Weather forecast for June 5 and 6, 1944 in the English Channel
The weather forecast for June 5 and 6, 1944 in the English Channel has been described as "perhaps the most important forecast ever made" because the Allied invasion of Normandy depended on it. For the day of landing ( D-Day ) , the military demanded a five-day forecast, which is still at the limits of the forecasting possibilities in areas with highly variable weather. At least the meteorologists were able to prevent a landing on June 5th, which would have failed due to the weather, and correctly predict a short period of good weather on June 6th. Those involved have left partly contradicting reports on the actual course of events.
Structure of the allied weather services
Three weather services were involved in the forecast for D-Day:
- The British Meteorological Office ( Meteorological Office ) based in Dunstable was headed by civilians CKM Douglas. It was subordinate to the Ministry of Aviation, but also provided the weather forecast for the British Army. He was only ever referred to as "Dunstable" by those involved.
- The Admiralty's Weather Service in London worked for the British and US Navy. Without a doubt, he had the greatest expertise in sea conditions - especially the swell - in the English Channel .
- The Meteorological Service of the US Army Air Corps ( United States Army Air Forces ) , most recently under Colonel Irving P. Krick, had a seat in Widewing in the UK. He provided the weather forecast for all US air and land forces in Europe.
The various weather forecasts converged at the chief meteorologist of Commander-in-Chief General Eisenhower , the British James Martin Stagg. An American liaison officer was assigned to him. Stagg was only formally qualified for his job, because he had mainly worked scientifically on geomagnetism and had spent his professional life in administration. The chief meteorologist did not have his own weather service. His task was to formulate a compromise from the various predictions, which in turn was issued to the Allied armed forces.
Those involved later criticized this structure. However, it can be justified by the fact that the German air force was unable to wipe out the entire weather service with a bomb strike.
Weather forecast requirements
Only one day with low tide shortly after sunrise was considered for the invasion , so that a second invasion wave could follow with the next low tide in the evening. In addition, it should be extremely low water , i.e. a day with a new or full moon, so that the underwater obstacles installed by the Wehrmacht were exposed as much as possible. For the same reason, the weather should be calm. The moon on June 6, 1944 was full.
In addition, each type of weapon had its own ideas about “good weather”. A full moon and a cloudless sky would have been ideal for the airborne operations; Morning fog would have hindered the paratroopers' gathering. The wind force in the target zones could not exceed 20 miles per hour. From the perspective of the bomber command, no more than 60 percent of the sky could be covered and the cloud base had to be at least 3,000 feet high. In addition, the weather in south and east England had to be taken into account for flight operations. The army wanted solid, stable ground, which made rain problematic in the days before the invasion.
The requirements of the navy were the strictest: for the landing operations, an onshore wind was not allowed to exceed ten to twelve miles per hour, on the other hand, calm was also a problem due to the risk of fog formation and gas attacks. Visibility had to be at least three miles. In the days before the landing, there should be no storm in the channel or on the Atlantic, because the landing ships sometimes had to cope with several days' approach. The swell also had to be taken into account.
The invasion was planned for May at the earliest. When evaluating historical weather data, chief meteorologist Stagg came to the conclusion that the chance of “good weather” in the sense of all branches of arms in early summer during a new or full moon was 1 in 25 to 30; if the full moon was insisted, it halved again. For him, however, it also unexpectedly emerged that the chances of good weather in June would be twice as good as in May, while in July they deteriorated again drastically.
Preparations
The military planners wanted a weather forecast for the four days before "D-Day", the day of the invasion itself, and the following two to three days, which would still be impossible today. It was not until 1939 that a working group at the British Meteorological Office began working on long-term forecasts. In 1944 the weather department issued a forecast for the next 24 hours and then named the “further prospects” for the following one or two days without any formal rules in place. The SHAEF High Command was finally satisfied with a five-day forecast.
The fact that the US Army Aviators in Washington regularly delivered precisely formulated five-day forecasts contributed to the mutual lack of understanding. They were based on a statistical analysis of historical weather data on the earth's surface, with the assumption that the weather for the next five days would follow the closest historical model. This "analog procedure" was worked out by Irving P. Krick of the California Institute of Technology , who was then appointed to the weather service of the US Army Aviation in Great Britain. The US weather service also published the weather maps of the northern hemisphere for every day for the past 50 years.
