02/06/2014 13:14 BST | Updated 02/08/2014 06:59 BST

D-Day Weather

The success and failure of invasions and military expeditions between Britain and Europe have always depended on the weather, from Julius Caesar onwards. But the role the weather and meteorologists played in the 1944 Normandy landings was critical.

D-Day landings, 6 June 1944 - the role of the Met Office

The success and failure of invasions and military expeditions between Britain and Europe have always depended on the weather, from Julius Caesar onwards. But the role the weather and meteorologists played in the 1944 Normandy landings was critical.

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. And the role of group captain Stagg of the Met Office in the dramatic weather-dependent decision to launch the D-Day assault, was unprecedented. To the extent that it could be said wartime meteorology laid the foundations of modern weather forecasting throughout the world.

The assault on Normandy needed a moonlit night, dawn at low tide, light winds and little low cloud throughout the day and for several following days. The broadly acceptable minimum weather conditions were:

  • 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 mph) onshore, and force 4 (13-18 mph) offshore.
  • Cloud should be less than 30% coverage below 8,000 feet. The cloud base should generally be above 3,000 feet.
  • Visibility should be more than three miles.

The tides and moonlight would be favourable on 5, 6 and 7 June. The tide, but not the moonlight, was right again two weeks later. 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'.

By the last week of May, the Supreme commander and his staff at Southwick Park, near Portsmouth, knew that the weather was going to be the most important unknown factor for the operation's success. 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 held regular 'scrambled' telephone conferences and presented their agreed weather forecasts to Eisenhower and his commanders, who then learned how to assess them.

Early June was unseasonably unsettled as depressions, fronts and strong winds moved over the UK. Late in the evening of Saturday 3rd June the forecasting centres had, for once, reached reasonable agreement.

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. This was moving south-eastwards and would clear the assault area within two or three hours. These conditions would have precluded airborne operations had the 5th June been chosen as D-Day.

A ridge of high pressure developing behind the front could provide a temporary window over the Channel and assault area for the critical hours of Tuesday 6th June. The strong winds would moderate and at H-hour on Tuesday morning the weather would be suitable for heavy bomber operations. Operations later on Tuesday could be curtailed by large areas of cloud, but this was likely to be high enough to enable the fall-of-shot to be spotted for the naval heavy guns.

Eisenhower asked Stagg how many hours he could count on for the attack. Stagg replied that the morning would be fair and good weather might last throughout the afternoon. Asked for his opinion by Eisenhower, Field Marshal Montgomery said: "I would say 'Go!'" The order for Operation Neptune to begin was given.

Less than five hours later, the first convoy left Spithead. The fresh west wind tested landing craft crews and was unexpected by many in the Army, who thought that an invasion needed fine weather and a calm sea. On 6th June, wind and weather permitted night-time airborne operations and, during the hour before the landings, large areas of clear sky allowed visual bombing of the shore defences.

The first Allied observation from the Normandy beachhead was from Sword Beach, north of Caen, 7.5 hours after the first landings. It was mainly sunny, with a moderate north-westerly wind, small amounts of broken cumulus cloud above 4,000 feet, good visibility and a temperature of 59 degrees Fahrenheit (15 degrees Celsius).

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."