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It was the raw courage of the more than 160,000 Allied troops who stormed an 80-kilometer (50-mile) stretch of heavily fortified beaches in Normandy, France, that made victory on D-Day possible. But without the sound advice of meteorologists and geologists working behind the scenes, one of the most consequential battles in human history could have gone quite differently.

On June 4, 1944, armadas of Allied forces were either amassed in Portsmouth Harbor or steaming toward France. Allied Supreme Commander Dwight Eisenhower had set the early morning of June 5 as the date for the massive amphibious invasion-a date military planners had carefully selected. It was one of just three days in June 1944 when the Moon would be bright enough for paratroopers and pilots to operate effectively and the tides would be low enough for the first landing parties to clear mines, barbed wire, and other defenses on the beaches.

But they also needed relatively clear skies and calm seas. The job of predicting the English Channel's famously fickle weather fell to teams of meteorologists from the U.S. Army Air Corps, the British Royal Navy, and the British Meteorological Office. At the time, weather forecasting was a young science; there were no satellites, no computer models, and no weather radar.

As D-Day neared, the American meteorologists predicted fair weather on June 5 and pushed for invasion, based on a forecasting method that gave great weight to historical weather conditions for a given date and location. The British forecasters took a different approach, focusing instead on analyzing measurements of temperature, pressure, and humidity to try to map out weather fronts. Unlike the Americans, the British teams predicted low clouds and stormy weather on June 5. At the last minute, Captain James Martin Stagg, the highest ranking of the meteorologists, convinced Eisenhower to postpone the invasion.

Meanwhile, on the other side of the English Channel, German meteorologists had come to the same conclusion-and then some. Their forecasters had predicted that gale-force winds would arrive on June 5 and persist until mid-June. The Germans were so confident that the Allies would not dare attack that they allowed many soldiers to leave their posts on the beaches and take part in war games in Rennes, France. Field Marshal Erwin Rommel felt comfortable enough to return to Germany to deliver a pair of shoes to his wife as a birthday present.

On June 4 and June 5, Allied scientists agonized over a few clues that suggested there could be a break between storms on June 6. Those few atmospheric pressure readings weren't much to go on, but the meteorologists thought there would be just enough fair weather to make an invasion viable. "Okay, let 'er rip." Eisenhower reportedly said during an early morning meeting on June 5 with top staff assembled at a house in southern England.

When the first paratroopers were dropped behind enemy lines around midnight and the first wave of Allied boats began to swarm the beaches at dawn on June 6, the weather was still far from ideal. Cloud cover meant many paratroopers ended up in the wrong locations, and rough seas and high winds made the task of landing boats and unloading tanks a terrible challenge. But by noon the skies cleared, just as the Allied meteorologists had predicted. The Germans, meanwhile, had been caught off guard. That day the Allies endured thousands of causalities, but they established a toehold in France that they would never give up.

An enormous amount of scientific expertise went into even the most unscientific of tasks, like rolling a tank up the Normandy beaches. Prior to the invasion, Allied military planners studied nearly one million aerial photographs of the shores of Normandy to find the best landing sites. The aerial photographs would have looked something like the Landsat 8 image shown above. Acquired by the Operational Land Imager (OLI) on July 15, 2018, the image offers a top-down view of the sandy Normandy beaches that were center stage on D-Day.

In January 1944, Allied forces even sent reconnaissance teams of divers to secretly collect samples of sand; the goal was to determine whether it was strong enough to support tanks and other heavy equipment. They divers came at night in small submarines, swimming to shore with sample tubes, augur screws, waterproof writing pads, and flasks of brandy. When scientific surveys had determined Normandy sand-mainly quartz, feldspar, and limestone-were quite similar to the beaches at Brancaster in Norfolk, military planners bombed the Brancaster beach to determine how easily vehicles could drive over bomb craters. The sand was strong enough, the Allied teams of engineers and geologists concluded.

Long after that battle and war, during a ride to the Capitol for his inauguration, President-elect John F. Kennedy asked President Eisenhower why the Normandy invasion had succeeded. Eisenhower's fitting response: "Because we had better meteorologists than the Germans!"
NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey. Story by Adam Voiland.

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It was the raw courage of the more than 160,000 Allied troops who stormed an 80-kilometer (50-mile) stretch of heavily fortified beaches in Normandy, France, that made victory on D-Day possible. But without the sound advice of meteorologists and geologists working behind the scenes, one of the most consequential battles in human history could have gone quite differently.

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Seventy-five years ago, in early June 1944, Allied forces landed in northern France as part of a military action that paved the way for the defeat of Nazi Germany, molding the course of world history.

While careful planning, demonstrations of collective and individual bravery and, in many cases, the ultimate human sacrifice contributed to the operation, much of its success came down to the weather.

A team of six meteorologists - two each from the U.K. Met Office, Royal Navy and United States military - worked for months honing forecasting techniques, many of which have stood the test of time, before advising Allied commanders, led by General Dwight D. Eisenhower, as to the optimal time for attack.

The Allies ended up sailing and landing in relatively calm waters, but documents released in the intervening years showed just how close bad weather came to making the operation a complete failure.

"I don't think people realize how close-run it was," Dr. Lawrence Hogben, one of the Navy meteorologists, told The Telegraph. "Not much would have to have changed for D-Day to have been a failure, and a failure caused by the weather."

Based on their own forecasts, two U.S. meteorologists, Irving Crick and Ben Holtzman, believed the attack should take place on June 5, an assertion Hogben and Geoffrey Wolfe of the Royal Navy and C. K. M. Douglas and Sverre Pettersen from the U.K. Met Office disagreed with.

