The Thirty-Six-Hour Window

The weather station at Dunstaffnage, on the west coast of Scotland, was recording something unusual in the first days of June, nineteen forty-four. Group Captain James Martin Stagg was not a general. He was a meteorologist, a tall, careful Scotsman who had spent the spring charting Atlantic pressure systems with instruments that were, by modern standards, primitive. He had no satellites. He had weather ships positioned across the North Atlantic, land stations dotted across Britain and Ireland, and a staff of forecasters at Widewing, in Bushy Park, southwest of London. His job was to tell Supreme Allied Commander Dwight Eisenhower when the English Channel would be crossable. The answer, for most of May and early June, had been: not yet. Operation Overlord required a narrow set of conditions — a late moon for cover, a low tide at dawn for the engineers to clear the beach obstacles, and at least two days of tolerable weather for the air and naval forces to operate. The tidal window that met these requirements fell on June fifth, sixth, and seventh.