4

NUMBER OF THE MONSOON

Arriving in India in 1903 at the age of thirty-five, Gilbert Walker traveled directly to the foothills of the Himalayas. His destination was Simla, the summer capital of the British Empire in India, and the year-round location of the Meteorological Department. He was on his way to assuming the most important job he would ever hold, Director-General of Meteorological Observatories.1

He had spent most of the past decade and a half at Trinity College, Cambridge, living among other single men in rooms set off private courtyards. There, he had devoted himself to the study of mathematics. Though he was about to take over the world’s most extensive meteorological network, he knew next to nothing about the weather, still less about managing an organization that included hundreds of observatories and tens of thousands of observers. That, in a sense, was the very reason he’d be summoned to India. Things had gotten so desperate, and the problem facing those who hired him so intractable, that Walker’s ignorance and lack of experience had started to seem almost desirable. When little else had worked, perhaps it was time to try something completely new. There was one single, highly reassuring fact about Walker: He was one of the best mathematicians of his generation. That was enough reason for Walker’s predecessor, John Eliot, to stake his own reputation on the tall, thin young man who had undertaken the long journey to India.

Simla, the city where Walker was headed, was just as paradoxical as Walker’s appointment. It was a cool place in a hot land, a small city in a wildly populous nation, a remote enclave from which to rule a voluminous empire. Located just over 7,000 feet above sea level and nearly 1,000 miles from Calcutta, Simla offered relief from the stupefying heat of the plains. The city had proved its worth by providing a safe, comfortable, and healthy location from which the British could rule their largest colony during the oppressive summer months. Arrayed on a steeply terraced hillside, and governed by the rhythms of the court, the town was as much redoubt as refuge, deriving its power from its very remoteness and exclusivity. “Here,” in the words of one visual atlas of the Empire, “wars are planned, peace is made, famines fought.”2

That last, alliterative phrase was the reason Walker had been summoned.

FIG. 4.1. Simla and surrounding mountains in the Himalayas. Credit: Wellcome Collection.

FIG. 4.2. Portrait of Gilbert Walker, who arrived in India at the height of famine hoping to identify the causes of the monsoon.

That remoteness could signal power—that power could be enacted from a distance and that its effects might be amplified rather than attenuated by that distance—was a thing that the British in India already knew. What they didn’t know, and what they hoped Walker would tell them, was what powers gave rise to the monsoon deluges upon which the well-being—indeed the survival—of tens of millions of Indians and the wealth of the Empire depended.

* * *

The idea that mathematics might be able to cut to the heart of a problem was not a new one in 1903. Mathematics had long been granted a special authority among scientists, dating back at least as far as Galileo’s measurement of the acceleration of falling bodies. Used properly, numbers could reveal the laws of nature. That mathematics was essential for understanding the atmosphere of the earth had also been long understood, ever since observers had gathered quantitative measurements of temperature and pressure. In the final three decades of the nineteenth century, however, both the mathematical tools, and perhaps even more importantly, the amounts of data to which they could be applied, had increased dramatically. The automatic observatories set up by the Met Office in the wake of FitzRoy’s death—recording the traces of the weather day in and day out—were generating data from which it was hoped patterns could emerge. In addition, legions of observers were diligently recording observations by hand. These observations—many of which were made by sailors in the navies and merchant marines of the ruling nations of the globe—had grown exponentially in the decades preceding Walker’s arrival in India, for the simple reason that the management of empires depended on the management of the weather. As empires had grown to subtend ever-larger portions of the globe, so the collection of meteorological data had spread to keep pace.

Climatology was the name of the field dedicated to the collection of data about the weather. Originating in German, the term made its way into English and French usage in the first decades of the nineteenth century, rising in usage from the 1840s onward. Its rise can be traced, as so much, back to Alexander von Humboldt’s influence at the start of the century (one of the earliest uses of the word was in a French translation of Humboldt’s work).3 Humboldt’s concept of climates was related to his insight into the unity of nature. This unity, according to Humboldt, did not produce climatological uniformity. Instead, myriad physical forces combined to generate climatological difference—distinct zones, often defined vertically, in which certain conditions of temperature and precipitation persisted and where certain plants and animals thrived accordingly. If the physical forces of nature were always in flux, the wonder was that they combined in such a way as to produce stable, geographically fixed climates that could be measured, described, and in a certain sense safely stowed away. Safe, in this sense, meant reliable. Humboldt expected them to endure for a long time.

The exploratory and unity-seeking arm of climatology that claims a lineage from Humboldt must be reconciled with the part of climatology that was, as it were, born statistical. In both England and Prussia, national departments for gathering meteorological data emerged from or were closely linked to state statistical offices. There, the practical benefits of knowing the changing weather patterns—and determining some general climatological rules—were enormous. Understanding climate was of practical benefit in the same way that astronomy, botany, magnetic studies, and surveying were—they allowed state holdings to be mapped and understood in order that they could be exploited. But perhaps more than any of these allied disciplines, the collection of weather data that could be transformed into climate averages was a nation-building exercise. By gathering data on both citizens and the weather, governments hoped to control these often-unwieldy phenomena.4

Julius von Hann, director of the Central Office for Meteorology and Geomagnetism in Vienna, and Wladimir Köppen, director of the German Marine Observatory in Hamburg, were the primary inheritors of Humboldt’s confidence that the earth’s face could be measured and known. They differed from Humboldt, however, on the question of how climate should be defined. Armed with the desire, the money, and the institutional capacity, Hann and Köppen transformed Humboldt’s mantra of singular exploration into the basis for a systematic discipline. In the process, the Humboldtian emphasis on the interrelationships between living creatures and the environment in defining climate gave way to a definition of climate in terms of fixed zones, identified on the basis of averaged meteorological data. Climate, as defined by Hann and Köppen, was built on the foundations of meteorological data. It was the weather averaged.

More important than the precise definition of climate (as it turned out) was the foundation that these men established for climatology. Under the direction of Hann and Köppen, climatology was a science of rigorous measurement carried out in robust institutional settings. Hann promoted his approach through a Handbook of Climatology, published in 1883, which laid out the method by which climatology could advance, step by step, as an invading army. The natural direction for this science was cartographic, and in due course Wladimir Köppen extended climatology by introducing the graphical power of the climate map, on which averaged climatic zones were represented in visually distinctive areas. The “tropical climate” was born on Köppen’s maps, alongside the “polar climate,” the “subtropical climate,” and the “Mediterranean climate.”

