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The Met Office played a crucial part in deciding the timing
of Operation Neptune which would take troops and equipment across
the English Channel and lead to Operation Overlord - the D-Day
landings in Normandy.
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Operation Overlord needed specific minimum conditions:
- D-Day should be within one day before, to four
days after a full moon
- D-Day itself should have quiet weather, followed
by three quiet days
- Winds should be less than force 3 (8-12 m.p.h.)
onshore, and force 4 (13-18 m.p.h.) offshore
- Cloud should be less than 30% coverage below 8,000
feet
- Visibility should be more than three miles
- Cloud base should be generally above 3,000 feet
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The tides and moonlight would be favourable on 5, 6 and 7 June,
and General Dwight Eisenhower set the 5th as the date for Operation
Neptune to commence 'subject to last-minute revision in the event
of unfavourable weather'.
Eisenhower really needed seven-day forecasts, but these were
impossible with the available knowledge and tools. Representatives
of the forecasting centres of the RAF, USAF and the Royal Navy
presented weather summaries to him and his commanders, who then
learned how to assess them. The weather would be one of the most
important factor for the success of both Neptune and Overlord.
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Early June was unseasonably unsettled as depressions, fronts
and strong winds moved over the UK. The weather forecasting
centres combined their efforts and Group Captain James Stagg
(RAF and Met Office) told General Eisenhower that the weather
would be unfavourable for operations on the 5th. With ships
already loading and putting to sea, Eisenhower decided to
delay Neptune on a day-to-day basis, and those ships at
sea returned to port.
By late evening on the 5th there was driving rain from
a cold front that had been unexpected earlier, but Stagg
and the other meteorologists advised that it would clear
the Overlord area within two or three hours, and a following
ridge of high pressure could provide a weather window for
6 June. Early on the morning of the 6th, having taken and
carefully considered the best meteorological advice, Eisenhower
ordered Operation Neptune to begin.
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Later, Stagg's memorandum to an official report to Eisenhower
on the meteorological implications of 6 June stated that
had Neptune been delayed until the next suitable tides the troops
would have met the worst Channel weather for 20 years. Eisenhower
wrote across the bottom of the memo:
"Thanks, and thank the Gods of war we went when we
did."
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