Words in bold are explained in the D-Day Glossary.
How important was good weather for D-Day?
The decision about the best time to launch an invasion relied on studies of the tides, the phases of the moon and weather records.
As well as reasonable conditions at sea and a favourable tide to aid the landing of infantry forces by sea, the safety and success of air operations also depended on good weather, good visibility and bright moonlight for Horsa Glider and parachute landings in the very early hours of 6 June, and for the 14,000 air support sorties flown by Allied air forces on D-Day.
Meteorological reports from land, sea and air crews were crucial throughout the course of the war, providing information to troops waiting to advance or attack. They were often dangerous missions over enemy territory providing secret encrypted reports to military planners.
Click here for more about the role of the Met. Office on D-Day
D-Day was originally planned to take place on the night of 5 June, but a severe storm caused it to be delayed until 6 June. The storms meant that the sea was rough and that the tides on 6 June were especially high, hiding some of the beach obstacles that the landing forces had hoped they would be able to see and avoid or destroy on their way to dry land.
The temporary Mulberry Harbour at St Laurent, which was meant to supply US forces, was severely damaged by a storm on 19 June The second Mulberry Harbour at Arromanches (designed to supply British and Canadian forces) survived, and was used to unload 11,000 tons of stores per day!
