How the success of D-Day hinged on a weather forecast

If it weren’t for a weather forecast, D-Day—the largest seaborne invasion in history—would have taken place on June 5, as originally planned. And if that had happened, the invasion would have ended in disaster. Thousands of men would have been swamped by storm-whipped waves. Instead Allied forces waited a day, and the rest is history.

The story of this pivotal moment in World War II, which gave the Allies a foothold in mainland Europe and spelled the beginning of the end for Adolf Hitler’s forces, has been recounted in countless books, movies and miniseries. But one crucial ingredient in the invasion’s success—that forecast—is still little known to the broad public.

The story of that history-bending prediction is the subject of Pressure, a new movie out today. The film, adapted from a play with the same title, covers the tense, make-or-break forecasting and decision-making that happened in the 72 hours before the first troops set foot on Normandy’s beaches. It is, of course, a dramatized version of events. But the film shines a light on the underrecognized effort to gather weather data, the importance of paying attention to what the evidence showed, and the very little that separated success and defeat that day.

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“D-Day hinged on the weather, and there were some people who had to make incredibly difficult decisions with what would now be considered a handful of data points,” says Catherine Ross, library and archive manager at the U.K.’s Met Office. “They had the fate of thousands of people’s lives in their hands.”

A War of Data

Both the Allies and the Germans went into the war understanding how crucial forecasting would be to their side’s success. Both employed meteorologists within their military structures to provide forecasts for everything from engaging in hours-long bombing raids to accurately aiming artillery.

And both sides scrambled to gather weather data from whatever sources they could, including planes, military and merchant ships, meteorological units deployed near battlefronts and regular readings taken by civilians. Later in the war, after they’d broken the Enigma code, the Allies even folded in German weather data. “They understood that the data was paramount,” Ross says. Or, as the movie’s protagonist James Stagg (played by Andrew Scott) says, “Get me the data; that’s what counts. If we’ve measured it, then I want it.”

One key data source was the radiosonde—a box of instruments attached to weather balloons that measures temperature, pressure and other parameters. (The movie includes colorized archival footage of some actual balloon launches from the war.) Radiosondes are still used today—but back then forecasters didn’t have computers to crunch the data they gathered and spit out likely future scenarios. Instead meteorologists plotted the data on hand-drawn maps and connected those points to show areas of high and low pressure. Based on how those changed over the course of several hours, the forecasters would try to make a prediction. And making predictions more than a day or two out was mostly guesswork.

That meant that the forecasters advising Gen. Dwight D. Eisenhower, who led Allied forces in Europe, had much less to work with than modern meteorologists. “They didn't have a lot of data, but they did a lot with it,” says weather historian James Fleming, who has written about the D-Day forecasts.

“I Need a Forecast”

In early June 1944, more than 150,000 men were ready to cross the English Channel on thousands of ships and planes. The intricacies of the invasion required very specific conditions: a full moon for troops to see, low tide so boats could avoid coastal defenses—and decent weather. As Brendan Fraser’s Eisenhower tells Stagg in the movie, “I need a forecast.”

The Allies and the Germans both knew that the moon and tide would be aligned from June 4 through June 6. The Allies had set up decoy armies to throw the Germans off of their plans. If the Allies had had to postpone the invasion for the next moon-tide alignment later in the month, the subterfuge would have been exposed. So the pressure was indeed on for generals and forecasters alike.

In the movie, all the key players are kept in the same place for simplicity and tension. In reality, there were three forecast teams—one American and two British—in three different locations in case one of them were to be bombed.

As in the movie, Lt. Col. Irving Krick and his fellow Americans used “analogs,” or weather charts from past periods that matched the then current meteorological setup. Based on those analogs, they forecast fine weather. But as Stagg tells Krick in the movie, “The weather never replicates its own history.” Very small differences can grow into very large ones over time.

The British teams, meanwhile, forecast stormy weather. They were right, and the planned invasion for June 5 was called off. In fact, they thought the whole June 4–6 window was a wash. But—just as in the movie—at the last minute the forecasters spotted a break in the storms that would be just calm enough for the invasion force to make the crossing. As Stagg says in the film, “The weather won’t be perfect, but it’ll do.” So at 6:30 A.M. on June 6, tens of thousands of Allied forces invaded the six designated beaches on the French coast, catching the German military on the back foot.

There is some evidence that the German meteorologists saw the same break in the weather, as Heinz Lettau, a German Weather Service meteorologist during the war, noted in a 2002 interview. “The Germans had very good talent,” Fleming says. But the German commanders either didn’t heed the forecast or didn’t think the Allies would go for it, so they were away from their posts, leaving their response disorganized.

After the war, Krick, who launched a dubious cloud seeding venture, claimed the Americans had made the successful forecast. Stagg, meanwhile, had written a letter panning that office’s work and claiming that the unit led by Norwegian Sverre Petterssen made the correct call. Petterssen himself avoided the limelight and had preferred for all the forecasters to get credit as a group, Fleming says: “Petterssen said we should just give credit to everybody that contributed. We shouldn’t try to take credit for the heroic forecast that saved the world.”