Crick and Holtzman were so adamant their appraisal was correct that they phoned Douglas and Pettersen in an attempt to make them change their conflicting forecast.

But the U.K. Met Office pair refused to relent, forcing Eisenhower to postpone the landings for 24 hours. It was a decision that would prove crucial as, on the morning of June 5, there were rough seas and 25-30 mph winds in the English Channel.

"The weather was terrible that morning," recalled Hogben. "The outcome of D-Day, perhaps the whole future of the western world rested on those forecasts, so I think you could say there was some pressure."

Despite correctly shelving the June 5 plan, the mood was far from celebratory that day. Unless the weather conditions changed to allow the attack, the operation would have to be postponed until June 19 when the tides would be right for the landings.

The six-strong meteorology team then produced what would prove to be one of the most significant forecasts in modern history, declaring June 6 suitable for the Allied attack.

History reflects that despite a terrible human toll, the D-Day landings spelled the beginning of the end for Hitler's reign of terror in Europe.

But, as Hogben explained, things could have been considerably, and tragically, different, or, "utter catastrophe, complete failure," as he put it. "If we had been a little less certain and said no again, it would have had to shift to the 19th," he added.

"As it happened, on the 17th, all six of us produced a forecast for the 19th for almost perfect conditions, so they would definitely have gone ahead," said Hogben. "On 19 June the biggest storm of the 20th century came up. If they had landed that day, I doubt many landing craft would have even made it to the beaches. It does not bear thinking about."

Back in the 1940s and during the Second World War, weather forecasting was still in its infancy. There were no satellites, weather radars or "supercomputers," and simply managing to get weather observations in a timely fashion was a challenge. The Weather Channel U.K.'s chief meteorologist, Leon Brown, takes a closer look at the weather conditions that prevailed that fateful day.

An Expert View of the D-Day Forecast

The map below shows an approximation of the weather pattern in the run up to D-Day. The jet stream must have been farther south and stronger than normal, steering deep depressions past northwestern Europe in early June.

Back in 1944, meteorologists didn't know much about the upper atmosphere or that jet streams existed, and there were very few observations from the Atlantic.

At the time the D-Day landings were being planned, Captain James Stagg was the Chief Meteorological Adviser to General Eisenhower and, along with a team of six specially selected meteorologists, had the unenviable task of trying to forecast the weather several days ahead. In fact, Eisenhower insisted on a five-day forecast, which can sometimes be a challenge even today.

Back then, forecasts were essentially nothing more than a guess beyond 24 hours ahead, which makes the story of the D-Day forecast all the more remarkable.

The Americans wanted the invasion to start early, and no later than May, while the British were more inclined to wait until August. Ironically, May 1944 was a month of fine, settled conditions, and then the weather suddenly changed dramatically in early June.

The initial plan was to invade the Normandy coast on June 5. However, on June 4, an unusually deep area of low pressure tracked over Ireland and then across northern Scotland with gales and heavy rain. Gusts over 60 mph were recorded around coasts in the west and north, and the English Channel would have been very rough.

So, it quickly became clear that the invasion had to be delayed, and the American and British meteorologists did their best to collect surface observations and plot them to create surface-pressure charts, then estimate the positions of any fronts.

In the fast-moving weather pattern at the time, this would have only given them a rough idea of the weather to come.

Captain Stagg advised that there should be a weather window for June 6. But there were a wide range of forecasts, with the British expecting more wind and rain and the Americans predicting three days of more-settled weather. The weather window was short during the event and the invasion was only just possible, but it was a great call from Captain Stagg's team.

Delaying much beyond June 6 would have meant waiting a few more weeks for the tides to be right, and by then, the Germans may have realized an invasion was imminent. In the event, the end of June was again quite stormy, so delaying may have been no better.

In fact, June 1944 was one of the windiest of the century in southeastern England, with only 1917 windier, going back to 1895.

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D-Day: An Expert View of the Weather Forecast That Changed History | The Weather Channel

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Seventy-five years ago, Allied forces landed in northern France as part of a military action that paved the way for the defeat of Nazi Germany, molding the course of world history in the process. - Articles from The Weather Channel | weather.com

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HOST: This is the NOAA Ocean Podcast, I'm Troy Kitch. D-Day, codenamed Operation Neptune, was the largest seaborne invasion not only in World War II, but in history. It was a massive, complex amphibious landing along the Normandy coast of France that began on June 6, 1944 - within ten days there were half a million troops ashore, and within three weeks there were two million. All told, D-Day marked the start of the liberation of German-occupied France (and later western Europe) and laid the foundations of the Allied victory on the Western Front. But most people don't know how ocean tides played a crucial role in the initial phase of the invasion. Joining us by phone today to tell us the story of D-Day from the perspective of the science of tides and tide predictions is Greg Dusek, a physical oceanographer and senior scientist at the Center for Operational Oceanographic Products and Services, or CO-OPS - the tides and currents office of the National Ocean Service. Greg, thanks for joining the NOAA Ocean Podcast. What sort of conditions were the allies looking for as they planned the invasion of Normandy?

