. This is where Sverdrup's clout came in. Harold Sverdrup was world renowned. He commanded respect. His influence got them a seat at the Operation Torch planning table. With their new method, they produced a forecast for the North African landings. It indicated a small window of opportunity. Outside that window, the waves would be too large for the Higgins boats. The commanders decided to run the operation within that narrow gap. The predictions were accurate. The Higgins boats made it to the beach. But as predicted, the waves began to rise on the first afternoon after landing. The landing craft already on the beach and in the surf suffered heavy losses as the waves' size increased. By the end of this assault's second day in November 1942, a staggering 64% of the 378 landing craft deployed at Casablanca's beachhead had broached, stranded, or sunk in breakers that reached six feet or more. The losses terrified the Allied forces because the stakes were about to become much, much more dire. An even larger landing was in the works, Operation Neptune. The Allied commanders were betting everything on this operation, and everyone at the highest levels knew that surf could make or break the war. As President Roosevelt wrote in a message to Churchill, "Bad surf on the Atlantic beaches is a calculated risk This wasn't just about dropping troops off. It was a 15-week logistical operation of transporting men, equipment, and supplies over the beaches. Munk and Sverdrup returned to La Jolla in May 1943 to train a class of meteorological officers, weather warriors. Month after month, they taught, refined, and evolved their method, graduating about 100 officers ready to apply this new Sverdrup-Munk surf prediction method. These men were about to become central to the new offensive brewing off the coast of England. Operation Neptune under Eisenhower, who was the supreme commander of the Allied forces, was to be an audacious invasion of coastal France, the largest military buildup of the war. If the Higgins boats didn't make it to the battlefield, the battle would be lost before a single shot was fired. Winston Churchill wrote in his diary, "The destiny of two great empires are seemingly tied up in some goddamn things called Higgins boats." Wave prediction was central to avoiding catastrophe. Because weather can only be predicted a few days out, the rough schedule was dictated by the seasons. They opted for the placid summer. The beaches of Normandy generally face north, protected from distant Atlantic swell and simplifying the prediction to one that relied more on local winds. But waves weren't the only factor. The military needed a full moon to drop thirteen thousand paratroopers in the dark. The Germans had planted a dense network of obstacles in the intertidal zone, so the army wanted to land at high tide to minimize the length of the beaches for the infantry to cross. The Navy wanted to go at low tide, exposing the obstacles so the army engineers could blow them up. The compromise: time the dawn invasion to coincide with the incoming tide. It would propel The boats towards the shore, the tide would be low enough to blow the obstacles, and the beach would shorten as the morning advanced. To recap, Eisenhower needed a low tide occurring just before dawn on a full moon in the mild summer season. Finding a day that met all of these requirements severely limited their options. Eisenhower chose June 5th, 1944 The Royal Navy set up a swell forecast section using fifty-eight observation stations off the coast of Western Europe. They tested both the British and the American forecasting methods. The Sverdrup-Munk method won out for its accuracy and simplicity. As June fifth approached, a joint team of officers made their predictions. On June third, weather maps showed a gale developing in the North Atlantic and heading east. In Blacksod, Ireland, a twenty-one-year-old postmistress named Maureen Flavin Sweeney awoke at one AM on her birthday to make weather observations. Blacksod was the most westerly station. She measured a drop in air pressure and an uptick in wind and a slight drizzle. These were the first whispers of incoming inclement conditions. The gale was real, and large waves were predicted in the English Channel. It became incredibly obvious that while the tide was optimal, the seas would be too high for the Higgins boats, a bloodbath waiting to happen. However, the Sverdrup-Munk model predicted that the following day, the seas would fall to six feet, the absolute razor edge of the operational limit. Eisenhower had a brutal choice. Every day that they delayed, the tidal minima shifted by 30 minutes. A 24-hour delay meant low tide was no longer perfectly aligned with dawn, but it was still workable. A 48-hour delay meant the tide was just totally wrong, and the next viable pre-dawn low tide wasn't for another two weeks, and that would be on a new moon. Terrible for the paratroopers. Most of all, Eisenhower was terrified of losing the element of surprise. With disaster on his mind, he made the call. He pushed the invasion back by one single day. He was so anxious, he actually drafted a resignation letter to President Truman just in case About ten miles offshore of the French coast, well before sunrise on June sixth, Allied troops began to load into Higgins boats hanging off the sides of large battleships. As predicted, the wave height had dropped, but it was perfectly balanced on the knife's edge. The rough seas were a shock. Soldiers were unsettled by the cold, the wet, and they struggled against violent seasickness. It was a miserable start, but they were alive. If you've ever seen the famous photograph taken by Robert F. Sargent titled Taxis to Hell and Back, you know exactly what this looked like. The jaws of the Higgins boats dropping open into the churning surf, soldiers wading out into the water toward beaches of Normandy. This was the first of many perils for those troops, but almost all of the 133,000 of them made it to the beach June sixth, nineteen forty-four became known as D-Day. It was perhaps the most crucial turning point in the war against the Nazis. It is no exaggeration to say that this single decision to delay the invasion, a decision weighed by the mathematics of wave prediction, it changed the world. For the rest of his life, Walter Munk remained humble about his role, claiming they were just doing "lucky extrapolation from inadequate data." But in nineteen sixty-five, Professor Blair Kinsman from John Hopkins University gave Munk's work the proper gravitas. He said, "There are some thousands of World War II veterans alive today who would have been dead in the surf had Sverdrup and Munk not done their best with what they had." Munk took great pride in that quote, and rightfully so. But this story, it has one final twist. It crosses from the realm of scientific ingenuity straight into the realm of pure cosmic fate. Remember how Eisenhower felt incredible pressure to go on June sixth to maintain the element of surprise? Had he not gone on June sixth, the next opportunity based on the tides wasn't for another two weeks. We can't know for sure if the Germans would have discovered the armada waiting in England over those two weeks, but we do know what the weather did. Exactly two weeks after D-Day, a massive, unpredicted storm slammed into the English Channel. It was entirely missed by the meteorologists. The surf prediction for that day would have indicated completely benign conditions. When that storm hit on D-Day plus thirteen, it wrecked the newly installed artificial harbors and destroyed countless ships. If Eisenhower had delayed the initial invasion to that second tidal window, The fleet of Higgins boats would have been sent blindly into a devastating storm. The losses would have been catastrophic. The invasion would have failed. Upon hearing about the havoc wreaked by that storm, Eisenhower looked at the wreckage and said, "I thank the gods of war we went when we did." As for his resignation letter, it remained unsent Thanks for listening. I hope you enjoyed this story about how surf science saved the world
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