Three separate teams of meteorologists worked in secrecy on the high-stakes weather forecasts required for the Allied invasion of Normandy during World War II. But punishing storm fronts in 1944 left them little to work with: Only four possible, but less-than-ideal, dates appeared feasible for “D-Day” from May into July that year—with each nerve-racking option presenting 13-to-one odds against success.
“So, meteorologically, D-Day was bound to be a gamble against the odds,” as Royal Navy meteorologist Lawrence Hogben recollected. And now scientists with the University of Reading and the European Centre for Medium-Range Weather Forecasts (ECMWF) have provided detailed visualizations of just how small this crucial window of clear skies actually was on the historic morning of what would become D-Day, June 6, 1944. Working with old weather records and a modern weather data-synthesis tool known as reanalysis, the researchers have effectively generated what satellite images might have looked like that day, when nearly 160,000 U.S., British, and Canadian troops stormed the beaches of Nazi-occupied France.
“Troops set off into a sky that was still breaking up after days of storms into a narrow window of better weather,” climate scientist Ed Hawkins explained in a statement from the university, where he collaborated on these historical reconstructions with the ECMWF.
“A day earlier, or a few hours either way, and the weather could have stopped the invasion in its tracks,” Hawkins said, “and changed the outcome of the war.”
A perfect storm
The reconstructed satellite images depict a high-altitude view of the clouds that stretched across the British Isles and into the English Channel toward northern France on June 6, 1944. Reanalysis tools fill in or interpolate gaps between real measurements in climate data to faithfully recreate fuller images of historic weather patterns—a process that the U.S. National Oceanic and Atmospheric Administration calls “gridding.”
The images created by Hawkins and his colleagues show the cloudless patches of southern England and northern France critical to making D-Day possible. While powerful rain storms and roiling seas were major concerns, high winds and even low-lying cloud cover above Normandy were also major concerns, according to John Ross, author of The Forecast for D-Day: And the Weatherman Behind Ike’s Greatest Gamble.
“The weather over Normandy contained too much cloud cover for Ike’s [General Dwight D. Eisenhower’s] greatest strategic asset, the Allied air forces, to effectively protect the landings from German armor, artillery and infantry reserves,” Ross told History.com in 2014. On some of these other potential dates, he noted, the 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.”
In the line of fire
Judged against data from the Copernicus Climate Change Service, the reconstructed satellite images match well with historical reports of the weather that morning, according to the university. The first weather report from that day noted primarily sunny skies and a northwesterly wind, despite ominous clouds in the distance and conditions a bit less favorable than what Allied military planners might have found comfortable.
Perhaps obviously, the new weather images conveniently follow this past May’s release of the StudioCanal period drama Pressure, in which Brendan Fraser from The Mummy plays General Eisenhower in debates over weather reports with Royal Air Force meteorologist Captain James Stagg. (Stagg is played by Andrew Scott, who you might recall as either the “hot priest” from Fleabag or Moriarty from the BBC’s Sherlock.)
More importantly, Hawkins noted, the new images help us understand what threats Allied forces actually faced that day, alongside the dangers of German bullets and mortar shells.
“They show just how narrow the margin was between success and failure,” Hawkins said, “and how much was riding on a small break in the cloud.”
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