The Weather on D-Day
Perhaps the most important weather forecast ever made was the one for D-day, the Allied invasion of France. It succeeded not because of the brilliant work of any solitary forecaster, but because a group of forecasters imitated the weather. They jostled, yelled, scribbled, and cast malevolent looks at one another. They fought it out and voted. And in the end, they were just right enough.
The invasion of France had been scheduled for June 5, 1944. To bring off the invasion, General Dwight D. Eisenhower needed a full moon, a low tide, little cloud cover, light winds, and low seas. (The low tide was necessary to allow soldiers to see, avoid, and disarm the mined obstacles that the Germans had placed in the surf.) He could have had the full moon and low tide on June 5, 6, or 7. He could have had the low tide without the full moon on June 19 or 20. But what about the weather?
Outside, on the morning of June 4, the weather was mostly clear, with a light breeze blowing. May had been a pleasant month on the English Channel, but June was not shaping up to be so. The Azores High, a semipermanent high-pressure zone in the mid-Atlantic that moved north during the summer and south during the winter, had not come as far north as usual. When its influence dominates the weather, Europe and the south coast of England experience dry warm summer days. Where it stood then, however, it was steering a series of low-pressure troughs across the North Atlantic and into the English Channel.
The first ships for the invasion had left Scotland, steaming south, on May 28. They were now in midcourse, forming their convoys, and the preparation to move three million armed souls from the south of England to the north of France was far advanced. Landing craft were jostling in Portsmouth Harbor, which was so crowded that you could walk from one shore to the other without touching water. Sherman tanks had been fitted with flotation skirts to help them wallow ashore. Supply vessels were following to bring in howitzers and materiel. Other ships were towing long jetty-like caissons to create temporary ports.
Into the middle of this armada came the chief meteorologist, Group Captain James Martin Stagg, a terse Scot with a long, thin, pale face, closely cropped hair, and a severe mustache, to report that his three teams of meteorologists—conferring and arguing by telephone—had grudgingly reached agreement. The weather for June 5 would be bad, very bad. Winds in the channel were likely to be force 5 on the Beaufort Scale—a stiff breeze, not yet a gale, but enough to set up a swell that would trouble ordinary ships, never mind the landing craft. Worse, the sky would be overcast, and the cloud bases at only five hundred feet, making the launching of paratroopers impossible, rendering precision bombing of the defenses out of the question, and making it too difficult for naval gunners to judge the accuracy of their salvos.
Thousands of troops remained on their landing craft in the harbor, seasick with the swells and sickened by the smell of vomit.
The operation was put on hold. Radio silence was in effect, so destroyers were deployed at flank speed to head off convoys, signaling them with flags or lamps or handing out cryptic messages like “Post Mike One,” meaning return to harbor. That night thousands of troops remained on their landing craft in the harbor, seasick with the swells and sickened by the smell of vomit.
Would the weather improve?
Although the military commanders had routinely been asking for weather forecasts a week into the future, they were told again and again that such forecasts are uncertain. Only the team of American forecasters, led by Irving Krick, was confident that they could see that far forward. Comparing past weather systems to the current ones, they contrived to predict how the present weather would evolve. Alone among the three teams forecasting, they suggested that June 5 would be just fine.
The two British teams—one from the navy and one from the Meteorological Office (called the Met)—were far less sanguine about June 5, and they were not so sure about June 6 just yet. In the Met Office was Sverre Petterssen, a Norwegian meteorologist who had been a student of Tor Bergeron, the man who discovered how raindrops form and who was himself a student of Vilhelm and Jacob Bjerknes, the meteorologists who first described the birth, life, and death of the kind of storms now crossing the Atlantic.
For June 5, the British teams prevailed. They used methods developed by the Bergen School, gathering measurements of temperature, pressure, and humidity from stations on land, at sea, and in the air. (They were even incorporating data gathered from German U-boats, whose code had been broken so that their weather reports to Zentralwetterdienstgruppe were immediately available to the Allies.) With this data, they tried to map the systems of counterclockwise-spinning storms, finding their warm and cold fronts, the pressure drops around their lows, and the position and direction of following highs. Where the lows passed, bad weather was to be expected. When the ridges of highs moved in, fair weather should follow.
The British made no pretense at being able to see beyond twenty-four to forty-eight hours into the future. Nevertheless, they were pessimistic on the night of June 3. It looked as though the cyclone system that would be bringing bad weather on the 5th would be followed by another such system on the 6th. The American Krick was infuriated by their reticence, and would later claim that it was only the farsightedness of the American team that allowed the invasion to succeed.
Petterssen told a different story. According to him both British teams had been quite pessimistic until the afternoon of June 4, when the weather had indeed begun to deteriorate, as they had predicted. A single ship stationed six hundred miles west of Northern Ireland to record the weather, however, began to report a rising barometer. Out there in the mid-Atlantic, the pressure kept rising. Perhaps, they reasoned, the Azores High is moving north. Perhaps it will shunt the coming storm to the north, or at least stall it for a day. They detected a break in the weather.
The Met Office team still voted no for June 6, but the British navy team and the Americans carried the day. Late on the evening of June 4, Captain Stagg met again with the Allied commanders. Outside, the trees were swaying in the wind, and a hard rain fell. Stagg told Eisenhower that they thought the weather would improve. There might be winds of force 3 or 4, with a few excursions to force 5, but the sky should be clear. It might cloud later, but the cloud bases should stay high enough for the naval gunners to spot their shots. Not ideal, but good enough.
