Meteorologists played a crucial role in D-Day invasion

June 6th is D-Day, the anniversary of the largest seaborne invasion in history that almost didn't happen. But meteorologists on the Allies' side made a weather forecast that would prove successful.

A lot of factors had to come together for a successful invasion. The Allies needed low tides, light winds, and little cloud cover to allow troops to make it to the shores with as little impediment as possible.

Group Captain James Stagg, the chief meteorological adviser to Dwight D. Eisenhower from the British Meteorological Office, was tapped for the monumental task of identifying a window of time for the invasion. Stagg consulted meteorologists from Britain and the United States, who at times provided conflicting forecasts.

Meteorology in the mid-20th century was less advanced than it is today without modern computers. Forecasters in the United States employed analog forecasts, which compare current weather to past known events. The British relied on handmade atmospheric analyses and observations. These methods of forecasting are still used today.

The group identified June 4th, 5th, and 6th as a possible window for the invasion. However, an already busy start to that summer would bring another blustery and stormy day on the 5th, which ultimately delayed preparations. As the next opportunity would not come for weeks, Eisenhower decided to go through with the invasion on the 6th.

While this decision was backed by good science, conditions did not turn out perfectly. High winds blew some aircraft slightly off course and slowed down the deployment of parachutes. Troops crossing the English Channel also reported seasickness from choppy waters.

Nonetheless, by the end of the 6th, over 150,000 allied troops successfully stormed Normandy's beaches, in what many call the beginning of the end of World War II.