by Sarah Dry
He never did learn what caused the monsoons. In 1941, nearly twenty years after he had left India, he received a letter from then-director of observatories Charles Normand, informing him that the monsoon forecast for that year based on Walker’s work was “little or no better than will be given by the intelligent layman who knows no meteorology but does know the monsoon frequency curve.” Normand was, understandably, reluctant to issue an official forecast on this basis. “I much prefer not to speak,” he explained, “unless the correlation forecast is appreciably more useful than the intelligent layman’s.” Walker could only concur. He’d never placed much store in the forecasts himself. “I fully agree with your policies of not making fuss about monsoon forecast,” he wrote back.52 The truth, as Walker was the first to admit, was that the Southern Oscillation was “an active, and not a passive feature in world weather, more efficient as a broadcasting than an event to be forecast,” as Normand put it.53 By 1950, the dream of forecasting the monsoon had been, if not fully abandoned, put on indefinite hold. Not only was it clear more data was needed, it was looking increasingly possible that data alone would never be enough. S. K. Banerji, who became the first Indian head of the Indian Meteorological Department in 1945, was clear-eyed about the limitations of an effort into which an “enormous amount of labour” had been poured. “The results obtained are not satisfactory. We do not, however, know yet all the factors which control the Indian rainfall. . . . It seems unlikely that a complete solution will be achieved in the near future. It is possible that part of the seasonal rainfall is not predictable in advance.”54
Walker never presumed that success was inevitable. Still, it is hard to read this story without feeling a sense of disappointment. The man who had definitively ended the search for correlations between sunspots and monsoons had been unable to find his own holy grail—a means of predicting the monsoon. In the process of looking, he had discovered something very important, a way to begin investigating the links between distant parts of the global atmosphere by validating which statistical connections were most likely to indicate physical connections. The exact nature of those physical connections remained unclear to Walker, and indeed, impossible to discover by the methods he had developed. It was only in 1969, ten years after Walker’s death, that a further veil on the mystery of the monsoons was removed when the Scandinavian meteorologist Jacob Bjerknes showed what was missing from the landscape of Walker’s world weather.55 The ocean was the enormous elephant in the room. It provided the necessary other half of what Bjerknes showed was a grand global cycle whereby ocean temperatures affected the temperature of the air above it. He named this cycle of east- and westward motion the Walker Circulation. Its basic mechanism was this: Cold water that welled up from the depths of the eastern Pacific cooled the air above it, preventing it from rising and therefore allowing it to be blown westward by the trade winds, where it eventually warmed enough that it rose above the western Pacific. It was then able to return eastward in the upper atmosphere, where it closed the circle by sinking back down over the Pacific. Variation in the degree of cold water that upswelled, unexplained in 1969 (and still mysterious today), seemed to be the reason why some years the circulation “failed” to bring monsoon rainfall to India.
Walker and Bjerknes did work which, laid end to end, would eventually solve at least some of the mystery of the monsoon. This tells us something important about the way our understanding of the earth has evolved. It is the movement between observation, calculation, and theorizing that produces insights. No prescription can set the order in which these different ways of knowing can, or should, proceed. And no reliable method has yet been devised which can forecast from which quarter an important new piece of work will emerge. Too late for Walker, the great arcing trajectory of his monsoon research did, eventually, find its return. The monsoons were part of a global system by which heat travels through the oceans and the atmosphere, making its way around the complexities of water and air as surely as the boomerangs Walker threw found their way back to him.
5
HOT TOWERS
Joanne Gerould, aged twenty-one, stood in front of a roomful of aviation cadets at the University of Chicago. The year was 1943 and the United States was at war. Though she was young, and more unusually still, a woman, she had good reason to be standing where she did. She knew more than the cadets did about the movement of air and moisture through the atmosphere. That one thing was reason enough to grant her the authority to lecture them. The men needed to learn, as quickly as possible, the fundamentals of weather forecasting.