The success of the program and its weakness lay in the same fact: Knowing the climates of the earth in this way required an avalanche of data. Climatology, in the sense that Hann and then Köppen practiced it, was a science of the telegraph, the postal system, and the publishing house. It relied on an extensive system of measurements for compiling maps, and just as significantly on a system by which such maps could be printed and distributed. This climatological project motivated and absorbed an incredible amount of energy in the decades following its ripest definition.

Despite its productivity, Hann was almost aggressive in setting out the limitations of climatology. He was self-consciously anti-theoretical in his claims for what climatology could do. “Climatology,” he cautioned, “is but a part of meteorology when the latter term is used in a broad sense.” Climatology, he clarified, was descriptive, while meteorology, which aimed to “explain the various atmospheric phenomena by known physical laws,” was theoretical. Nevertheless, the two fields were intimately related. Climatology was an essential part of meteorology, and what it lacked in explanatory power it made up for in its breadth. As a primarily visual science, it provided the means to build up a “mosaic-like picture of the different climates of the work.” This was a very orderly patchwork, in which facts were presented systematically. In this way, “order and uniformity are secured, the mutual interactions of the different climates are made clear, and climatology becomes a scientific branch of learning.” As Deborah Coen notes, this made climatology a science (at least potentially) of “complex wholes,” while meteorology concerned itself instead with reducing atmospheric phenomena to “simpler, theoretically tractable elements.”5

Much was presumed in the elision here between Hann’s admission of the descriptive nature of climatology and his ambitious hope that it could become a true science.6 How exactly it would be possible to get from the compilation of average values of rainfall and temperature to a science of physical laws was left unclear. There was a deep tension in relation to the nature of change buried in the text of Hann’s Handbook. Change was essential to this transformation, but just how much change should be defined or investigated remained unclear. Hann included a final section on “Changes of Climate” in his handbook, in which he considered geological changes in climate, such as the ice ages, and the theories of Croll and others who tried to explain them. He also considered the search for shorter, so-called oscillations of climate that could be linked to sunspots. In both cases, however, Hann revealed his overriding commitment to  the method of averaging, with its strong assumption of the explanatory validity of a period of stability. To generate an average, as Hann explained, required identifying a period of time across which the average would apply. Using this method, it was eminently possible to identify oscillations either above or below an average value. Change, in other words, was here understood in relation to some fixed period within which averages could be constructed.7

Hann’s approach was predicated on a commitment to stability in the form of averages, but in practice it enabled him (and others) to identify climatic anomalies. It did so by establishing a rudimentary form of climate system. The statistical table, in this sense, was an inchoate climate model. By bringing together averaged weather data from distant parts of the planet, the table enabled Hann and others to search more easily for patterns within the numbers. Since those numbers corresponded to average weather values—what Hann had defined as climate—they enabled researchers to find connections between longer-term features of the atmosphere. To be sure, the process of averaging elided the dynamism that it was the business of meteorology to uncover and describe—what Hann called the “causes underlying the succession of atmospheric processes.”8 But Hann urged that attention be paid both to the averages and to the deviations from them. This was, Deborah Coen argues, part of a desire to forge a national Austrian identity out of a range of diverse climate zones. To do so required blending distinctive local settings into a harmonious imperial whole. Statistically speaking, this meant attending both to the deviations and to the average, the local and the global. In a real sense, it was the climate averages that made the deviations—and the dynamism they implied—more visible. As a result, and somewhat counterintuitively, Hann’s approach ultimately paved the way for a new kind of climatology focusing precisely on the variability within a climate system rather than the stability of individual climatological zones.9

When Walker arrived in Simla to take up the post as Director-General of Indian Meteorological Observatories, he walked into precisely this uncertain space between the stability that averaged climate statistics generated and the variability those data could be used to search for. Whether the climate system was seen as inherently stable or inherently variable depended on the perspective of the person inspecting it. Prerequisite to either approach was the concept of a system itself. That system was a product of the Empire as surely as was Gilbert Walker, or the bales of wheat upon which so many depended for sustenance and for profit.

* * *

The challenge that faced India and, in particular, the British in India was really a set of challenges. When Walker arrived in 1903, the summer monsoon rains had failed in India for three of the past seven years, in 1896, 1899, and 1902. The brevity, and simplicity, of such a statement belies the grand scale of human suffering it unleashed.

Millions had died. Of the things that were countable in the universe, the number of dead was not among them at that time, so it was impossible to know precisely how many. A Lancet article in 1901 put the number who had perished in the past five years at nineteen million, half the total population of the UK at the time and roughly eight percent of that of India.10 Crops had failed completely across areas totaling more than three times the size of the entire UK.

This was not a natural disaster. The language of “failure” which was (and still is) used to describe the lack of monsoon rainfall in some years implies that the rains were to be expected every year. In fact, variable rainfall—including the complete absence of rain in some years—was a normal, rather than an abnormal, feature of the Indian climate. The monsoons had always come and gone, sometimes providing life-giving moisture, sometimes withholding it. Famines had accompanied such droughts in the past, but the number of deaths from starvation had increased dramatically during British rule. This reached a peak between 1876 and 1878, when some six to ten million had died (across both British and non-British territory).

FIG. 4.3. An American tourist and an unidentified woman pose with a famine victim, India, 1900. Credit: John D. Whiting Collection/Library of Congress Prints and Photographs.

The British Empire was largely at fault for this harvest of death. In the process of imposing a cash economy on India, the British had dismantled traditional systems of mutual relief and grain storage that had allowed farmers to build up grain reserves during “fat” harvest years that could be drawn upon to make it through the lean years.11 In the name of productivity, the British had encouraged the destruction of countless individual safety nets. In their place, they had offered ready cash for this year’s crop and little else.

In the midst of the famine, Queen Victoria was proclaimed Empress of India. Imperial pomp was blind to the suffering, willfully, even righteously so. Lord Lytton, poet and viceroy of India, proudly wore the mantle of Adam Smith, who had claimed, in relation to the Bengal famine of 1770, that famines were worsened by “improper” and violent government interventions. According to Smith, so-called “humanitarian hysterics” who insisted on sending money for famine relief were in fact contributing dangerously to the possible bankruptcy of India. The best thing to do was nothing. By letting the famine run its “natural” course as quickly as possible, a natural correction in the economic cycle would be effected and, like a series of frequent brushfires that prevent a catastrophic blaze, thereby limit the potential for the worst losses. Famines were in this sense natural events, in social and economic terms, a kind of built-in mechanism to keep the population of India in balance with its size, and, not incidentally, to keep grain prices high. Any attempt to limit the effects of famine, claimed Lytton to the Legislative Council in 1877, only added to the problems of overpopulation.12 In a brutal bit of human calculus, Lytton pointed out that since the overwhelming majority of those who died in the famines were poor, any policies that had the effect of saving their lives only increased the proportion of the population living in poverty. It might be better that the poor should die, was the unstated conclusion, than left to live lives which were subhuman.