GREG DUSEK: "So the allies were planning an amphibious assault. They were going to cross the English Channel to the French coastline, near Normandy. Because they were going over the ocean, they needed good weather, so they needed to find a time where the waves were going to be minimal and the winds were going to be minimal, and obviously that's something they really can't plan ahead of time. But they knew that, in the summer months, you were more likely to have good weather, so they wanted to plan a time in the summer for the assault. They also wanted to have a time period where you had a full moon or close to a full moon the night before the assault, and the reason for that was, they were going to have airborne infantry sent behind enemy lines the night before, and to do that, you needed some sort of lights for them to be able to figure out where they're going. So they wanted close to a full moon the night before. And then, lastly, they were looking for a time with low tide shortly after dawn. And the reason they needed it shortly after dawn, was because they needed a couple hours of time for the amphibious assault groups to travel across the English Channel in darkness, but then enable the Naval bombardment to have daylight to be able to target initial areas of interest to bombard, before the amphibious assault began. Those criteria, you know, didn't have a lot of times to work with, and June 5th, 6th, and 7th of 1944 were the three days that were identified."

HOST: That is a lot of environmental factors to have all fall into place. That last part, looking for a time with low tide shortly after dawn, is where we get to the science of predicting the tide for a particular location.

GREG DUSEK: "So tide predictions were top secret during WWII, and the reason for that was you wanted to limit the axis information about allied-held coasts, you didn't want to divulge any information that they might not already have about the tide or our own coastlines, so they couldn't plan their own attacks. And then the other important part was that, if we were generating predictions for positions we were likely to attack, if the enemy found those predictions, it might tip them off as to where we were thinking about attacking next. So all of the work done relating to tide predictions was really secretive and it was a lot of work to make sure that none of that information escaped and was available to the enemy."

HOST: And this was of course well before computers. Can you tell us a bit about the basics of what tides are and how people predicted tides in the past, leading up to WWII?

GREG DUSEK: "Tide predictions enable us to tell when is high and low tide and what time is high and low tide going to occur at different coastal locations. The tide is related to the position of the sun and the moon relative to the Earth, and so going back even to the mid-1700s, people understood that when you had high tide every day, and how high the tide was, relates to, in particular, the phase of the moon. And so, even in the mid-1700s in colonial America there were tide predictions of the timing of the tides at various harbors. They weren't terribly accurate, but they did provide some information which would be useful to mariners and to citizens. But it wasn't really until the late 1800s that a few folks - Sir William Thompson, who later became known as Lord Kelvin in England and William Farrell of the U.S., who was at the U.S. Coast Survey, which later actually became NOAA much later in time - they were the first ones to figure out if we go and collect the observations, go measure the water level for a month at a time at a certain location, you could then figure out what frequencies make up the tide, what are known as tidal constituents. And if you knew those tidal constituents at a particular location, then you could use that information to generate tide predictions, or very specific water levels and times of the tide at a certain location even months or years into the future."

HOST: So how were tide predictions calculated once this was figured out in the late 1800s?

GREG DUSEK: "Understanding that there were these specific frequencies that related to the tides, these tidal constituents, you could recreate those mechanically. So you could have these different gears and pulleys represent specific frequencies and then use what's known as a tide machine to, basically, you put in those constituents, and that tide machine would spit out a tidal curve telling you exactly the times and water levels associated with certain tidal constituents. Some of the first tide machines were created in the late 1800s, and in the U.S., really kind of the best tide machine that was really ever created was finished in about 1912 and, in fact, if you ever visit NOAA, you can find it right in one of our buildings. We still have it there today. It was called 'old brass brains,' and it was this metal machine about eleven feet long, about as tall as a person, and people would operate that all the time, generating tide predictions from tidal constituents - and you can generate those predictions for anywhere in the world, as long as you had the information about the constituents. And the big thing with this machine is that this used to be a process that was done by hand. There was a quote in a New York Times article when the machine was first put out, where they say, 'the machine turns out in ten to 15 hours the work that would keep a mere human calculator busy for six months.' So, you know, we used to have human calculators, and it would take about six months for them to do one set of tide predictions, and now we could do it in maybe a day or so."

HOST: I'm still not clear on the idea of tidal constituents.

GREG DUSEK: "So tidal constituents are specific frequencies that represent the position of the moon and the sun relative to the Earth, and how that influences water level. You can calculate the tide with not too many tidal constituents, maybe 20 or 30. The tide machines of the time could solve for about 37 tide constituents, which is usually more than enough to get a really accurate prediction, but there are several hundred known constituents that we can solve for today if needed."

HOST: How many of these mechanical tide machines did the Allies have during the war?

GREG DUSEK: "There were a number of these tide machines by the time WWII rolled around. We had one in the United States, there were two in the United Kingdom. Pretty much every maritime nation at that point had some sort of tide machine to enable them to generate tide predictions. But because we only had three primary machines within the allied nations, it was really important to keep them safe. So in the UK, for instance, they had these two machines which they had in two different locations, because they were really afraid of the Germans figuring out where the machines were and then firebombing them to destroy them, because they would've been high value targets. So the location was a closely held secret and the really tried to keep them hidden. If they lost both of them, that could be a huge implication on the war effort, so it was really important to keep the machines safe."

HOST: And this leads us back to planning for the D-Day invasion. What were some of the challenges of predicting the tides along the Normandy coast where the Allies planned to land?

GREG DUSEK: "So the tide range around the Normandy beaches was around 20 feet, because you have this really large tidal range, the beach you're going to have to traverse is going to be wildly different depending on if you're at low tide or high tide. At low tide, you might have 2-300 yards more of beach to cover, during an amphibious assault, then at high tide. So because of that, you really want to minimize your exposure. The other important part about the tide range is that the water level is changing very rapidly. So *you could be gaining or losing about a foot of water every 15 minutes. *On top of that, the Normandy coastline is kind of complex, and that influences the tides fairly substantially as you go along the coast. So, the allies had five locations they wanted to land at. Utah and Sword were the two furthest apart, about 100 kilometers apart, and the tide could vary by more than hour between those two locations, and so knowing the precise time of low tide was going to be really important."