What the young woman needed was less clear, but she already had a strong sense of what she didn’t want. She didn’t want to be dependent on a man. She’d learned from her mother the emotional damage it could do to be bright and unable to follow one’s dreams. Her own mother had trained to be a journalist but never managed to pick up the threads of that ambition after she’d given birth to Gerould. She’d taken out her frustrations on her daughter, and Gerould, in turn, had struggled with the weight of that bitterness. She’d searched for escape, finding it in the teeming confusion of the marshy estuaries in which she’d played as a child on Cape Cod, in the coastal waters upon which she’d sailed and in which she’d swum, and finally, in the skies through which she’d flown. Aged just sixteen, Gerould had obtained a pilot’s permit. It was both a metaphorical and a very real form of escape, out and up, into the skies.
When it came time to choose a college, Gerould had followed the same urge, flying away from her home in Cambridge, Massachusetts, and from Radcliffe, where both her mother and grandmother had studied. She went west, to the University of Chicago. There, a course that included plenty of science classes appealed to her. She thought she might study astronomy. But the time was right not for the study of the heavens, but of earthly skies. World War II was, famously, an airman’s war. Flat navigational charts, plotted with straight rulers, were set aside in favor of spherical globes on which bits of string were laid to trace the curving paths taken by planes that found their target with unerring directness. Old European battle lines looked set to disappear in the face of the new Pacific arena and the northern pathways that touched the whitest parts of the planet. The geography of the world looked as if it could be remade by these airmen, and the countries for which they flew, if only they could win the war.
At the start of the war, the Germans had more than 2,700 trained meteorologists available to advise their pilots on how to stay safe in the air. The United States had just thirty.1 To rectify this alarming imbalance, the air force had gone straight to the person who could make the fastest and greatest difference. Carl-Gustaf Rossby was a maelstrom of meteorological theory and administration, a thinker and a doer, with energy to spare. A Swede, he’d received his own meteorological education in Bergen, Norway, at the time and place in which meteorology had come of age professionally and started to deliver on promises long in the making.
In Bergen, a man named Vilhelm Bjerknes had managed to cleave the practical necessity for daily forecasts to the clarifying mathematical equations of physics. The theories of the weather that Bjerknes had helped develop, and which Rossby had learned better than anyone else, were well suited to explaining the skies above Scandinavia. Having come of age during World War I, these men naturally saw the battlegrounds of northern Europe projected on the cold skies overhead. “We have before us,” wrote Bjerknes, “a struggle between a warm and a cold air current. The warm is victorious to the east of the centre . . . The cold air, which is pressed hard, escapes to the west, in order suddenly to make a sharp turn towards the south, and attacks the warm air in the flank; it penetrates under it as a cold West wind.”2 These organized lines of clouds could be deduced from observations at regularly spaced intervals, then tracked as they moved across Britain, the Netherlands, and into the skies above Denmark, Sweden, and Norway.
Rossby was the man the U.S. Air Force trusted to bring the necessary meteorological know-ho
w to the pilots in time for it to matter. To do that meant getting training programs (one would not be enough) up and running as soon as possible.3 And to do that, as was so often the case during wartime, meant calling upon women to do jobs that they would not ordinarily be offered. So when Gerould went to see Rossby about the possibility of doing some meteorology courses alongside her astronomy degree, she ended up with an offer instead: to teach on the cadet training course that Rossby had established to quickly boost the military’s forecasting capability. While she had not gone seeking such an opportunity, she was more than ready for it when it arose. And while she had not yet, in fact, fallen in love with the study of the clouds, stepping into Rossby’s office was a fateful step on her journey toward an intellectual passion that would last a lifetime.
FIG. 5.1. Carl-Gustaf Rossby with the rotating tank used to study the motion of fluids in the atmosphere and ocean. Credit: NOAA/Department of Commerce.
FIG. 5.2. Graduation and commissioning of U.S. Army Air Force meteorology cadets at the University of Chicago, September 6, 1943. Credit: University of Chicago Library, Special Collections Research Center.
