With the release of the new film ‘Pressure’, audiences are being introduced to one of the most important forecasting challenges of the Second World War.
The film tells the story of the meteorologists whose weather forecasts helped Allied commanders determine when to launch the Normandy landings on 6 June 1944.
This is explored further in a special exhibition on display onboard HMS Belfast, which delves deeper into the events behind ‘Pressure’.
But while weather forecasting played a crucial role, another prediction was equally important to the success of the operation: the tide.
Chanmi Kim, Permanent Service for Mean Sea Level (PSMSL) Data Manager at the National Oceanography Centre (NOC) looks back at the role of tidal prediction, and the connection to the NOC's scientific heritage.
The D-Day Decision: The role of tides in one of history's most important operations
On 6 June 1944, Allied forces landed on the beaches of Normandy in one of the largest amphibious operations in history. More than 150,000 Allied troops landed across five beaches, Utah, Omaha, Gold, Juno and Sword, as part of Operation Overlord.
When we think about D-Day, we often think about the weather forecast. And for good reason: the decision to launch the invasion depended heavily on weather conditions with meteorologists led by Group Captain James Stagg advising Allied commanders.
More than a weather forecast
But weather was only part of the story. The success of the Allied landings in Normandy depended on another forecast too, the tide. The Normandy coastline experiences a large tidal range, with the difference between low and high water reaching around 6 metres or more. At low tide, large areas of beach were exposed, but this also exposed the thousands of obstacles placed across the intertidal zone to damage landing craft. This meant that knowing exactly when the tide would fall and rise was critical.
The first landing forces needed to arrive shortly after low tide, giving demolition teams time to clear pathways through the obstacles. But the tide also needed to be rising. Landing craft had to be able to approach the beach, unload troops and equipment, and then leave without becoming stranded as the tide changed. And the timing wasn't the same everywhere.
The five landing beaches (Utah, Omaha, Gold, Juno and Sword) experienced low water at different times, so the landings had to be carefully staggered according to the predicted tides. The tidal predictions also had to work alongside the Moon, sunrise and daylight, as well as the weather forecast. The Allies needed a very specific combination of environment conditions to come together at the right time.
The science behind the tides
The Admiralty's tidal and tidal current predictions for the war effort were produced by Arthur Thomas Doodson at the Liverpool Tidal Institute (now the National Oceanography Centre, Liverpool), using tide predicting machines operated at Bidston Observatory.
Doodson produced predictions for the Normandy landing beaches using the best available tidal information at the time. These predictions were then incorporated into planning diagrams combining tides with information about moonlight, sunrise and sunset.
Why ocean predictions still matter
The D-Day landings provide a remarkable historical example of why understanding and predicting the ocean matters.
Accurate observations and predictions of the ocean are essential for understanding what will happen at the coast.
Today, tide gauges provide continuous measurements of sea level around the world. These observations, together with ocean and atmospheric models and other sources of data, help us understand and predict coastal conditions. From the mechanical tide predicting machines of the 1940s to today's global ocean observing networks, the science has evolved enormously, but the fundamental need remains the same: To understand the ocean, we need observations.
For us, here at NOC, we continue to evolve the science and technology we need to accurately inform models, forecast for predictions in a modern world.
Together, the HMS Belfast’s D-Day exhibition and the new blockbuster film ‘Pressure’ (released across the UK on 9 September) brings the extraordinary forecasting story of D-Day to the life. It is also an opportunity to remember the important role that tides, tide predictions and ocean science played in making the landings possible.
Be part of the community helping our ocean thrive.