As visible as this failure of government was, British rulers went to great lengths to fail to see what was happening around them. In a state where ten percent of the population had perished from starvation, a glance from the window of the vice-regal train was sufficient to reassure Lord Elgin of the “prosperous appearance of the country even with the small amount of rain that has come lately.”13 Notwithstanding such heroic acts of self-deception, death on such a biblical scale demanded a response. Following the catastrophic loss of life in the so-called Great Famine of 1876–1878, a commission had been established to determine what steps could be taken to avoid such a disaster in the future. Experts in fields such as medicine, economics, and agriculture were consulted, and special regional famine laws, or codes, were written to ensure that aid would be delivered locally in a timely manner. The famine commissioners lamented the inadequacy of meteorology to the task at hand. Whatever clarity the future might bring about the “true periodical fluctuation” in the rainfall, it was painfully clear that contemporary scientific knowledge was sorely lacking as a basis for forecasting. Though famines were inevitable, the depressing truth was that “they will come upon us with very little warning and at very irregular intervals.”14 Forecasts had been issued for monsoons since the 1880s, but the decision was made to cancel them following their failure to predict the absent monsoon in 1901–1902. This decision disappointed many who felt the forecasts were useful even if they were not always accurate. One commentator in the Times of India argued that “in a country so essentially agricultural as India the myriad cultivators may not unreasonably ask why such help as the Meteorological Department may be capable of rendering to them . . . is suddenly to be denied.”15 Such requests fell on deaf ears, and instead the commissioners expressed the hope that other technologies of distance that had served the empire would now serve the people. The railroads and telegraph system that were normally used to regulate the flow of commerce—specifically the grain which was the greatest export crop of the Indian empire—would be used, in the case of future droughts, to deliver relief food where it was needed and, by evening out supply and demand, ensure that grain prices did not spike, as they had during the last famine.

In the event, precisely the opposite happened. Trains were used to transport grain not to where it was needed by hungry people, but to where it could be sold for profit. Telegraphic news enabled speculators to corner the grain markets. Local charity was grossly inadequate to the task of caring for large populations of starving people. Desperate parents, unable to feed their children, sold them for pennies each, or, failing that, tried to give them away. One correspondent reported visiting an orphanage where he encountered children whose arms were no bigger than his thumb, and whose ribs showed through their skin “like a wire cage.”16

* * *

Such horrors formed the backdrop against which Walker took up his post on the first day of 1904. As desperate, and even absurd, as it may have seemed to recruit someone like Walker to achieve the seemingly hopeless, there were several reasons to think that Walker was in a better position than anyone ever had been to study and eventually be able to predict the rhythm of the monsoons. He’d been living with high expectations for most of his life. Prognostications of his promise dated back to a mythic mistake he’d made in school, when he’d bungled the declination of a Latin verb. The mathematics teacher who took him in after his subsequent banishment from the classics could not stop marveling at what Walker was able to do, mathematically speaking, with very little effort.

From the age of seventeen, there is evidence that he loved anything that was spinning or turning. A gyroscope he made with his own hands won him a prize in school, and more notice. At Cambridge, he studied applied mathematics with the leaders in the field, J. J. Thomson and G. H. Darwin. In his spare time, he threw a boomerang on the wide green lawns that rolled down to the river from the backs of the great colleges. “Boomerang” Walker could make the curved piece of wood fly far away before it made an improbable, arcing turn and came to roost once more in his hands. It was a noticeable eccentricity at a time when most young men put their bodies to the test in the mosquito-thin rowing boats on the Cam.

Mostly, he devoted himself to mathematics, and in particular to mathematical physics—the study of how objects (those boomerangs) moved through abstract, geometrical space—which formed the backbone of the Cambridge program. In a sense, he pulled it off. At the end of three years of study, he’d not only taken the notoriously difficult Mathematical Tripos exams but come first, living up to the promise that so many—his schoolteachers, his tutors, his coaches, and his parents—had laid a claim to. But the achievement had taken a serious toll on him. He had what was delicately referred to as a “breakdown” in his health, necessitating removal from the location where he’d reached debilitating heights. Three winters at a sanatorium in Switzerland were required to smooth out the mental kinks, the places deep inside him where tension, a necessary quality if one was to marshal numbers at the heights he’d scaled, had become crippling.17

John Hopkinson, a fellow Cambridge mathematician turned engineer, had once said that “Mathematics is a very good tool but a very bad master.”18 By the time Walker arrived in India, he’d learned for himself what that meant. Mathematics alone was unwieldy, dangerously consuming, while being simultaneously useless. Boomerangs and their reassuring returns were not enough to salve the hurts that mathematical intensity inflicted on him. In Switzerland, he found succor in ice-skating. The lack of friction, the cold air, and the clear skies rinsed his mind. The arcing boomerang found its echo in the curve of his skates on the ice. Inside his mind, some reciprocal curve began to grow, rebuilding the parts of him that had been broken by too much study, too much mental tension. He skated his way through, and eventually out of, his breakdown. He spent several years back in Cambridge as a college lecturer, seeking suitable material with which to ballast his flightier tendencies, something weighty enough to keep him from flying off into a mathematical abyss. Electrodynamics, and a problem suggested to him by a senior mathematician, kept him tethered for a while.