Clouds, Gerould would later write, were more complicated than almost anything else. The only thing more complicated, she conceded, were human beings. “The mysteries of cloud formation, and the precipitation that can follow, have proven to be one of the most challenging aspects of the global climate system. Except for man himself, the weather is probably the most variable, unreliable, and fluctatory phenomenon of which human intelligence has dared to attempt a science.”4 A cloud suffers the buffets of the atmosphere around it, retaining its shape for a while before becoming utterly changed. To become entrained, in meteorological terms, is to be taken up by a pre-existing air current or cloud. It is what happens to air in the environment that comes close to a cloud. “Within ten minutes, I was entrained in his orbit,” was how Gerould described her first meeting with Rossby.5 It is no accident that Gerould used this term, since she herself had used the concept it described (though she did not invent it) to create a completely new way of thinking about clouds and, as a consequence, a new way of thinking about the circulation of the entire atmosphere.
Part of what Bjerknes and Rossby had begun, which Gerould would continue, was the project of moving the study of clouds beyond the scientific equivalent of stamp-collecting. By the 1930s, when Rossby arrived in America, the study of meteorology had different ambitions. Those were twofold: first, and most urgently, to provide operational support to the military in order to help pilots make informed decisions about where and when to fly. Second, Rossby and those who worked with him wanted to transform meteorology into a physical science. By this, they meant a science at the heart of which lay physical equations that described the movements of the atmosphere. There were obvious linkages between these two desires, but they were also, perhaps to a surprising extent, distinct. It was possible to make meteorological forecasts in the absence of physical theory. And physical theories were not always that useful when it came to making practical forecasts. It wasn’t clear, then, in which direction progress would first be achieved, and by whom.
Gerould taught for a year on the course, from the fall of 1943 to the summer of 1944. That was enough. After that, she’d been entrained by the study of meteorology. She then enrolled in a one-year master’s program and continued to take classes after it ended. So it was that she found herself still studying in 1947, listening to a series of lectures on a topic that had been more or less ignored by the Scandinavians who’d put together the modern science of frontal weather systems: tropical meteorology. What Gerould heard electrified her and caused her to abandon once and for all her ambivalence about a field that she had good reason to doubt could ever provide her with an all-important steady income and intellectually rewarding work. The sense of an exciting new area of study opening up before her eyes was almost palpable and too compelling to resist. “Almost immediately,” she later remembered, “a bolt of lightning struck me and I said to myself and my colleague, ‘This is it—tropical cumuli are what I want to work on.’”6
The lecturer to whom Gerould owed this electric realization was Herbert Riehl, a man just eight years her senior. A Jew, he had been forced to flee Germany as a young boy, traveling first to England and then to America, where he himself became entrained by meteorology, almost by chance. He’d come to the United States with the idea of becoming a screenwriter and had pursued that passion for a few years. But success was not forthcoming and, seeking more practical employment, he applied to enter a U.S. Army Air Corps training program. The electrical engineering course he applied to was full, so he settled for meteorology. After completing the one-year program at NYU, he too went to see Rossby, and Rossby offered him the same opportunity he’d offered Gerould. Riehl accepted, and he taught on the training course at the University of Chicago the year before Joanne Gerould, from 1941 to 1942.
By 1942, the war in the Pacific had taken a dangerous turn. The Japanese had taken Burma, Malaysia, the Dutch East Indies, the Philippines, and Thailand. To combat the Japanese threat, military pilots desperately needed a much better understanding of the meteorology of the tropics. Thousands of military sorties over the tropical Pacific made it glaringly obvious that the weather worked very differently there than it did in Northern Europe. Sudden squalls arose in the absence of obvious fronts and demanded explanation. Rain fell from skies far too warm to sport ice crystals. The situation wasn’t simply confusing; it was potentially dangerous. To fly safely, pilots needed better predictions of bad weather. The U.S. Army Air Corps agreed when Rossby proposed adding a special tropical institute to the nine-month cadet program. Arrangements were made as quickly as possible, and in the summer of 1943 the Institute for Tropical Meteorology in Puerto Rico was created in hopes that new observations and concerted effort might come up with something useable in time to help the war effort.
