One of the key figures at Allied headquarters was Captain Jim Stagg, a Scotsman who was the chief meteorological officer to General Eisenhower, the Allied commander. Senior officers awaited his forecasts with bated breath.
The Allies had selected early June for Operation Overlord because records showed that the weather was most likely to be fine then. But 1944 defied statistics and a series of depressions more typical of midwinter swept across the North Atlantic.
The ideal conditions for a successful invasion were several days of reasonable weather, light winds, calm seas, broken cloud no lower than 1,000ft and visibility of at least three to five miles. This would enable the huge fleet to cross and land safely with close air support.
Getting predictions right was not easy for Stagg. Weather forecasting was not a highly developed science and during the crucial days in early June there were gaps in the information received.
However, the Germans were worse off; they could do little more than look out of the window. Enemy weather stations in Iceland, Greenland, and Spitzbergen had been seized and the Allies had a much wider area to take readings from.
D-Day was originally scheduled for 5 June but two days before, Stagg gave a gloomy briefing to Allied commanders. He feared that a ridge of high pressure was moving south, leaving the Channel more vulnerable to Atlantic depressions.
His next briefing was even more pessimistic and convinced Eisenhower to overrule his army commander, General Sir Bernard Montgomery, and postpone Overlord for 24 hours. Warships were recalled and troops on cramped landing craft faced extended misery.
But when the weathermen and the commanders next met, Stagg said that there would be an interlude of a day between depressions. The gap was agonisingly narrow, Eisenhower knew that if he postponed Overlord, conditions would not be favourable again until 19 June.
It was as well that Eisenhower did go on 6 June. On 19 June, the longest summer Channel storm for decades began. In three days it caused five times as much damage to Allied shipping as the Germans had been able to inflict since the invasion.
Had D-Day been delayed again it could have been an absolute disaster.