HOST: So how did the Allies get the information they needed to do their calculations for these extreme tide conditions?

GREG DUSEK: "At this time in history, there was tidal information at lots of the major coastal cities. There were tide constituents generated from water level observations at nearby cities - Cherbourg to the West and Le Have to the East - but now both of these locations were about 100 kilometers away from the landing beaches, and so the predictions that you generated at those locations would probably not be very accurate for where the landings were going to occur. And so they had, in the tide tables at the time, there was some information closer to the landing zones, but it wasn't nearly as detailed and there was even a note in the tide tables that said, 'these predictions may not be accurate.' So they really didn't have much to go on for the exact landing locations. Apparently, the allies actually sent in some special forces, night reconnaissance, ahead of the attack, to look at the bathymetry (the shape of the bottom), the type of sand that was there, and to even collect a little bit of information about the tides, which could then support calculating new predictions for those specific landing zones."

HOST: And why was it so important to stage the invasion at low tide?

GREG DUSEK: "They knew they wanted to land at low tide, so that they could send their initial forces to clear out the number of objects on the beach, but if you can imagine that they got tides just a little bit wrong, say they were off by 30 minutes, 45 minutes, and they landed just before low tide. Well, because of the tide range, water's *dropping about a foot every hour right before low tide, *and so your amphibious craft would've arrived, unloaded the troops, the tides would've dropped, and all the craft would've been stuck on the beach - and then, you know, you'd be gumming up the whole operation, you wouldn't be able to have reinforcements come in, and it would have been a disaster. And so they needed to arrive just after low tide, so that water levels are *rising about a foot an hour, *and it would enable their crafts to drop the troops and then get back out of the way for the next round of troops to arrive."

HOST: At the time, German forces knew that the allies would likely try an invasion of the French coast from across the English Channel. Can you talk about how the Germans were planning for this?

GREG DUSEK: "The Germans and Gen. Rommel were really expecting the allies to attack at high tide, because at low tide there would be maybe 2-300 yards of beach that the amphibious forces would have to traverse, leaving them exposed for an extended period of time. Because of that, Rommel had all of these obstacles placed along the beach - millions of obstacles along the French and Belgian coastlines - and so, they were convinced that an attack would happen at high tide. Now, the allies saw these obstacles and decided that a high tide attack wouldn't be possible, and instead they would have to plan their attack at low tide, giving their initial troops a chance to move obstacles out of the way, blow up obstacles, and clear a path for the heavy infantry and tanks and things like that, that would follow the initial attack."

HOST: So you said that all the conditions that needed to fall in place - the moon, the weather, the tides - meant that the small window between the 5th and 7th of June were the best dates for the Allies to launch the invasion. How did the Allies settle on June 6th to commence operations?

GREG DUSEK: "On June 4th, the weather was going to be too bad, so they didn't attack. They waited until June 6th. The weather was still not great and it was very questionable, but Eisenhower made the decision to attack because he was worried that, if it failed here, they'd have to wait at least two weeks and maybe a month or more to go forward with the assault, and then it could've been figured out. They didn't want to wait that long, because they were losing lives like crazy. Even though the weather was rough getting across the Channel, it actually worked in our favor because Rommel - because the weather wasn't any good and because it was low tide at first light and he was anticipating a high-tide assault - he was actually not even in Berlin, he was visiting his wife for her birthday somewhere else, and so wasn't prepared even for the assault at all. So we actually caught them off guard by choosing to attack that day."

HOST: Well the invasion of course succeeded, so among all of the factors that led to this success, I guess that means that the Allies got the tide predictions right for the Normandy coast on June 6th, 1944?

HOST: GREG DUSEK: "Later on, people went back and using computers and using hydrodynamic models re-ran a simulation to look at how accurate the predictions were around the Normandy coastline for the assault and found them to be really quite accurate, you know, using a mechanical machine and data collected from a few hours in a midget submarine or something, was almost as accurate as we can determine today. I would say anecdotally in our office, we've looked at a lot of historical predictions going back to the late 1800s, early 1900s, and compared them to what we can find today with modern instruments and modern computers and are always amazed at just how accurate, you know, what people were able to figure out with really minimal amounts of information and technology, and could get pretty darned close to what we can measure today with all of the technology we have."

HOST: From your perspective as an oceanographer at NOAA's tides and currents office, what's your takeaway from the D-Day invasion?

"The important thing to think about with this is that tides really were one of the pivotal roles with one of the most important days in human history. I think it shows just how important it is to understand our natural environment and to be able to make prediction about the natural environment. It seems simple, in terms of the rise and fall of the water level due to tides, but it's actually quite complex. It can take a lot of work to figure out and can be important to understand for a whole range of reasons."

HOST: It's truly a remarkable story. I'm curious how you discovered the historical role of tide predictions for the invasion of the Normandy coast in WWII?

GREG DUSEK: "Our former chief scientist of the National Ocean Service, Bruce Parker, was really on the forefront of tide prediction and tide analysis when he was here at NOAA, and did a lot of research into tide history. He wrote a book a number of years ago, The Power of the Sea, where he goes into a lot of stories and other interesting historical facts about the tide and about the ocean. He's been a great source for this kind of historical research and it's really interesting."

HOST: I imagine tide prediction in the 21st century has come a long way from the Old Brass Brains machine the U.S. used back in the day.

