In the first days of June nineteen forty four, in a country house behind Portsmouth, a tall gloomy Scotsman walked into a room twice a day and told the most powerful commanders in the western world what the weather was going to do.
His name was James Martin Stagg, he was a group captain in the Royal Air Force, and he was by training not a soldier at all but a geophysicist who had once spent a year on an expedition to the Canadian Arctic.
Everything was waiting on him. Nearly seven thousand vessels. Twelve thousand aircraft. A hundred and fifty thousand men in the first day alone.
And weather forecasting in nineteen forty four was, by the standards of anything we would now accept, barely a science at all.
The invasion needed a set of conditions that almost never occur together.
It needed a late-rising moon, so that the airborne divisions could fly in darkness but have moonlight to land by.
It needed low tide shortly after dawn, so that the landing craft would ground short of the obstacles the Germans had planted in the surf, and so that engineers could see and clear them.
It needed reasonably calm seas, because flat-bottomed landing craft are appalling in rough water and the men inside them would arrive unable to fight, and it needed cloud high enough for the bombers and the spotting aircraft to see the coast.
And it needed all of these along fifty miles of coast at once, at a particular hour, on a particular tide, in a month when the Channel does more or less whatever it likes.
Moon and tide together left only a handful of possible days each month. In June nineteen forty four they were the fifth, sixth and seventh. After that, the next window with the right moon and tide was more than a fortnight away.
There were no weather satellites. There was no radar imaging of storms. There were no computers.
A forecast was built from observations: readings taken by ships, by weather stations, by aircraft flying long patrols out into the Atlantic specifically to measure pressure and wind, all of it telephoned or radioed in and plotted by hand onto a chart.
The critical shortage was observations from the west, because weather in northern Europe arrives from the Atlantic, and the Atlantic in nineteen forty four was mostly empty of anybody taking measurements.
There is a detail here worth knowing. Some of the most important readings came from a small station on the west coast of Ireland, at Blacksod Point, in a country that was neutral. Ireland quietly continued passing its weather observations to Britain throughout the war, and the report from Blacksod on the morning of the third of June, taken by a lighthouse keeper and his wife, was among the first indications of the front that changed everything.
Stagg also had a further problem, which was that he did not have one team. He had three, two British and one American, all of them within reach of London, and they used different methods and they disagreed with each other constantly and vehemently. His job was to give one answer.
The invasion was set for the fifth of June.
On the evening of the third, Stagg told the commanders that a depression was moving in, and that the fifth would bring high winds, rough seas and low cloud. He said the conditions would be, in his word, impossible.
The American teams disagreed. They expected the weather to hold.
Eisenhower postponed by twenty-four hours. Ships that had already sailed were recalled, some of them in worsening seas, and the men who had been loaded stayed loaded, seasick, in ships that were rolling badly, in harbours all along the south coast.
On the fourth of June the weather was exactly as Stagg had said. It was the worst Channel weather in twenty years for that time of year, and had the fleet sailed, the landing would have been a catastrophe.
That did not make his next forecast any easier. It made it harder, because he was now a man whose reputation was riding on the previous one.
Late on the evening of the fourth of June, in the library at Southwick House, with rain hitting the windows hard enough that people in the room commented on it, Stagg told the commanders that he could see a gap.
The front would pass through overnight. Behind it there would be an interval of perhaps thirty-six hours of improved conditions on the sixth, before the weather closed in again.
It would not be good weather. The sea would still be rough, the wind would still be strong, and the cloud would still cause serious problems for the bombers. It would be, just barely, possible.
The commanders argued. Montgomery wanted to go. The naval commander was concerned but willing. The air commander thought the cloud made it too risky.
Eisenhower listened, and then there was a silence in the room that several people who were there described afterwards as lasting a very long time.
Then he said: OK, we'll go.
There were about a dozen people in the room and every account of it says the same thing, which is that nobody cheered and nobody said anything at all. They stood up and went to work.
That was it. Stagg walked out and the orders went out behind him, and the fleet sailed into weather that was bad enough that a great many men in the landing craft were too ill to stand up.
The Germans were forecasting the same weather, from far fewer observations, because Allied action had systematically removed their Atlantic weather ships and stations.
Their meteorologists saw the storm and concluded, correctly, that the first week of June was unsuitable for a landing. They did not see the gap, because they did not have the readings from the west that would have shown it.
So the German command relaxed. Units in Normandy were stood down from the highest alert. A major exercise was scheduled for the sixth, taking senior officers away from their commands.
And Rommel, who commanded the coastal defences, drove home to Germany on the fourth of June to spend his wife's birthday with her. It was the sixth.
He was in Germany when the landings began, and it took him most of the day to get back.
Eisenhower wrote a short note in advance, in pencil, to be released if the landings failed. It said that the troops had done all that bravery and devotion to duty could do, that if any blame attached to the attempt it was his alone, and he put it in his wallet. He misdated it, writing July instead of June, which tells you something about the state he was in. It was never sent, and it survives.
Stagg was not thanked at the time in any dramatic way. Weeks later Eisenhower sent him a note about a later spell of bad weather, saying that he thanked the gods of war that they had gone when they did. Stagg kept it.
The lighthouse keeper at Blacksod Point was Ted Sweeney, and he and his wife Maureen were taking hourly readings as an ordinary job. They were asked to check and repeat their reading, twice, which they found puzzling. Maureen Sweeney lived until twenty twenty three and was formally honoured in her nineties.
Here is something to think about tonight. Stagg had to give one answer, out loud, in a room full of people who outranked him and who disagreed with each other, knowing that he might be wrong and that being wrong would cost thousands of lives. What is the hardest thing you have ever had to say out loud to people who did not want to hear it?