D‑Day: How Did the Allies Know the Weather Would Be Right?
Operation Overlord, the cross-Channel attack that hit the Nazi-occupied beaches of Normandy in 1944, was the culmination of a grand strategy adopted early in the war, followed sporadically during the years of conflict, and aimed at defeating Hitler’s Reich by striking directly at Germany by invasion. As such, it was the culmination of a long series of difficult negotiations among the Allies, ultimately setting the stage for final victory. For the Anglo-Americans, Overlord was the decisive campaign of the European Theater.
The Allied invasion of Normandy on June 6, 1944, represents one of the most monumental weather-sensitive military operations ever undertaken. The planners and decision makers required weather assessments and forecasts in order to bring off such a huge operation successfully. The actual weather constraints for the invasion were critical and complex. Airpower needed clear skies to be effective. Naval forces needed calm winds and seas. Airborne forces wanted low winds for their drop but also clouds to hide them from the Germans.
In 1948 General Dwight D. Eisenhower, the Supreme Allied Commander for Overlord, described the problem this way: “We wanted to cross the Channel with our convoys at night so that darkness would conceal the strength and direction of our several attacks. We wanted a moon for our airborne assaults. We needed approximately forty minutes of daylight preceding the ground assault to complete our bombing and preparatory bombardment. We had to attack on a relatively low tide because of beach obstacles which had to be removed while uncovered. These principal factors dictated the general period; but the selection of the actual day would depend upon weather forecasts.”
These factors presented Allied meteorologists with a very difficult, vitally important task at a time when the scientific state of the art was quite limited. There were no weather satellites, no weather radars, no sophisticated computer forecasting techniques, and no large network of stations providing upper air data critical to longer range forecasting. Despite these limitations, the Allied weather team was successful.
The fundamental military problem with the Overlord assault was to establish and maintain a reasonable margin of local superiority over the enemy. The naval plan, called Neptune, was vital and complex. Five hundred warships and 3,000 landing craft were needed for the initial assault alone. The U.S. and British navies provided, assembled, and escorted the invasion fleets, kept the English Channel open, and provided effective onshore bombardment. Yet, insufficient shipping existed to develop a flow of troops and supplies across the Channel to match what the Germans could bring to bear if they were allowed to marshal forces unmolested. This is why Eisenhower put so much emphasis on an air campaign to cut potential reinforcement routes. It is also why the Allies undertook a massive disinformation campaign to deceive the Germans as to the actual landing area, holding many of the best German divisions in the vicinity of Calais.
In retrospect, one is tempted to view the success of Overlord as a foregone conclusion. In reality, the margin of error between success and failure was slim. In the end victory came because of the overwhelming Allied logistical base, unchallenged airpower, and grave German strategic mistakes. But the success of the initial assault was the crucial ingredient. Eisenhower correctly counted on airpower, intelligence, surprise, and knowledge of the weather to even the odds.
Meteorology: A Weapon in the Allied Arsenal
World War II demonstrated the need for an adequate weather service for the military and made it an indispensable part of combat operations. Two factors accounted for this. The first was the extensive use of airpower, which was highly weather dependent. The other was the high premium placed on both the strategic and tactical application of mobility. It was to be a war of blitzkrieg, aircraft, and armor. The lessons of World War I had been learned by both sides. Therefore, no World War II commander worth his salt would commit forces without considering the effects of weather on his operations.
The invasion of Europe constituted a combined operation on a scale that had never before been attempted and involved all the armed services of the United States and Great Britain. Therefore, it was necessary to establish an organization for providing weather services to the supreme commander and component commanders concerning operational decisions about the invasion. Besides providing the best possible advice, weather services needed to ensure complete coordination so that decisions at all echelons were based on the same weather outlook.
Three months before General Eisenhower arrived in England to take over as supreme commander, U.S. and British authorities discussed the part to be played by meteorologists in Overlord. Of immediate importance was the development of a single, all-encompassing climatic assessment about the period of the year when weather conditions were most likely to be favorable. The chief of staff, Supreme Allied Commander (COSSAC) (soon to become Supreme Headquarters Allied Expeditionary Force, SHAEF), British Lt. Gen. Sir Frederick Morgan, wanted an independent weather officer to develop the assessment and to advise the supreme commander at the time of the invasion. To conform to the Anglo-American nature of Morgan’s combined staff (his G‑3, strategic planning officer, was an American, Maj. Gen. Harold R. Bull), two meteorologists, one U.S. and one British, were to be appointed under the G‑3. The senior post was to be British while the deputy was to be an American.
A civilian meteorologist from the British Meteorological Office (BMO), James Martin Stagg, was appointed to the senior post. Initially, Colonel Cordes Tiemann of the United States Army Air Forces (USAAF) was named his deputy, but he was soon to be replaced by Colonel (later Lt. Gen.) Donald Yates. The selection of the weather team did not come without controversy. U.S. Army officials in England were not pleased with the appointment of a civilian to the top post. This led Lt. Gen. Walter Bedell Smith, Eisenhower’s chief of staff, and Air Chief Marshall Sir Arthur Tedder, the deputy supreme commander, to apply pressure, successfully, to the British Air Ministry to at least frock Stagg as a group captain in the Royal Air Force (RAF), giving him the rank and professional stature needed to work in the secret, high-level military circles associated with planning for Overlord.