GREG DUSEK: "Today, we have the benefit of having a lot more data. You know, instead of maybe collecting information about the water level a couple times a day, a few days, or a few months, we have instruments that measure water levels every six minutes for years and years. And so, we can take that time series and run it in a computer, and take four, five, six years of data or more, run it through a computer, and figure out very precisely what the tidal constituents are for a particular location, and then generate tide predictions from those constituents, again, every six minutes, at a very high level of accuracy going out years into the future. From a time standpoint, we can do that in a matter of minutes. Whereas before, even after the tide machine was created, it was taking people days to generate these predictions. So we've advanced quite a bit."

HOST: What are some of the reasons that we need such detailed information about water levels today?

GREG DUSEK: "The importance of tides and currents for NOAA and the nation really comes down to a couple of things. One is safe and efficient navigation. So if you're pulling in a large commercial vessel or Navy warship, you need to know precisely when low and high tide are going to be, and high it's going to be, so you know you're not going to run aground or run into a bridge, things like that. On top of that, tide predictions are really important to things like high tide flooding. So one of the benefits of being able to predict the tide really well means that we can figure out what days you're likely to see flooding in certain locations. So it's really important for public safety and or providing awareness regarding potential hazards. So tide predictions is something we know how to do really well, but it remains something that is really important for a wide range of uses."

HOST: That was Greg Dusek, a physical oceanographer and senior scientist at the Center for Operational Oceanographic Products and Services, or CO-OPS - the tides and currents office of the National Ocean Service. And that's all for this episode. If you like what you hear, head to iTunes and leave us a rating. It'll help more people find our show. And head to oceanservice.noaa.gov or check your podcast app for our show notes - and be sure to subscribe to the NOAA Ocean Podcast so you don't miss any episodes.

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national oceanic and atmospheric administration

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Turning the Tide: D-Day and Tide Prediction

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D-Day, codenamed Operation Neptune, was the largest amphibious landing not only in World War II, but in history. It marked the start of the liberation of German-occupied France (and later western Europe) and laid the foundations of the Allied victory on the Western Front.

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D-Day Landing (4:15minutes) [The Longest Day - movie]

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D-Day Landing (4:15minutes) [The Longest Day - movie]

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D-Day: How The Normandy Beaches Were Stormed | D-Day's Sunken Secrets | Channel 5 #WW2

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In contrast to the bright morning about to dawn over Portsmouth, England, on June 4, 1944, gloom settled over the Allied commanders gathered inside Southwick House at 4:15 a.m. Years of preparation had been invested in the invasion of Normandy, but now, just hours before the launch of D-Day operations, came the voice of Group Captain James Stagg urging a last-minute delay. As Operation Overlord's chief meteorological officer, the lanky Brit was hardly a battlefield commander, but the ultimate fate of D-Day now rested in his decision-making.

The disappointed commanders knew that the list of potential invasion dates were only a precious few because of the need for a full moon to illuminate obstacles and landing places for gliders and for a low tide at dawn to expose the elaborate underwater defenses installed by the Germans. June 5, chosen by Allied Supreme Commander Dwight Eisenhower to be D-Day, was the first date in a narrow three-day window with the necessary astronomical conditions. The massive Normandy landings, however, also required optimal weather conditions. High winds and rough seas could capsize landing craft and sabotage the amphibious assault; wet weather could bog down the army and thick cloud cover could obscure the necessary air support.

The critical, but unenviable task of predicting the English Channel's notoriously fickle weather fell to a team of forecasters from the Royal Navy, British Meteorological Office and U.S. Strategic and Tactical Air Force, and as D-Day approached, storm clouds brewed inside the meteorological office.
WATCH 'D-Day: The Untold Stories' on HISTORY Vault.

Ben Birchall/PA Images via Getty Images

Observations from Newfoundland taken on May 29 reported changing conditions that might arrive by the proposed invasion date. Based on their knowledge of English Channel weather and observations, the British forecasters predicted the stormy weather would indeed arrive on June 5. The American meteorologists, relying on a differing forecasting method based on historic weather maps, instead believed that a wedge of high pressure would deflect the advancing storm front and provide clear, sunny skies over the English Channel.

In the early hours of June 4, Stagg believed foul weather was only hours away. He sided with his fellow British colleagues and recommended a postponement. Knowing that the weather held the potential to be an even fiercer foe than the Nazis, a reluctant Eisenhower agreed in the early hours of June 4 to delay D-Day by 24 hours.

On the other side of the English Channel, German forecasters also predicted the stormy conditions that indeed rolled in as Stagg and his fellow Brits had feared. The Luftwaffe's chief meteorologist, however, went further in reporting that rough seas and gale-force winds were unlikely to weaken until mid-June. Armed with that forecast, Nazi commanders thought it impossible that an Allied invasion was imminent, and many left their coastal defenses to participate in nearby war games. German Field Marshal Erwin Rommel even returned home to personally present a pair of Parisian shoes to his wife as a birthday present.

German Luftwaffe meteorologists, however, relied on less sophisticated data and models than their Allied counterparts, says John Ross, author of The Forecast for D-Day: And the Weatherman behind Ike's Greatest Gamble. "The Allies had a much more robust network of weather stations in Canada, Greenland and Iceland; of weather ships and weather flights over the North Atlantic and observations by secret agreement from weather stations in the neutral Republic of Ireland," he says.
WATCH: 'A Distant Shore: African Americans of D-Day' on HISTORY Vault

Those weather stations, in particular one at a post office at Blacksod Point in the far west of Ireland, proved crucial in detecting the arrival of a lull in the storms that Stagg and his colleagues believed would allow for an invasion on June 6. As rain and high winds lashed Portsmouth on the night of June 4, Stagg informed Eisenhower of the forecast for a temporary break. With the next available date for an invasion nearly two weeks away, the Allies risked losing the element of surprise if they waited. In spite of the pelting rain and howling winds outside, Eisenhower placed his faith in his forecasters and gave the go-ahead for D-Day.