That time ended when, aged just thirty-five, he was recruited to be the new head of meteorological observatories in India and to join the cadre of scientific professionals who populated a thin but growing strata in the great laminated system that was the British Raj. Those who had come before him in the meteorological field had tried, in their way, to master the weather from the ground up, with maps of storm systems and theories about the effect of snowfall in the Himalayas on next year’s monsoon. But the conclusion to which they’d come was that the connections between aspects of the weather that mattered most to India were too complex to reveal themselves to even the most intuitive and insightful of scientists. In the past, the basis of meteorology had always been physical. Scientists had always tried to picture things visually, to imagine the way different masses of air might interact to push and pull each other around the ocean of air that was the atmosphere (to say nothing of the masses of moving energy in the ocean itself). They had failed, and now they hoped that someone like Walker, for whom numbers acted like a lever with which to pry open otherwise closed systems, could do the same for India. On hearing news of Walker’s appointment, Cleveland Abbe wrote to congratulate him and expressed his hope that “by suggesting a new class of problems, your thoughts may be centred on dynamic meteorology, to the great advantage of this difficult branch of science.”19

As it happened, opening up India to Walker’s penetrating gaze was no different than offering him the world.

* * *

By 1904, both the British Empire and the discipline of meteorology were edging toward the farthest boundaries of the planet—they were nearly, if not yet completely, global in extent. The British Empire was close to its peak of influence and power, when it encompassed nearly a quarter of the earth’s landmass and a fifth of her population. In India, its largest colony by far, the British controlled an area of some 1.5 million square miles, ten times the size of Britain itself. This kind of lopsided rule was inherently precarious, as the bloody Mutiny of 1857 had shown.

The challenges the Empire and meteorology faced were remarkably similar. Both sought to understand and control a set of unruly phenomena unfolding in locations that were often remote from the offices where calculation and coordination occurred. The notion of imperial meteorology, championed by Nature editor Norman Lockyer, was, therefore, something of a redundancy. Empire was meteorology, and meteorology was empire. To put it more practically, as India’s finance minister Guy Fleetwood Wilson memorably did in 1909, the “budget of India is a gamble in rain.”

Only with the leveraging power of certain technologies was British rule in India even thinkable. Much has been made of the importance of railways, telegraphs, and steamships in drawing the Empire together across time and space. Just as essential but often overlooked were the tools of bureaucracy itself. These took the form of central offices where information could be gathered, sorted, and acted upon. Such offices were the nodes of the great imperial network. They reached their apotheosis in London, but were necessarily to be found also in Calcutta, in Simla, and in remote field stations from which telegraphic messages were sent and received. In these small and well-organized spaces, a few workers with the ability to move information around with as little friction as possible could contribute to the governing of millions of subjects of the crown.

Thanks to the power of technology and bureaucracy, distance, once a foe to be vanquished, became something rather more interesting and much more valuable to the British Empire. Rather than a challenge to the exercise of power, it came to be seen as a mark of that very power. A tiny post office on an Indian tea plantation, set beside a stream in which an elephant might peacefully bathe, could reveal itself to be, on closer inspection and by dint of a small wire emerging from it, a node in a global network of imperial connection and control. As a result of scenes such as this, often reproduced in imperial gazettes and albums, distance became the leitmotif of the Empire, an expanse on which the sun famously was given no opportunity to set.

Distance wasn’t just symbolic of the great power of the Empire, it also created value where none had existed before. Reliably fast steamships that muscled their way across the seas meant that English citizens could eat bread made from Indian grain grown year-round (or nearly so) at a comfortable and safe distance from the land in which it was grown, from both the sunlight and the rains upon which it depended. India became both Britain’s bread box and its money box. By 1904, it was Britain’s greatest source of imported goods and the largest market for Britain’s own exported goods.20 India’s value to the Empire arose not in spite of, but because of, its distance from London.

Distance was what made the Empire work. It was as much a part of the logic of its success as any local control. In many ways, it was inevitable that Walker’s greatest achievement would be the discovery of something he called world weather. The greatest distances the world could offer were available to Walker, and he took them and put all his skills to bear in meeting the correspondingly enormous challenge that had brought him from the peace of Cambridge to the monsoon wars.

* * *

The very meteorological facts that made India such a challenging place to govern made it singularly ripe for meteorological investment and study. India offered a fantasy geography for the meteorologist. This was partly a matter of scale. Everything in India was oversized. Conceptually separated from its neighbors in the region by dint of its special relationship to Britain, it was also physically separated by the extreme vertical boundaries of the Himalayas at its northern edge, by the coasts on its east and west sides, and by a southern tip that, reaching to the equator, tapered into nothingness. Straddling a quarter of the earth’s latitude, India demonstrated an enviable range of climatological phenomena. Its scale and climatological features meant that unlike Britain, where weather varied from day to day, in India the weather conspired to generate longer-term patterns: months, rather than days, formed convenient units of measurement. This made calculation vastly more manageable. As a result, India was a place where the patterns of the weather could make themselves more legible than almost anywhere else on the planet. Walker’s predecessor Henry Blanford wrote without irony of India that “Order and regularity are as prominent characteristics of our atmospheric phenomena, as are caprice and uncertainty those of their European counterparts.”21 This was partly a matter of geographical extent. India was a place where “general laws have a sufficient space to produce general results,” and so-called “disturbing influences are regular and well-ascertained.”22 For anyone who had ever grown first irritated and then maddened by the changeability of English weather, India presented a compellingly bold picture. Inundations were almost normal in parts of the country, while desert conditions prevailed elsewhere. More than 460 inches of rain fell in an average year on the village of Cherrapunji in the Assam hills, while parts of the Upper Sind garnered less than three inches. Impossible things seemed to happen frequently in India. In the wettest regions, it wasn’t uncommon for twenty-five inches of rain to fall in a single day—the same amount that normally fell on London in a year.23 During extremely hot weather, instruments recorded negative readings for humidity in some places. It was common for cyclones to hit the Indian coasts that were stronger than any which had ever been experienced in Europe.

This meteorological profusion took many and complex forms, but the most dominant was that of the monsoon, an alternating pattern of dry land winds that persisted for half the year and high humidity, cloud, and heavy rainfall that lasted for the other half. During the cold season, from October to April, the winds blew in dry and cold from the northeast. They reversed direction from May, bringing from over the oceans the wetter air that bore the heavy rains that fell from June to September or October.