Though not an experienced operational forecaster, Stagg was a capable meteorologist and well qualified to do his job of pulling together the forecast and presenting it to Eisenhower and his subordinate commanders in a manner that they could use for military operations. His American deputy, on the other hand, was an experienced operational Army Air Force pilot who had been trained in meteorology at the California Institute of Technology (Caltech). Yates’s appointment to the COSSAC staff dual-hatted him since he was already the senior AAF weather officer in the European Theater. General Bull instructed him to provide interpretations relating to weather forecasts and their operational implications for Army ground and air units.
Because of the need to keep the SHAEF team small, it was decided that Eisenhower would be briefed by only Stagg and Yates, who would synthesize the data provided by the three major Allied weather facilities, called weather centrals or centers, already in England. These included the BMO forecasting unit in the London suburb of Dunstable (primarily supporting the RAF), the British Admiralty Weather Central in downtown London, and the U.S. Strategic Air Force (USSTAF) at Bushy Park, Teddington (code name Widewing).
Organizing the Air Force’s Weather Command
The weather centrals were the major sources of meteorological expertise for their component or service. They performed the large-scale weather analysis and forecasting functions for operations. Weather stations at each RAF and AAF base provided local weather observing, takeoff, and recovery forecasts and briefed the mission and target weather provided by the centrals. To accommodate Stagg and Yates, a special encrypted telephone system was established to permit frequent conferences among the various weather centrals and staff weather officers then operating in England. Besides Stagg and Yates and the centrals, participants included the staff weather officer to the air commander in chief at Stanmore, Middlesex, and the staff weather officer to the naval commander in chief at Southwick, Sussex.
As Allied forces began to gather in England for the invasion, fighting units and their supporting headquarters expanded greatly. In January 1944, the headquarters of the USSTAF was created under command of Lt. Gen. Carl “Tooey” Spaatz. USSTAF assumed control of both the Eighth and Ninth Air Forces. The AAF undertook a coordinated expansion of weather support in England in early 1944 by establishing a forecasting central and overall administrative headquarters known collectively as the USSTAF Directorate of Weather Services. Both the directorate and the USSTAF weather central, which it included, were under the direction of Colonel Yates. He had been sent from the headquarters AAF weather facility at the Pentagon and was the senior U.S. weather officer in Europe.
According to Yates, General Henry “Hap” Arnold, chief of the AAF, personally told him about the Eighth Air Force and his future assignment to USSTAF before he left Washington: “Their bombers are getting weathered out. They can’t get back in. They have lost more bombers that can’t get back in than they have lost to enemy fire. There is something wrong with the weather service over there. I told Tooey [General Spaatz] that I was going to send the best weathermen in the Air Force … I want you to organize a group of any forecasters you want … and take them over and report to Tooey Spaatz and do it in two weeks.”
Yates took two lieutenant colonels, Ben Holzman and Irving Krick, as the chief forecasters for the new central and Captain Robert Bungaard as the chief upper air analyst. Both Holzman and Krick were experienced in peacetime and wartime weather services and were among the foremost practioners of long-range weather forecasting in the United States before the war. Bungaard had been schooled in the relatively new techniques of upper air analysis and prediction, which were key to extended-range forecasts. Colonel Yates himself had previously served in weather posts in headquarters AAF and had won the Distinguished Service Medal for leading the U.S. meteorological mission to Moscow during 1942.
The primary task of the USSTAF weather central was to furnish weather services in support of Eighth Air Force strategic bombing missions, Ninth Air Force tactical operations, SHAEF invasion preparations, and the headquarters, USSTAF decision makers. Its activities were concerned with tactical and strategic targeting, takeoff and recovery weather, reconnaissance coverage, winds aloft, and the myriad other weather interests of a fighting air force. The central was also involved with trafficability estimates for Army vehicles, smoke screen effectiveness studies, and similar issues.
On the British side, the BMO was at this time a department of the Air Ministry. It controlled weather services to both the British Army and the RAF and was closely linked with the latter. In contrast to U.S. practice, the British meteorologists were civilians who were temporarily commissioned when they served overseas with the military forces. The British central at Dunstable was represented in the Overlord period by C.K.M. Douglas and Sverre Petterssen. Douglas had served extensively in World War I as a pilot and weather observer. As a professional meteorologist since the Great War, he had become an expert in weather over England and northern France. Petterssen had been a weather expert in Norway before taking a university teaching position in the United States. He had a deservedly high reputation as a leading theoretical meteorologist.
According to Stagg, the forecasters at the Admiralty center normally accepted the weather forecasts from the BMO center at Dunstable. Throughout the course of the Overlord operation they did not contribute substantially to the development of the overall forecasts other than by interpreting the weather patterns in terms of sea state and swell for naval operations.
Merging Climatology with MeteorologyThe Weather Conferences
This article by Gene J. Pfeffer first appeared in the Warfare History Network on July 7, 2016.
Image: “Into the Jaws of Death — U.S. Troops wading through water and Nazi gunfire”, circa 1944-06-06. U.S. Coast Guard/Robert F. Sargent.