The weather during the initial hours of D-Day was still not ideal. Thick clouds resulted in Allied bombs and paratroopers landing miles off target. Rough seas caused landing craft to capsize and mortar shells to land off the mark. By noon, however, the weather had cleared and Stagg's forecast had been validated. The Germans had been caught by surprise, and the tide of World War II began to turn.

Weeks later, Stagg sent Eisenhower a memo noting that had D-Day been pushed to later in June, the Allies would have encountered the worst weather in the English Channel in two decades. "I thank the Gods of War we went when we did," Eisenhower scribbled on the report. He could also have been thankful for Stagg overruling the advice of the American meteorologists who wanted to go on June 5 as planned, which Ross says would have been a disaster.

"The weather over Normandy contained too much cloud cover for Ike's greatest strategic asset, the Allied air forces, to effectively protect the landings from German armor, artillery and infantry reserves. Winds were too strong for the deployment of paratroopers to secure bridges and crossroads inland from the beaches thus preventing German reinforcement of coastal positions. Waves were too high for landing craft to put soldiers and supplies ashore. The key element of surprise-location and time-would have been lost, and the conquest of western Europe could well have taken another year."
Video: Charles Norman Shay, Combat Medic

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In contrast to the bright morning about to dawn over Portsmouth, England, on June 4, 1944, gloom settled over the Allied commanders gathered inside Southwick House at 4:15 a.m. Years of preparation had been invested in the invasion of Normandy, but now, just hours before the launch of D-Day operations, came the voice of Group Captain James Stagg urging a last-minute delay.

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Blacksod, Blacksod calling ... Here is the weather report for June 3rd, 1944.

n the history of mankind, few weather forecasts have carried such import. As he cranked the telephone and delivered his news over a crackly line from Co Mayo's most westerly point, Irish Coast Guardsman and lighthouse keeper Ted Sweeney had no idea the lives of more than 150,000 Allied troops would hang on his words.

It was a fateful call. As he watched the barometer fall precipitously, Ted Sweeney's report from the Coast Guard station convinced General Dwight D Eisenhower to delay the D-Day invasion for 24 hours - a decision which averted a military catastrophe and changed the course of the Second World War.

This Friday, June 6, marks the 70th anniversary of the invasion of Normandy - codenamed Operation Overlord. The assault on Hitler's 'Fortress Europe' signalled the beginning of the end of the Nazi regime in Europe with the German surrender less than a year later.

Some 5,000 ships and over 11,000 aircraft carried approximately 156,000 Allied troops into battle on D-Day across a 60-mile beachfront and into the interior of Normandy's Cotentin peninsula.

Now, after a meticulous examination of the 70-year-old records conducted by Met Eireann forecasters, new details have emerged of how the Blacksod Bay forecast changed the course of history.

Years of planning for the Allied invasion came down to one crucial but uncontrollable factor in the tense days before D-Day - the weather.

Separate observations were taken at various locations by Royal Air Force, Royal Navy and United States Army Air Force meteorologists.

But an accurate forecast from the Irish Meteorological Service, based on the observations from Blacksod on the Mullet peninsula, was the most vital.

Blacksod's importance was due to the fact that it was the first land-based observation station in Europe where weather readings could be professionally taken on the prevailing European Atlantic westerly weather systems.

Despite Ireland's neutrality during 'The Emergency', the State did continue to send meteorological reports to Britain under an arrangement which had been agreed since Independence. Irish weather reports, however, were not passed on to Germany.

In August 1940, the future chief meteorologist for the Normandy Landings, Scotsman James Stagg, then representing the British Met Office, visited Irish met observation stations. Later in the war, in early 1944, and with planning well advanced for D-Day a Captain Robert Bundgaard, a meteorologist with the United States Army Air Force, reputedly visited Valencia Weather Station in Co Kerry.

D-Day was originally planned for June 5, but the following two days, June 6 and 7, were also pinpointed as possible dates because moon and tide conditions were deemed ideal for seaborne and airborne landings.

However, as the planned day of the greatest invasion in history approached, British and American forecasters could not agree on the likely weather for that date.

On June 2, the Americans were optimistic for a 'go', having faith in a ridge of high pressure dominating the original D-Day date of the 5th, whilst the British were "unmitigatedly pessimistic" according to Stagg's memoirs. Agreement could not be reached and Stagg was in a serious dilemma.

Then, just after 2am on June 3, Irish Coast Guardsman and lighthouse keeper Ted Sweeney sent his latest hourly weather observation report. It contained an ominous warning of "a Force 6 wind and a rapidly falling barometer" at Blacksod.

At Southwick House outside Portsmouth, Group Captain Stagg studied the Blacksod report and strongly advised General Dwight D Eisenhower to postpone for 24 hours. Eisenhower heeded the advice and postponed to Tuesday June 6. The postponement was due to the slowing down of the westerly weather system bringing in clear weather from the Atlantic. Two cold fronts were approaching Ireland in quick succession.

Eisenhower also strongly faced the possibility of having to postpone the invasion altogether until July when moon and tide conditions might once again be suitable but, as he later reminisced, that was an option "too bitter to contemplate".