The monsoon was a perfect example of the paradoxes of India. The very thing that caused so much suffering—the unpredictability of the monsoons—might turn out to be the key that could unlock the secrets of the weather more generally. The monsoon was as strong a signal as a meteorologist could ask for, writ as it was in the suffering or prosperity of millions, dutifully reported by those thousands of rain gauges and the busy barometers and thermometers whose readings were also faithfully recorded. And strong signals were the best chance that meteorology had of transforming itself into a more reassuringly predictive science. Norman Lockyer made it seem almost self-evident: “surely in meteorology, as in astronomy,” he urged his fellow scientists, “the thing to hunt down is a cycle.” Geography should be no obstacle, and indeed need not be, given the great reach of the British Empire. If a cycle is “not to be found in the temperate zone, then go to the frigid zones, or the torrid zones, and look for it,” urged Lockyer, “and if found, then above all things, and in whatever manner, lay hold of, study it, record it, and see what it means.”24

If the monsoon’s variable cycle was about as hard to miss as an elephant at close range, it was a more difficult matter to determine what caused the rains to come when they did. The starting point was the sun, the only thing more visible than the Indian monsoons. It had already offered up a well-characterized cycle of its own, during which its spots waxed and waned. These black spots, first noticed by Galileo, had been studied since by scientists seeking to understand what effect they might have on the earth. In the eighteenth century, astronomer William Herschel compared the historical index of grain prices in Adam Smith’s Wealth of Nations with sunspot data, looking for correlations. In the 1830s, the Magnetic Crusade to map the magnetic currents of the earth had sent observers with magnetic instruments to the four corners of the globe. They’d hit the jackpot when they discovered that the earth’s magnetic field fluctuated in time with the sun’s. Interest in sunspots took off further in 1850, when Heinrich Schwabe published nearly twenty-five years’ worth of daily records he’d made of sunspots —the best data set so far—which he used to identify a ten-year cycle of waxing and waning spots. That figure was soon revised to eleven years, and the sunspot cycle seemed even more likely to have definitive impacts on Earth. More grist was added to the mill in 1859, when a very strong solar flare had caused magnetic instruments to go haywire, sent telegraph communications offline (and even set some telegraph stations on fire), and generated visible aurora even by the equator. Thanks to the sense of urgency occasioned by such events, funds were made available to build a series of special observatories that were meant specifically to observe the sun and to collect and analyze data about possibly related phenomena on earth (on Piazzi Smyth’s expedition to Tenerife, he carried many requests from leading scientists to make solar observations). Caught up in the sense that certain natural mysteries were on the cusp of being revealed, physicists sought, and to a certain extent found, links between sunspots and magnetism, sunspots and temperature, sunspots and wind, and sunspots and rainfall. That these relationships could often be summed up in almost disarmingly simple terms was part of their allure. The links seemed obvious. Charles Meldrum, the government astronomer at the official observatory in Mauritius, summarized his own findings thus: “many sunspots, many hurricanes; few sunspots, few hurricanes.”25

But despite the energy that went into solar physics, by the early twentieth century no further direct physical connections had been discovered between the earth and the sun to rival the findings of the Magnetic Crusade. Interest gradually waned. Sunspottery, as detractors called it, came to look dangerously like a dark art, its practitioners finding patterns where none rightly existed, in a morass of confusing detail.

Among scientists, a small group remained faithful to the search for links between the sun and the earth. These cosmic physicists were less interested in cracking a secret code of nature than they were with understanding the fundamental physical connections between phenomena. Unlike most physicists, who concerned themselves with the behavior of electricity, magnetism, and heat at very small scales, these physicists probed nature at the very largest of scales, that of the solar system and beyond, on the assumption that “a force not less universal than gravity itself, but with whose mode of action we are as yet unacquainted, pervades the universe, and forms, it might be said, an intangible bond of sympathy between its parts.”26 It was undeniable that the sun affected some aspects of earthly phenomena. The hunt was on to figure out what precisely was the nature of the force “not less universal than gravity” which was responsible for such effects. They were convinced that physical connections between the earth and the sun (among other celestial objects) were profoundly important to the unfolding of meteorological, magnetic, and electrical phenomena on earth. Though the distances they worked with were great, they thought in surprisingly sensuous terms. Like lovers, the sun and the earth were exquisitely attuned to each other. “Mutual relations of a mathematical nature we were aware of before,” wrote two leading cosmic physicists, “but the connexion seems to be much more intimate than this—they feel, they throb together, they are pervaded by a principle of delicacy even as we are ourselves.”27 Tiny, ramifying perturbations could arise anywhere in the solar system, not just on the sun itself. Like the trigger of a gun, small changes in the gravitational fields of other planets in the solar system could cause sunspots that could themselves have huge effects on earthly weather. The sun was therefore able to produce incredible variation in earthly weather “by falling at different times on different points of the aerial and aqueous envelopes of our planet, thereby producing ocean and air currents, while, by acting upon the various forms of water which exist in those envelopes, it is the fruitful parent of rain, and cloud, and mist.”28 Such a passionate belief in these connections gave cosmic physicists patience and hope in spite of the lack of results to date. The seemingly impenetrable variability of the weather was a measure “not of its freedom from law,” wrote Lockyer and Hunter, “but of our ignorance.”29 All natural things, including that most fickle of phenomena, rainfall, would eventually be shown to obey the laws of nature. Only more time was needed.

* * *

And data, lots and lots of data. Of that, at least, Walker had as much as, if not more than, he could have hoped for. Walker was not a cosmic physicist, by training or inclination, nor was he (anymore) susceptible to the fever for cycle-hunting. He was, instead, a man for whom numbers were tools that could be put to particular uses. The disciplining of numbers was essential. Just as Walker had put himself to the ultimate test of disciplined study as a student at Cambridge, so he submitted his numbers to the test of reliability and meaningfulness.

He’d inherited the concerns of those who’d come before him, and he would be conscientious in addressing those concerns. It would be wrong to say he wasn’t guided by the past. In many ways, the questions he asked of his numbers were questions others had already raised, about the relationship between distant phenomena, about the way in which things that are very far apart can, indeed, be linked. This characteristically imperial notion was both enabled and promoted by all the structures—the railways and telegraphs, and bureaucratic structures—which made the empire possible. Walker was, like anyone, influenced by the world around him, by what had come before, and by what he was hired to do in the moment.