From the point of view of British meteorologists, US forecasts of British weather were worthless. Since the weather depends not only on the heat and moisture exchange processes on the earth's surface, but also in the higher air layers, similar weather patterns on the surface can be driven by different weather processes in the upper air layers and therefore diverge in the further course. That is why the Norwegian Sverre Petterssen - a specialist in the upper layers of the air and who discovered the jet stream - was brought to Dunstable. It was not until the first years of the war that wind and temperature data were regularly collected from 18,000 to 20,000 feet.
The British Meteorological Service in Dunstable ventured into three-day forecasts on this basis, with the forecast for the third day being very general. As a result, the five-day forecasts that have now been requested were almost completely determined by the US colleagues towards their end. Their forecasts were precise, but mostly turned out to be wrong, while the forecasts made by British meteorologists familiar with their weather were very cautious. On the British side, only Petterssen was optimistic about the possibility of long-term forecasts, as he had already published four-day forecasts in Norway when the weather was right. Douglas also worked at Dunstable, probably the meteorologist with the greatest sense of British weather. However, his known good predictions were based on intuition, and he was usually difficult to justify factually. The team from Petterssen - based on its studies on the upper air layers - and Douglas also provided the most realistic weather forecasts in the run-up to D-Day.
From February 1944, the weather forecasts of the three weather services involved were coordinated in telephone conferences, and from March a five-day weather forecast was issued weekly. Since the end of April, the US Army Aviation Meteorological Service and the British Meteorological Service in Dunstable have not completely coincided on a single occasion at the conference calls. The Admiralty usually joined colleagues from one of the other two weather services. Since the meteorologists of the three weather services had only partially met each other personally, the telephone discussions became all the more toxic after Stagg's impression. His task was to formulate a compromise from the predictions, which sometimes differed widely from one another. On April 17, he issued a weather forecast for the first time in front of around 30 assembled admirals, generals and marshals of the Allied High Command. In the Monday May 1st forecast, Stagg predicted worsening by mid-week, after which an exercise was postponed. Indeed, the predicted deterioration came true.
It was particularly problematic that only a few weather reports were available from the Atlantic. Because they had to be kept secret, they were encrypted and decrypted again, which also consumed time. Regular weather reports from the continental margins were available from the US Atlantic side and through weather reconnaissance flights from Northern Ireland, West Scotland, Cornwall and Gibraltar to the East Atlantic, but the Central Atlantic and especially the coast off South Greenland were only recorded sporadically. The British Navy then stationed two ships south of Iceland and north of the Azores.
The forecast for D-Day
In April and May 1944 the weather in the Canal area and southern England had remained largely calm, but the preparations for the invasion had not progressed far enough. Around May 17th, a day in early June was envisaged; from the tide calendar only June 5th or 6th was an option. On May 22nd, June 5th was confirmed as the day of the invasion. Allied high command was transferred to Southwick House north of Portsmouth.
The three weather services differed greatly in their weather forecasts on May 31 and June 1. A large mass of cold air had accumulated over the Arctic and could not flow away, which posed the risk of stormy weather. On the Atlantic, a series of four low pressure areas emerged that extended as far as North America. The US meteorologists in Widewing, however, were optimistic that a high over the Azores would extend north, deflecting the expected storms north. The British Meteorological Service in Dunstable was forecasting stormy weather throughout the canal, while the Admiralty took a mediating position. To make matters worse, the weather maps were constantly updated as soon as weather reports came in, so that the various groups of meteorologists probably never argued on the same basis of data.
On Friday June 2, the first ships weighed anchor in Scapa , off western Scotland and Northern Ireland, and set course for the canal. Meanwhile, the weather on the Atlantic became so stormy as it had never been registered at this time of year in the past 50 years. Widewing remained optimistic; Dunstable became even more pessimistic.
In the night of June 3rd to 4th, the ships that had gathered on the south-west coast of England were supposed to cast off. The Azores high hardly played a role in the discussions, and the Admiralty became more pessimistic. On Saturday, June 3, at 9:30 p.m., Stagg made the following forecast: A series of three low pressure areas would pass through at least the north of the British Isles in rapid succession. For the canal, he predicted strong winds and overcast skies with rain. When the prospects had not improved at 4:15 a.m. on Sunday, General Eisenhower postponed the invasion for a day. This decision was associated with the risk that German reconnaissance aircraft could discover the invasion fleet at any time. From the tide calendar only June 6th was an option.