As it turns out, history records that the weather for the following month was deplorable for a sea and air invasion.

At 11am on June 3, 1944, nine hours after Sweeney's report, the telephone rang at the remote Blacksod Coast Guard station in Co Mayo which also doubled as the area's post office which was run by Margaret Hughes. The call was taken by her daughter Maureen Sweeney, who worked in the post office. Today Maureen is 90 years old.

She recalls: "That call was from 'a lady with an English accent' and she requested a repeat of the last weather observations Ted had sent earlier. So I called my husband Ted to the phone and he checked and repeated the latest information we had sent by then."

An hour later, shortly after 12 noon, the telephone rang once more and again Maureen answered it.

"It was the same lady. The lady with the English accent and she asked if we could please check and repeat the very latest weather observations we had sent from Blacksod. So Ted repeated those ones again."

Ted Sweeney died in 2001, but in a 1994 interview he stated: "I was sending an hourly report 24 hours a day and night. It had to be phoned into London, (Dunstable). We got a query back.

"They asked for a check. 'Please check and repeat the whole report.' I was wondering what was wrong. I thought I had made some error or something like that. They sent a second message to me about an hour later to please check and repeat again. I thought this was a bit strange so I checked and repeated again. It never dawned on me that this was the weather for invading or anything like that. When I checked the report, I said: 'Thanks be to God, I was not at fault anyway.' I had done my job and sent over a correct reading to London."

Ted Sweeney's crucial Blacksod reports from June 3, 1944, in conjunction with observations from other met stations, indicated a cold front lying halfway across Ireland and moving rapidly south eastwards and that a deep depression lay between Iceland and Scotland. Stagg could now see that gale-force winds, low clouds and heavy showers would still be affecting the English Channel in the early hours of June 5, new Met Eireann analysis has confirmed.

Then, at 12pm on June 4, Ted sent the report that offered hope to Eisenhower and the Allied commanders at Southwick House to launch Overlord. It reported heavy rain and drizzle cleared, cloud at 900ft and visibility on land and sea very clear. A full clearance of the weather would actually arrive at Blacksod one hour later.

In the early hours of the following day, at Eisenhower's morning briefing, the latest report from Blacksod confirmed the passage of cold front at Blacksod at noon on June 4. A loud cheer went up in the room which broke the tension momentarily.

Complete confidence was restored. Eisenhower's long awaited weather clearance had finally arrived and he gave the order for Overlord to proceed. D-Day would definitely be June 6. As the meeting concluded, the rain stopped outside Southwick House and the clouds began to disappear. The clear weather reported from Blacksod had reached the English Channel.

Next Tuesday - 70 years to the day Ted Sweeney gave his weather report - the D-Day Museum in Portsmouth will be presented with high quality copies of the Mayo man's original Blacksod observation sheets, collated by the Irish Met Service. These contain the actual weather observations which proved so crucial to Group Captain Stagg and his team in correctly forecasting the D-Day weather for General Eisenhower.

The presentation will be made by Irish members of the Round Canopy Parachute Team (RCPT) who will fly in a formation of C47 wartime Dakota aircraft from Portsmouth to Normandy on Wednesday morning to parachute jump into Carentan, Normandy, as part of the 70th anniversary celebrations.

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How Blacksod lighthouse changed the course of the Second World War

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Blacksod, Blacksod calling ... Here is the weather report for June 3rd, 1944. n the history of mankind, few weather forecasts have carried such import. As he cranked the telephone and delivered his news over a crackly line from Co Mayo's most westerly point, Irish Coast Guardsman and lighthouse keeper Ted Sweeney had no idea the lives of more than 150,000 Allied troops would hang on his words.

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On June 3, 1944, Irish Coast Guardsman and Blacksod Lighthouse keeper Ted Sweeney and his wife Maureen delivered a weather forecast by telephone from Co Mayo's most westerly point.
The report convinced General Dwight D Eisenhower to delay the D-Day invasion for 24 hours, potentially averting a military disaster and changing the course of World War II.

In 2014, 70 years to the day from when Sweeney delivered his weather report, the D-Day Museum in Portsmouth was presented with Sweeney's original Blacksod observation sheets, collated by the Irish Met Service, and including the actual weather observations which proved so crucial to the invasion.

The Irish Independent reported that evidence from Met Éireann forecasters reveals how the Blacksod day forecast changed the course of history.

Despite years of planning, in the days leading up to the attack, the Allied invasion would depend on one crucial and uncontrollable factor - the weather.

Although separate observations were taken at various locations by Royal Air Force, Royal Navy, and the United States Army Air Force meteorologists, an accurate forecast from the Irish Meteorological Service, based on observations from Blacksod on the Mullet Peninsula would be the most important.

Despite the country's neutrality during the war, Ireland continued to send meteorological reports to Britain under an arrangement that had been agreed upon since Independence. According to the Irish Independent, Blacksod was the first land-based observation station in Europe where weather readings could be professionally taken on the prevailing European Atlantic westerly weather systems.

The Normandy invasion was originally planned for June 5. Nearly 5,000 ships and over 11,000 aircraft would carry approximately 156,000 troops into battle on the day across a 60-mile beachfront and into the interior of the Cotentin peninsula. Because of the importance of the landings by sea and by air, the 6th and 7th were also pinpointed as possible dates because the moon and tide conditions were then deemed ideal.

However, British and American forecasters could not agree on the likely weather conditions for the planned date.

