The planning team responsible for the invasion of Normandy had to consider the weather, the moon and tides when assigning a date for D-Day. Air operations required clear skies and a full moon for good visibility. Naval operations required low winds and calm seas to safely transport troops ashore. Ground troops needed to land at low tide, when German beach obstacles were exposed and easier to deal with.
D-Day required the best combination of these factors. Military planners relied on information from meteorologists and other specialists, who advised that D-Day should fall somewhere between 5 and 7 June 1944. D-Day was set for 5 June, but Supreme Commander General Dwight D Eisenhower knew that the weather could be critical in determining whether the invasion went off as planned.
Group Captain James Stagg - Eisenhower’s chief meteorologist - and his team of experts regularly rehearsed for D-Day. They were asked to prepare trial forecasts, which would then be checked for accuracy as each week progressed. Meteorologists used a number of tools to measure temperature, humidity, precipitation and cloud cover, but collecting and interpreting accurate data was difficult and the weather remained hard to predict. In the days leading up to D-Day, Stagg and his team forecast that weather conditions would worsen and on 4 June Eisenhower postponed the invasion by 24 hours.
The decision to postpone was a difficult one, as any delay made it increasingly difficult to keep the operation a secret. If the weather did not improve, D-Day would have to be delayed until the tides were again the Allies’ favour. This would not happen for another two weeks. But over the course of 4-5 June, Stagg predicted a temporary break in the weather. Based on this information, Eisenhower ordered that the invasion proceed on 6 June.