Lawrence Hogben, Pressure and the meteorologists of D-Day

Lawrence Hogben (1916–2015) was of the meteorologists who produced the vital weather forecasts before the D-Day landings in Normandy in 1944. This, of course, is the process dramatised in this year's film Pressure.

In 1994, Hogben wrote about those days for the London Review of Books:

"Everything on the Normandy beachhead will hang on your weather," said the D-Day planners, assuming that we meteorologists had total control of the elements. "Just name us five fine, calm days and we’ll go."

A hundred years of weather records suggested there was no hope of their getting this. They limited their demand to "a quiet day with not more than moderate winds and seas and not too much cloud for the airmen, to be followed by three more quiet days." 

On top of this, the military insisted on a late full moon for the parachutists, plus a tide that would be high three hours after pioneer forces had landed at dawn to clear paths through the beach mines and other obstacles. 

Tides and moon being fully predictable, they would determine possible dates. July would be too late, and May too early. That left just four possible days: 5, 6, 19 or 20 June. We worked out the odds on the weather on any one of these four dates conforming to requirements as being 13 to one against. So meteorologically, D-Day was bound to be a gamble against the odds.

The planners had originally wanted to know how far ahead a correct weather forecast could be made and who should make it. "We can make it, one, perhaps two days ahead," said the Meteorological Office. "We can make it up to five days ahead," said the American Air Force. "But do they know anything about the sea?" asked the Royal Navy, who made no promises. 

To satisfy pride, and to get the best possible forecast, the solution was to form a committee made up of the three groups. However, fear that loud-mouthed Americans might shout down mild British civil servants, or even battle-hardened naval officers, brought about an administrative compromise. 

The D-Day forecast became a telephonic affair: the six forecasters would stay in three separate centres, joined to one another, and to Eisenhower’s headquarters, by GPO telephones (which worked). A non-forecaster 'presenter' would moderate our debates and then explain our agreed forecast to Ike and his staff.

And, as Hogben goes on to explain, this unlikely approach produced an an accurate forecast. Eisenhower was persuaded to stage D-Day on 6 June rather than his original preference of 5 June, and the waiting a fortnight until the next possible dates would have led to a military disaster.