It was the day that changed the course of World War Two and led to the final defeat of the Nazis.

But on June 6, 1944, the 160,000 troops massed on the south coast of England faced a less obvious but no less deadly adversary – the weather.

The extraordinary story of how meteorologist Captain James Stagg urged General Dwight D Eisenhower to delay D-Day is now being celebrated in a new film.

With storm fronts closing in and thick cloud over France and the English Channel, the date was still far from ideal. But the allies were running out of options and further delays could have been disastrous.

To mark the release of the film, scientists have created mock-ups of weather satellite images to show just how perilous the situation was.

The pictures show a small but crucial break in the cloud, which gave Allied commanders the opportunity they so desperately required.

‘Troops set off into a sky that was still breaking up after days of storms into a narrow window of better weather,’ Professor Ed Hawkins who co-created the images, said.

‘A day earlier, or a few hours either way, and the weather could have stopped the invasion in its tracks, and changed the outcome of the war.

‘Images like this help us understand what troops, sailors and pilots actually faced that day, in a way written reports alone cannot. They show just how narrow the margin was between success and failure, and how much was riding on a small break in the cloud.’

Professor Hawkins and his team atthe University of Reading created the images with forecasters from the European Centre for Medium-Range Weather Forecasts.

Meteorological observations from 1944 were fed into a modern system known as reanalysis and technology was able to fill in the gaps. The result shows just how narrow a window of fair weather that Allied troops had.

Parts of southern England and northern France are only partly obscured, suggesting the sky was beginning to clear, but had not yet become cloudless. This is consistent with what is already known about conditions that morning.

According to the Copernicus Climate Change Service, the first weather report from the Normandy beaches on June 6 described mainly sunny conditions with a north-westerly wind.

The weather was better than it had been the previous day, although conditions still fell short of those originally hoped for by military planners.

But despite these obstacles, most of the Allied troops arrived on Normandy’s beaches safely and in numbers – and carried out one of the most impressive feats in military history.

Pressure is released in UK cinemas on September 9.

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