The commanders met with Stagg again at 0430 on June 5. The high pressure was holding and building in.
“Halcyon Plus Five finally and definitely confirmed,” the word went out. This was the go code.
At 0900 on June 5, the convoys set out again in the face of force 5 winds. This was particularly troublesome to the landing craft and to the bellies of soldiers unused to the sea. Sea sickness continued throughout the seventeen-hour crossing.
Early on the morning of June 6, the Pathfinder planes for the paratroops ran into unexpected banks of clouds over the coast of France. They dropped their visual and radar beacons, but many fell in the wrong places. With the cloud cover, many of the visual beacons could not be seen. The paratroops themselves went in C47 transport, flying in a V of v’s formation. They too hit the cloud bank. The pilots had to break formation to prevent collisions. Some tried to get above the clouds. Some tried to go below. Some took evasive action as the antiaircraft fire began. The paratroops were dropped or glided in far from their intended targets.
As the landing craft moved in, waves were five to six feet high in midchannel, higher than expected but not impossible to survive. The wind had shifted to the northwest, driving the craft into the beaches with the wind at their backs. Some were swamped; some were wrecked. Of the thirty-two tanks going into Omaha Beach, twenty-seven were lost.
But the rest got in. At the end of the day, under a partly sunny sky, 59 degrees Fahrenheit with force 4 winds, the Allies had a firm hold on the beaches. They had lost twelve thousand men, but that is a fraction of the seventy-five thousand they were estimated to lose had they not had the element of surprise.
Why were the Germans surprised? They knew as well as the Allies that 5, 6, and 7 were the only days in June with the right tides and right moon. In May, they had stood on full alert at full-moon, low-tide times. What happened? Why was General Rommel—the man in charge of the Normandy defenses—in Berlin taking a walk with his wife, who was trying out her birthday shoes? Why were half the division commanders and a fourth of the regiment commanders at a war games exercise in Brittany planning for the invasion defense? Why were the torpedo boats in the harbor? Why had so many men been relieved of the heavy tasks of building the defenses and sent for a little rest and relaxation?
The Germans believed that the weather was too bad for the Allies to invade. This was not the fault of poor forecasting. Group Captain Heinz Lettau—later a revered professor of meteorology at the University of Wisconsin—saw the same succession of fronts as did the Allied forecasters. He may or may not have noted the marginal improvement of the weather on the 6th. Even had he seen this, however, his orders were clear. The High Command had decided that an invasion was not possible if there was a risk of the winds reaching force 4 or higher. (The Germans had put off their own planned invasion of Britain, Operation Sea Lion, in 1940, in part because they could never get what they felt was a calm-enough sea for the troops to cross.) Lettau was confident—and right—that there would be a force 4 wind on June 5, 6, and 7. Ergo, there could be no invasion. What the Germans failed to find out was that the Allies thought force 4 was just fine.
Just as important, Lettau did not have the benefit of colleagues to disagree with him. As contentious and nasty as the infighting had been among the three groups of Allied forecasters, each team got a vote, so the most persuasive case was likely to prevail. The Americans were most sanguine, believing they could forecast a week or better into the future by comparing weather maps for the previous days. The Met Office forecasters, using ground and upper-air observations and seeking to map the progress of fronts, believed they could go forward only a day or two at best. The naval meteorological service focused on wave heights and their effects on the invading landing craft. Among them, they came up with an ensemble forecast that allowed the generals to make intelligent decisions.
Lettau, following specific directives from higher-ups, had to predict that the invasion would not occur, regardless of any disagreements in his team. It is also likely that he caved in to military demands for a long-range forecast. He told them that the bad weather would continue for weeks. He himself knew that such a forecast was uncertain at best, and so did his colleagues. Left to himself, Krick would have done just the same—though promising good weather, not bad—telling the generals what they wanted to hear. Only the presence of his fractious colleagues prevented him.
The Allies appeared on the beaches of Normandy, just like a surprise storm.
In the end the Allies won the day because in order to predict the weather, they acted like the weather. Competing groups jostled and maneuvered, each trying to pressure the others into accepting their point of view. In just the same way, the high- and low-pressure cells fought and spun into one another over the Atlantic. The forecasters reinforced their own ideas, and none of their ideas was the winner, just as each gyre and each center of low and high pressure pressed against the others, squeezing out the future among them. The Germans, on the other hand, believing that they could conquer uncertainty by fiat, declared that weather and people would conform to their assumptions. They were proved wrong. The Allies appeared on the beaches of Normandy, just like a surprise storm.
After the war, German Admiral Friedrich Ruge praised Eisenhower for making “one of the truly great decisions in military history.” No German commander could have done so, he added, without seeking permission from higher up. The German brain was clogged and sclerotic with hierarchy. It should be remembered that Rommel was at the time part of a plot to depose Hitler. Perhaps his trip to Berlin was not just to give his wife a pair of shoes?
William Bryant Logan is a Quill & Trowel Award-winning writer, a member of the faculty at the New York Botanical Garden, a sought-after lecturer and teacher, and a practicing arborist. He is the author of Oak: The Frame of Civilization and Dirt: The Ecstatic Skin of the Earth, the latter of which was made into an award-winning documentary. He lives in New York City and the Hudson Valley.