D-Day and the weather forecast that changed the shape of history

D-Day required a precise set of conditions: low tides, moonlit skies and calm seas. And it was a very narrow window, as ITV News Health and Science Correspondent Martin Stew reports

Early on the morning of June 5 1944, General Dwight D Eisenhower, the supreme allied commander in Europe, finally gave the order to go ahead, telling his staff “I hope to God I know what I’m doing”.

Bad weather over the English Channel meant Operation Neptune, the largest seaborne invasion in history, had already been delayed for a nerve-shredding 24 hours.

Further postponement would have pushed the military move back at least another two weeks, which could have devastated morale among tens of thousands of troops assembled for the attack, potentially destroying any chance of surprise.

But, at a 4am briefing, Eisenhower’s chief meteorologist James Stagg, a 29-year-old Scot who had been given the temporary rank of group captain, told him – not without misgivings – that the next day would see a break in the skies.

That night, transport aircraft packed with 24,000 British and American paratroopers began taking off from airfields across southern England and heading for targets inside Nazi-occupied France.

Stagg had already halted Eisenhower's initial plan to invade on June 5 as low pressure threatened to bring strong winds and extensive low clouds across the English Channel.

However, as he drew up weather charts on June 4, a break of high pressure after the passing low pressure presented a chance for the invasion to begin on June 6.

Without satellites or supercomputers, Stagg's hand-drawn charts convinced Eisenhower to initiate Operation Neptune, and has been dubbed "the most important weather forecast ever produced".

E Company, 16th Regiment, 1st Infantry Division, participating in the first wave of assaults during D-Day in Normandy. Credit: AP

By comparison, the Nazis hadn't spotted the high pressure in the north Atlantic because they hadn't placed as many ships there.

They had been so convinced that the weather would be terrible that German Field Marshal Erwin Rommel left his position in France and returned to Germany to give his wife a birthday present.

And, the rest is history, the invasion went as planned for the Allies, a brief but important weather window opened, and soldiers etched a foothold in Normandy.

Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To know...