Riehl spent just two years in Puerto Rico, first as an instructor and then as director of the fledgling institute, before being sent back to Chicago at the war’s end. His time there was transformative. The meteorology that had been so proudly and confidently pioneered in Bergen was almost completely useless in the tropics. Tor Bergeron’s theory of rain formation, the pre-eminent such theory of the Bergen school, required the presence of ice crystals. Without ice, Bergeron had theorized, rain could not fall.7 That may have been true in Norway, but a single evening in Puerto Rico was enough to demonstrate how patently false that theory was in the tropics. Riehl vividly remembered such an evening, his first in Puerto Rico, when “some of the staff walked along the beach, and admired the beauty of the trade-cumuli in the moonlight. Well-schooled in the ice-crystal theory of formation of rain, they had no suspicions about clouds with tops near 8,000 feet where the temperature is higher than +10 C. Suddenly, however, the landscape ahead of them began to dim; then it disappeared; a roar approached as from rain hitting roof tops. When some minutes later they stood drenched on a porch, drenched and shivering, they had realized that cloud tops with temperatures below freezing were not needed for the production of heavy rain from trade-wind cumulus. There and then the question arose: How is it with the other theories in so far as they concern the tropics?”8
Back in Chicago, with his tropical experiences still fresh in his mind, Riehl raised this question with the students sitting in front of him. He described how at the end of the war, the navy had allowed a small group of researchers based at Woods Hole Oceanographic Institution (WHOI) to use some of their planes and ships to undertake some research on the trade winds of the North Atlantic. The project was charmingly informal, an example of the kind of do-it-yourself ethos that characterized WHOI at the time. Together, Jeffries Wyman, a physical chemist, and Al Woodcock, a self-taught jack-of-all-trades, made some of the first measurements of temperature and velocities both inside and outside the so-called trade-wind cumulus clouds (the clouds found in the region just north and south of the equator where th
e winds blow consistently from east to west, and toward the equator).9 Their data put to rest forever the idea that the tropical atmosphere was organized into fronts. Instead, Woodcock and Wyman had demonstrated that the equatorial atmosphere displayed what one scientist later described as a “disconcerting sameness,” with endless fields of trade-wind cumuli—isolated puffy clouds, like those in a children’s storybook—stretching to the horizon.10 This in itself was a strikingly different meteorological visage from that displayed by northern skies, where storms were common and clouds organized in long frontal systems. And there was more. Hidden within this seemingly calm atmosphere was the capacity for sudden, violent storms. Unlike in upper latitudes, when squalls arose in the tropics, they did so without any apparent provocation. Rarely, but unforgettably, they gave rise to a monster storm known in the Pacific as a typhoon and in the Atlantic as a hurricane. What caused these storms to arise when and where they did remained highly uncertain.
The data gathered by Woodcock and Wyman raised more questions than it provided answers. What caused the puffy trade-wind cumulus to form? Did the sea surface play a role in their formation? When and why did storms develop out of this seemingly uniform sea- and airscape? Like the atom, tropical clouds seemed to contain the hidden potential for dramatic transformation. The challenge was to explain what caused a seemingly harmless patch of tropical atmosphere to change into a violent squall and from there into an even more violent hurricane.
Listening to this news from a distant part of the planet, and the seemingly endless questions it raised about the basic workings of the atmosphere, Gerould had felt a rising sense of excitement, akin to an epiphany, that this was the work to which she wanted to devote herself. It remained far from clear whether that would be possible. In 1944, she had married a fellow University of Chicago student, Victor Starr, who had recently become the second person to be awarded a PhD in the University of Chicago meteorology program. Joanne Gerould had become Joanne Starr. She’d given birth to their son, David, in June, at the end of her master’s degree. When she told Rossby of her plans to study tropical cumulus clouds, his response was cutting: “That’s fine. An excellent problem for a little girl to work on because it is not very important and few people are interested in it, so you should be able to stand out if you work hard.”11 Undaunted, Starr immediately wrote to a friend of the family at Woods Hole, asking for a summer job. She got the job and spent the summer working on the Wyman and Woodcock cumulus data Riehl had lectured about.12