According to the memoirs of Scotsman James Stagg, the chief meteorologist for the Normandy Landings, by June 2, the Americans were optimistic for a 'go' on June 5, whilst the British were "unmitigatedly pessimistic." An agreement could not be reached.

Then, in the early morning hours of June 3, Irish Coast Guardsman and lighthouse keeper Ted Sweeney sent his hourly weather observation report, containing a warning of "a Force 6 wind and a rapidly falling barometer" at Blacksod.

Group Captain Stagg, stationed at Southwick House outside Portsmouth, studied the Blacksod report and advised General Dwight D Eisenhower to postpone for 24 hours. Eisenhower postponed the invasion to Tuesday, June 6.

IrishCentral History

Sweeney, who died in 2001, said in an interview from 1994: "I was sending an hourly report 24 hours a day and night. It had to be phoned into London, (Dunstable). We got a query back.

"They asked for a check. 'Please check and repeat the whole report.' I was wondering what was wrong. I thought I had made some error or something like that.

"They sent a second message to me about an hour later to please check and repeat again. I thought this was a bit strange so I checked and repeated again. It never dawned on me that this was the weather for invading or anything like that.

"When I checked the report, I said: 'Thanks be to God, I was not at fault anyway.' I had done my job and sent over a correct reading to London."

Met Éireann analysis has confirmed that the Sweeneys' June 3 reports from Blacksod indicated a cold front lying halfway across Ireland and moving rapidly southeastwards and that a deep depression lay between Iceland and Scotland. Gale-force winds, low clouds, and heavy showers would still be affecting the English Channel in the early morning hours of June 5.

On June 4, the Sweeneys sent a report saying that heavy rain and drizzle cleared, clouds at 900ft, and visibility on land and the sea was very clear. An hour later, Blacksod would receive full clearance of the weather.

The following day, at Eisenhower's morning briefing, the latest report from Blacksod confirmed the passage of a cold front at Blacksod at noon on June 4 and confidence was restored, reports the Irish Independent.

Eisenhower's long-awaited weather clearance had arrived and he gave the order for Operation Overlord, as the invasion was named, to proceed. D-Day would be on June 6.

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How the weather forecast from a Mayo lighthouse saved the D-Day invasion

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On June 3, 1944, Irish Coast Guardsman and Blacksod lighthouse keeper Ted Sweeney delivered a weather forecast by telephone from Co Mayo's most westerly point. The report convinced General Dwight D Eisenhower to delay the D-Day invasion, potentially averting a military disaster and changing the course of World War II.

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Normandy Tide Chart (April 2023) high and low tide times

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Normandy Tide Chart (April 2023) high and low tide times

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Vierville-sur-Mer: Full tide schedule in april 2023

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Tide schedule in Vierville-sur-Mer in april 2023. Find the tide schedules elsewhere in Normandy in april 2023. 100% reliable and updated daily.

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Normandy Sword Beach Tides - Present day sunrise, sunset times and moon rise and moon set times with a tidal chart.  Click on the left side tabs - especially the sun and moon link.

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Normandy Sword Beach Tides - Present day sunrise, sunset times and moon rise and moon set times with a tidal chart.  Click on the left side tabs - especially the sun and moon link.

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Sword Beach Tide Times, Tides for Fishing, High Tide and Low Tide Chart - Normandy - France - 2023 - TideKing.com

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Sword Beach, Normandy's tide times and tide chart including high tides, low tides heights, tides for fishing, fishing reports, surf reports, weather forecasts and solunar charts for the week.

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General weather during April in Normandy

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General weather during April in Normandy

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The climate of Rouen in april is tolerable

In april in Rouen, the climate is wet (with 3in of precipitation over 5 days). It should be noted an improvement from the previous month since in march it receives an average of 3.2in of precipitation over 5 days.

The climate is fairly cool in that area the month of april, but it is tolerable by dressing cosy. The thermometer averaged maximum of 58°F. The minimum is 43°F. Thus, the mean temperature average in april in Rouen is 50°F. These temperatures are far removed from those records observed in Rouen in the month of april with a maximum record of 76°F in 2018 and a minimum record of 25°F in 2013. You can plan to have about 5 days with temperatures up to 65°F, or 17% of time.

In april in Rouen, the days on average last 13:46. The sun rises at 06:03 and sunset is at 19:49.

With proper weather, april is okay (but not the best) to go there in Normandy.

Weather data for Rouen in april:

Weather data for Rouen for april are derived from an average of the weather forecast since 2009 in Rouen. There is a margin of error and these forecasts are to be considered as general information only. The weather in Rouen can vary slightly from year to year, but this data should limit surprises. So you can pack your bags or check for the best time in year to go to Rouen.

Weather in march in Rouen

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Weather in Rouen in april 2023 - Temperature and Climate in april

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The climate of Rouen in april is tolerable In april in Rouen, the climate is wet (with 3in of precipitation over 5 days). It should be noted an improvement from the previous month since in march it receives an average of 3.2in of precipitation over 5 days.

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Anatole France Danton-Douanes, Seine-Maritime, France Monthly Weather Forecast - weather.com

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Weather.com brings you the most accurate monthly weather forecast for Anatole France Danton-Douanes, Seine-Maritime, France with average/record and high/low temperatures, precipitation and more.

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Sunrise and Sunset Times in Normandy, France

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Sunrise and Sunset Times in Normandy, France

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Calculations of sunrise and sunset in Normandy - Calvados - France for March 2023. Generic astronomy calculator to calculate times for sunrise, sunset, moonrise, moonset for many cities, with daylight saving time and time zones taken in account.