First, he needed to gather the numbers themselves. That in itself was not difficult. He was in charge of the most advanced meteorological network in the world. Numbers—corresponding to facts about the weather—streamed into his office day after day, month after month. No data monsoon or failure thereof afflicted the Director-General of Observatories. In 1907, for example, his office in Simla received the records of rainfall from 2,677 rain gauges across India. He also received readings from several dozen meteorological observatories of pressure, temperature, and wind speed taken at eight-hourly intervals and, in some locations, recorded continuously by automatic instruments. He knew he needed data from the oceans if he was to crack the monsoons, and so he sent two full-time clerks to Calcutta and Bombay whose sole job was to visit ships as they came into harbor for the purpose of copying their meteorological logbooks and calibrating their barometers. The atmosphere—that ocean of air—wasn’t as easy to access, but it was critical to create a three-dimensional map, if possible, of the air currents that brought rain or dryness. By 1904, when Walker took up his job, it was generally agreed that more information on the middle and upper atmosphere was urgently needed, and should be acquired by any means—kites, balloons—necessary.30 Walker sent balloons and kites up from Belgium, and over the Bay of Bengal and the Arabian Sea. They flew as high as 2.5 miles into the atmosphere. From Simla, he sent up gutta-percha observation balloons that carried ultra-light instruments. These had to be recovered for their data to be useful, and he attached cards to the balloons, promising a reward for their safe return. The previous Director-General of Observatories, Henry Blanford, had pointed to snowfall in the Himalayas as an important factor in the monsoons, so Walker arranged for large-scale photographs to be taken of the snowfall visible from Simla, which could be compared year on year.31

And he corresponded. He set up telegraphic and postal correspondences with fellow observers around the world. The office in Simla, with its characteristic telegraph wire leading out of its windows, received weekly telegraphs keeping Walker informed of weather conditions at the Royal Alfred Observatory in Mauritius, a key location from which monsoon winds blew. Departmental observatories in Zanzibar and the Seychelles provided much-needed data on the Indian Ocean. For the southwest monsoon, he corresponded with Zomba, Entebbe, Dar es Salaam, Cairo, and Durban in Africa; with Perth, Adelaide, and Sydney in Australia; and with Buenos Aires and Santiago in South America.

All this data was promising, and seemed necessary. It was also potentially fatal to the dream of solving the monsoon mystery. Too much data could easily prove incapacitating. This was the dilemma. To understand the phenomenon, it needed to be observed. But it wasn’t clear precisely what the boundaries of the monsoon were. Where it started and where it ended was part of the answer Walker was seeking. So he needed, as had others before him, to cast his net wide. But the wider he cast, the more numbers he caught, the harder it would be to find the elusive signal amid all that noise.

“There are undoubtedly too many observations,” noted John Eliot, “and too little serious discussion of observations.” Instead of accumulating observations without consideration of how they might be used, the time had come for investigation of causes to “direct and suggest the task of observation.” A natural feature of a more thoughtful observing regime would be to consider allied sciences alongside that of meteorology—“there are undoubtedly definite relations between certain classes of solar phenomena and phenomena of terrestrial magnetism” and who knew what other links might be uncovered.32 What Eliot suggested was the creation of a central organization where observations taken throughout the British Empire could be compared.

As Arnold Schuster, a prominent cosmic physicist, put it, “observations are essential, but though you may never be able to observe enough, I think you can observe too much . . . It would not be a great exaggeration to say that meteorology has advanced in spite of the observations and not because of them.” There was always the danger that data collection would become an end in itself and science would become nothing more than “a museum for the storage of disconnected facts and the amusement of the collecting enthusiast.”33

Before the matter of the proper place of observation in meteorology could be settled, the nature of meteorology itself needed to be resolved. What meteorology was, exactly, was up for grabs. Should it consist of prediction? Of observation? Of theorizing? Or, as seems reasonable, a mix of the three? But if a mix of the three, in what sort of hierarchy should these different approaches to the atmosphere be ordered? The question of whether prediction could precede theorizing was a potentially explosive one, as the cancellation of weather forecasts in response to FitzRoy’s death makes clear. Some felt strongly that prediction without proper theory was a dangerous undertaking—for both the public who might be provided with faulty forecasts and, just as concerning, for the scientists who were wary of exposing what they saw as the weakness (or immaturity) of their discipline. American meteorologist Cleveland Abbe spoke for this point of view when he wrote in 1890 that “hitherto, the professional meteorologist has too frequently been only an observer, a statistician, an empiricist—rather than a mechanician, mathematician, and physicist.”34 Others felt just as strongly that theories based on too few observations were as useless as observations unleavened (as one commentator put it) by theory. As noticeable as the differences between what might be called theory- versus data-led meteorology might have seemed, there was not as much separation between these attitudes as might be thought. Indeed, depending on the problem at hand, the same person might advocate first a data-driven, and then a theoretical approach. Julius Hann, for example, did more than anyone to establish the descriptive, empirical tradition of climatology, but he also applied thermodynamics—a highly theoretical field—to problems of atmospheric phenomena.35

Just as Hann saw climatology as a helpmeet for meteorology, so others argued that physics was a needed stiffener that could transform the science of the atmosphere into a true science. The scales to which these disciplines directed their energies were largely as distinct as their methods. While climatologists set out to encompass the globe with imperial maps keyed to resource production and extraction, meteorologists focused instead on devising physical theories that could be regional, local, or even, in the case of clouds, hyperlocal in nature.

As these differences between meteorology, climatology, and an incipient physical geology such as informed the ice age debates indicate, the concept of change was itself unstable from the middle decades of the nineteenth century through to its close. Which kind of changes could be looked for, by whom, and using which tools had become very public and very controversial questions by the end of the nineteenth century. What it meant to be a science—how much it could be a function of data gathering and how much it required theory—was a primary question from which everything else, including what even counted as data, emerged.

These disciplinary anxieties formed the backdrop to Walker’s conundrum. How was he to escape the confines of Schuster’s meteorological “museum,” full of musty and disconnected facts? What Walker realized, thanks in part to the work of those who had come before him, was that solving the mystery of the monsoons would require two things. First, he would need to shift the scale of inquiry from local, regional, or even pan-regional studies to truly global surveys of world weather. Second, and just as importantly, Walker realized that he needed to ditch cycle-hunting in favor of something with a qualitatively different mathematical basis. He saw himself not as a hunter in search of one charismatic meteorological megafauna—the singular “link” between one cycle and another—but as a surveyor charting the landscape of the weather itself.

