“Well, for a start, he is shorter than my father, considerably,” said Peter Stagg of the actor Andrew Scott, who portrays his father, Group Capt. James Stagg, in the new film “Pressure.”
“He was 6-foot-4,” Mr. Stagg added. “Which, in those days, was very tall.”
A whole 3.93 inches, or 10 centimeters, separates Scott from the elder Stagg. It’s a small and faintly amusing discrepancy, and one of the few liberties taken in a film that is otherwise strikingly committed to historical detail. Because beyond the question of height, “Pressure” is a careful retelling of one of the most pivotal weather forecasts in history.
Set during the tense 72 hours leading up to D-Day, the film follows Stagg, the quietly burdened meteorologist tasked with advising Gen. Dwight D. Eisenhower on when the weather conditions would allow the Allied invasion of Nazi-occupied France to proceed.
The criteria for such weather conditions were extraordinarily precise. The invasion needed to take place during a low tide, in order to expose German defenses, and it needed to be just one day before or up to four days after a full moon. It also had to align with a Soviet summer offensive from the east, to maximize pressure on German forces.
There were other strict requirements:
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Before the landings, the weather needed to have been calm for 48 hours.
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Parachutists and other air support needed less than 30 percent cloud cover below 8,000 feet, with a cloud base no lower than 2,500 feet and visibility over three miles.
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For the three days after, the wind needed to stay below a moderate breeze, to keep the landing craft from capsizing while crossing the English Channel.
Stagg was also responsible for producing a unified forecast based on input from three independent groups, two British and one American, who often clashed in both methodology and interpretation.
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