On Sunday, June 4th, the weather developed unexpectedly for everyone involved. A depression in the Atlantic off Newfoundland became stronger than expected, which would hold up its course. That would leave time for two waves of landing on Tuesday, June 6th. On the conference call that afternoon, the British and US weather services fell out more than ever before. Everyone had agreed that a cold front would cross the canal on June 5th; the dissent was whether the calm weather that followed could last long enough to risk the invasion. To make matters worse, a weather report from a critical area in the Atlantic contained an inexplicable discrepancy of around 20 millibars of air pressure. Here the meteorologists could not judge whether the air pressure rose or fell, whether the low pressure area had already passed through or had simply disappeared. On the evening of June 4th, the weather map showed two low pressure areas over the Atlantic, one northwest of Scotland, the other south of Greenland. Landing seemed possible at least for Tuesday morning. At 9:30 p.m., Stagg predicted the passage of the cold front and the subsequent favorable weather on Monday and Tuesday. At this conference the decision to invade was made.
Actual weather on June 5th and 6th, 1944
During the night from Sunday to Monday and into Monday morning, there were strong onshore winds on the French coast and high waves with poor visibility, which would have made landing impossible. In addition, the low-lying, closed cloud cover would have made bombing more difficult. To that extent, the one day postponement had prevented a catastrophe. When the low pressure area crossed Scotland on the night of June 4th to 5th, it produced 967.8 millibars, the lowest air pressure recorded in the British Isles since the turn of the century.
On the day of the invasion, the sky cleared during the night so that the bombing could begin. There was a wind force of 3 to 4 on the beaches, which, together with the swell from the storm of the previous day, hampered the landing operations somewhat. During the day the cloudiness increased, but never more than three-quarters of the sky was overcast, and the clouds were never below 1,000 feet. As a result, the large low pressure area between Scotland and Norway did not move eastward as expected, but southeast towards Denmark. The resulting northwest winds made landing operations difficult in the evening. But this also had the advantage that the subsequent low pressure area with its low-lying cloud cover was held up further.
If Eisenhower had let the short good weather period of June 6th pass, the next possible date would not have come until June 19th (new moon) because of the tides. On that day and the following day, there was a storm on the French coast. The meteorologists involved never claimed that they could actually provide a five-day forecast, and none of the three weather services always forecast the correct weather. But they were able to advise against landing on June 5th a day and a half in advance and to advise landing on June 6th a day later with the same deadline.
The German side
It was clear to the German meteorologists under Major Lettau that the 1944 invasion had to come before the rainy autumn days. During the quiet days of May, they had expected them almost every day. In the first few days of June the weather turned so stormy that they no longer expected an invasion. Some of the troops were deployed inland for exercises; General Rommel left the headquarters in Paris for a trip to Germany. The June 4-5 storm kept the reconnaissance planes on the ground and the naval patrols in the ports. At the crucial point in time, the German meteorologists did not have any data from the Atlantic, where the weather was developing. In this respect, the surprise was bigger than if the weather had been consistently nice.
The representation of the weather forecast in history
Immediately after D-Day, US magazines and newspapers presented events as if the British weather services had failed and a disaster had only been avoided thanks to US meteorologists. The source of these rumors has never been determined. This misconception was reinforced when Irving P. Krick first published his account of the events in 1954; he had made the predictions that were furthest off. It was only through the late publications of James Stagg and Sverre Petterssen that the performance of the two British weather services became known - even if they covered each other with accusations.
Web links
- Weather maps of the German and British weather service dated June 6, 1944 with section for the canal / northern France area and section of Europe , published in June 2014 by the German weather service (ZIP)
literature
- Irving P. Krick and Roscoe Fleming. Sun, Sea and Sky. Weather in our world and in our lives Lippincott, Philadelphia 1954 (historically misleading account by the leading meteorologist on the American side)
- Sverre Petterssen: Weathering the Storm. Sverre Petterssen, the D-Day Forecast, and the Rise of Modern Meteorology . American Meteorological Society, Boston 2001. ISBN 1-878220-33-0 . (Memoirs of the Norwegian meteorologist Sverre Petterssen, who together with CKM Douglas was responsible for the weather forecast from Dunstable)
- en: James Martin Stagg : Forecast for Overlord . Ian Allan, London 1971. ISBN 0-7110-0251-7 . (Depiction of the chief meteorologist at the Supreme Commander's, who had to formulate a consensus based on the contradicting forecasts of the three weather services involved)
- Giles Foden: Turbulence . Faber and Faber, London 2009. ISBN 0-5712-0522-4 . (novel-like representation)
Individual evidence
- ^ Philip Ball: D-Day forecast fictionalized . In: Nature . Vol. 460, No. 7257, 2009, p. 799 f.
- ↑ http://www.vollmond.info/de/vollmond-kalender/1944.html