Here is where Walker’s ignorance of the weather may have been his greatest asset. Without any pre-existing assumptions about which aspects of the atmosphere might have the most bearing on the monsoons to guide him, he realized he needed a tool with which to evaluate all factors and to help him determine which, if any, were the most important. The tool he had was statistics. Specifically, he developed a technique for calculating what he called the reliability of the correlation coefficient between two factors. What this meant was that he had a device by which to sift the vast mountain of data. Before Walker’s innovation, the best tool cycle-hunters had was visual. They plotted charts comparing one signal against another (such as barometric pressure against sunspot appearances) and looked at the resulting curves to see if any pattern emerged—either an especially close fit or an especially poor fit, evidence perhaps of an inverse relation. Walker realized he could sharpen a device developed by statistician Karl Pearson, called a correlation coefficient, to sift through numbers statistically. Pearson’s correlation coefficient was a tool for identifying patterns—degrees of correlation—that linked two sets of data. This was a very helpful tool for sorting the vast reams of statistics which came flooding into Walker’s office.

A problem arose when it came to looking for real patterns in weather data: Pearson’s correlation coefficient tool was sometimes too good at finding patterns. When comparing two sets of random data, there is always a certain likelihood that you will find a relationship between them. The same is true when comparing real data, such as, for example, barometric pressure in different parts of the world. Pearson’s tool was unable to distinguish between the real correlations—that is, those that indicated underlying physical connections—and those that arise purely as a function of the quantity of data being compared. When comparing dozens, or even hundreds of data sets, as Walker was, the chance of false positives is large. Walker’s tool provided a measure of just how much correlation was required to balance out the likelihood of false positives in large data sets.

By applying his criterion of reliability to Pearson’s correlation coefficient, Walker was able to generate a quantitative measure of the likelihood that a correlation between two series of numbers was not due to chance. Instead of eyeballing a series of curves, Walker could rank relationships numerically and identify which were statistically robust and therefore likely to reflect something happening in the real world and those that were less strong and therefore more likely to be merely random. His technique was both more accurate and vastly more efficient at sorting through the huge data sets with which he was confronted than that of his predecessors. It was, as Napier Shaw, another leading researcher, recognized, a “kind of searchlight for sweeping the meteorological horizon from some selected point. The principal features of the otherwise invisible landscape, which in this case extends over the whole globe, can thus be located.”36

The landscape of Walker’s investigation was global. This was, again, as much a function of Walker’s ignorance as it was a calculated decision. Without a clear sense of where to shine his spotlight, he needed to shine it everywhere. Any correlation he found was, as Napier Shaw put it, a “very sensitive plant, it is much easier to kill one than to make one; whatever happens in the way of accidental errors, it must suffer.”37 That was the idea. If Walker were to find real relationships in the vast sea of data, he had to be merciless with any putative links. Only the strongest, most statistically resilient could be allowed to survive. These could lead the way for scientists who had a physical theory of the circulation of air, wind, and rain to explain what Walker had merely indicated.

And what he had helped reveal was this: There was something called world weather. It consisted in large regions of alternating high and low pressure that spanned the globe and changed with the seasons. There had been theories before of what had been called the general circulation of the atmosphere, dating back to Hadley’s theory of the trade winds in the eighteenth century. More recently, a spate of work done during the 1880s and 1890s, drawing on the same sorts of telegraphic correspondence networks that Walker did, had begun to pick out a series of such oscillatory, or seesaw, relationships between areas of characteristically high or low pressure. These papers, many of them by cosmic physicists, blended the tools and approaches of the cycle-hunters with those of the physicist accustomed to thinking about physical connections between matter. They generated maps, often of pressure but also of temperature, which demonstrated intriguing, even astonishing, connections between distant parts of the earth’s atmosphere. The term oscillation was used early on to describe the inverse relationship between pressure in different parts of the globe that many of these studies found. Léon Teisserenc de Bort, an architect of universal cloud studies, had shown that there was a relationship between the average pressure in Europe and that in certain “centres d’action” in Iceland, the Azores, and Siberia. Henry Blanford had done similar work for the Southern Hemisphere, showing that pressure in India, Siberia, and Mauritius was linked. H. H. Hildebrandsson, a round-faced Swede, had gone much further with his monumental series of five memoirs presenting a ten-year run of average monthly pressure data from no fewer than sixty-eight locations from all quarters of the globe. He used this data to push even further from these still hemispheric centers of action to suggest that there were what he called “intimate relations” between all of the centers of action on the globe.38 And finally, Hildebrandsson and de Bort’s Cloud Atlas of 1896 had shown that it was possible to leave behind what Julius von Hann called “church steeple politics” in meteorology (what could be seen from a church tower) and move toward ambitious, global projects.39 Clouds, to state the obvious, obeyed borders not at all, so any project to map them had to be similarly wide-ranging.

* * *

Here, then, was the landscape upon which Walker could shine his searchlight. From a tradition that stretched centuries into the past, which had built up interest in and knowledge of storms, to more recent attempts to collect and compare data at hemispheric scales, Walker had managed to arrive at precisely the right moment to submit a truly global data set to scrutiny. Like Blanford, Teisserenc de Bort, and Hildebrandsson, Walker found evidence of oscillations in the pressure data he’d collected. But while they had been limited by their visual techniques to making vague statements about the nature and degree of these connections, Walker’s correlation coefficients allowed him to eliminate those connections that were less meaningful. He found 400 significant relationships—correlation coefficients worth paying attention to.40 Subtracting the spurious connections left him with “three big swayings,” or inverted relationships between pressure. The biggest was between the Pacific and Indian Oceans. This Walker named the Southern Oscillation. Two smaller swayings, between Iceland and the Azores and between parts of the North Pacific, he named the North Atlantic Oscillation and North Pacific Oscillation.41 In these locations, pressure existed in inverse relations. When the barometric pressure rose in Iceland, it seemed to fall in the Azores, and vice versa.

One of the first questions to which he put his correlation coefficients was that of sunspots. In a 1923 paper, he demonstrated that there were no meaningful correlations between the eleven-year sunspot cycle and that of the monsoons.42 He seemed to recognize the discomfort, and even disappointment, he may have caused. It was natural, he acknowledged, “after long ages of belief in the control of our affairs by the heavenly bodies,” to believe in natural cycles. But the urgent need for good monsoon forecasts, and the terrible suffering the famines had caused him, had nevertheless driven him to “replace instinct by valid quantitative criteria.”43 Eliot’s gamble in hiring Walker had paid off. Sort of. For even as Walker brought the edges of meteorology and empire to their ultimate endpoint—the entire earth—his achievement also represented a retrenchment and scaling back of ambitions. In gaining world weather, he had sacrificed the cosmos. Taking away the hope that secret cycles might unlock the monsoon was just about acceptable if, in return, Walker could offer something better.

That something better was, of course, his original goal of predicting the monsoons. The monsoon forecasts, which had begun in the 1880s and had been suspended in 1902 following the disastrous famines, had been reinstated on the basis of Walker’s findings. Walker’s predecessor, Eliot, had emphasized how dangerous “the striving after perfection in short-period forecasts was.”44 There was too much imperfect information and experience with failure to treat forecasts as anything other than probabilities. But Eliot’s cautious words were hard to hear against the background of famine and economic imperatives, and the government pressured Walker to publish forecasts once again. Walker was the first to be cautious, and even critical, of the forecasts, which he emphasized were only as good as the correlation coefficients he was able to find. These varied from year to year, sometimes dramatically. He urged that forecasts be issued only with strong provisos. Foreshadows, rather than forecasts, would, he thought, be a more appropriate, modest name for them.45 But the stronger term had stuck, and the tendency, or desire, for these pronouncements to be powerfully predictive was as great as it had ever been. There were some successes in prediction, but it seemed there were just as many failures, and it was embarrassing, after so much time and expense, and in the face of such evident need, that professional meteorology was often unable to offer better prognostications. The fear of making inaccurate predictions could lead to an absurd state of affairs where experts were less adept at forecasting than simple folk. It was unfortunate, commented one writer, Charles Daubeny, when “the untutored peasant sometimes would seem to possess an intuitive insight, whilst the philosopher, although he may plume himself on his acquaintance with the general laws of atmospheric phenomena, is often at a loss to unravel the entangled skein of effects connected with it which daily observation brings before him.” Meteorologists were damned if they did and damned if they didn’t. A bad prediction could tarnish their name, while too much reserve was also unacceptable. Daubeny continued that while charlatans had no compunction about making predictions, “a Herschel or an Arago declare themselves incompetent to anticipate what may chance to supervene within the space of the next four-and-twenty hours.”46

The ironic fact of the matter was that it was easier to use the monsoon to predict what would happen elsewhere in the world than it was to predict the rains themselves.47 Why that was the case, Walker the mathematician was unable to say. Luckily, though the monsoon continued to flummox and tantalize hundreds of millions of Indian farmers and those who relied upon their grain, no more famines occurred on the scale of the terrible death and suffering that had preceded Walker’s arrival. Changes in economic and social policy on the part of the British, and a run of good monsoon years, were to thank for that.

If Walker had failed in his primary objective of predicting the monsoon through statistical means, he had also failed to provide any physical explanation for the discovery he’d made. It was, in a way, like throwing a boomerang without understanding the physics beneath it. In that case, his lack of knowledge hadn’t kept him from excelling, but he’d nevertheless been driven to try to describe precisely how the device worked. Though he’d embraced his task in India using the most effective means that were available to him, he never forgot what he’d lost in the bargain. In a 1918 lecture to the Fifth Indian Science Congress, he emphasized how important it was to have a grasp of the fundamental principles driving phenomena under study. “What is wanted in life,” he urged the students, “is ability to apply principles to the actual causes that arise . . . When Pasteur as a chemist was asked to find a remedy for the pest that was ruining the French silk industry, he knew absolutely nothing of silkworms; yet he solved the problem, and it was general understanding of Nature’s methods that brought him success.”48 Walker knew better than anyone that that general physical understanding was precisely what was missing from the world weather he had discovered.

Just as Walker had failed to find a way to predict the monsoon, the larger project of melding meteorology with astronomy that went by the term cosmical physics had, by the end of World War I, largely faded from view. In its place was a new branch of meteorology. Instead of trying to link the heavens and the earth, as cosmic physicists had, this new type of meteorologist tried to link the lower atmosphere, to which most meteorological measurements had long been confined, with the upper, which was becoming gradually more accessible. Charles Piazzi Smyth’s expedition to Tenerife was an early example of the push to establish mountaintop observatories that, in addition to offering better views of the stars, made it possible to take readings of the upper air. Mountains had obvious drawbacks when it came to tracking the movements of a free-flowing atmosphere. After a series of spectacular and dangerous balloon ascents into the upper atmosphere, notably by English meteorologist James Glaisher, researchers sought safer ways of taking readings of the air high overhead. One way was to observe the motions of clouds, as the organizers of the International Cloud Atlas understood. But these observations could only reveal so much about the atmosphere. More precise data would require sending instruments themselves into the skies. Kites and unmanned balloons soon became the prime instruments for plumbing the ocean of air. In the late 1890s, Teisserenc de Bort, who had retired from his post as director of the Central Meteorological Office of France, established a meteorological field station at Trappes, southwest of Paris. There, he pioneered techniques for launching the large and delicate balloons needed to reach the upper atmosphere, using a large hangar set on a rotating platform, which could protect the balloon from ground winds until it was safely launched. Using this apparatus, and a self-registering device to record temperature, pressure, and moisture, Teisserenc de Bort carried out dozens of soundings in the years around 1900. The traces recovered from the self-registering device—which scratched its readings into lampblack that was impervious to the damp conditions—revealed a new aspect to the atmosphere. The temperature of the atmosphere fell in a uniform manner until the balloon reached some eight kilometers high, at which point it stopped decreasing. In 1902, Teisserenc de Bort named this region of the upper atmosphere the stratosphere, and coined a new phrase for the layer closest to the earth, the troposphere.49

Walker himself was well aware of the need to understand the upper atmosphere better. “I think the relationships of world weather are so complex that our only chance of explaining them is to accumulate the facts empirically,” he wrote at the end of his life, “and there is a strong presumption that when we have data of the pressure and temperature at 10 and 20 km, we shall find a number of new relations that are of vital importance.”50 During his tenure as Director-General, he established an upper-air observatory in the northern plains of India, at Agra. Starting in 1914, a ten-year experimental program was carried out. Among other things, the balloons sent up by Walker and his men showed that the stratosphere—the zone of constant temperature—started much higher in the atmosphere above India than it did in Europe.51

Walker left India in 1924 after twenty years of service. His achievements (including helping hire increasing numbers of Indians into the Met Office) were lauded, he was awarded a knighthood, and he took up a position as professor of meteorology at Imperial College. He soon joined the Imperial College Gliding Club. And though he complained that his reflexes were not sharp enough for successful gliding, he accompanied the younger gliders on several expeditions in the South Downs. He sometimes took his boomerang with him and sent the device flying far above him in the gentle air of southern England before it began its perfect return, vibrating cleanly through invisible turbulence before coming to rest in his long, elegant fingers.

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.