The Search for Understanding - Windswept: The Story of Wind and Weather - Marq de Villiers

Windswept: The Story of Wind and Weather - Marq de Villiers (2006)

Chapter 3. The Search for Understanding

Ivan 's story: All the thunder cells caused by the great furnace of the Saharan summer are tracked by the one-man weather office in Timbuktu and by the slightly more sophisticated operation in Niamey, Niger's capital. Timbuktu's sole meteorologist, Bandiougou Diallo, observes the weather the old-fashioned wayby going onto the roof in a thunderstorm, eyeballing its extent, and hoisting aloft a handheld wind velocity meter. He is there mostly to warn the pilots of Air Mali's venerable Fokker aircraft on their thrice-weekly runs to the city from Bamako, Mali's capital, if it is safe to proceed or more prudent to turn back. But his handwritten notes, forwarded later to his bosses in Bamako, are assembled into broader databanks and used by others to track storm patterns, and thus become part of the global struggle to understand weather systems. Not all the storms he watches cause him or his distant correspondents much concern. Some dissipate locally. Others lose their energy after a day or so. Others persist. A few coalesce into violent weather systems big enough to alert American meteorologists who are monitoring satellite images across the Atlantic.

All thunder cells, though, travel, for that is the nature of the air that has become wind that has transformed itself into a storm.

If you were on the high dunes north of Timbuktu, the wind would have come from the northwest but you could have watched the storm approaching from the east. Saharan storms are easy to see: Sulfurous clouds of yellow sand swirl up into the angry blacks of the clouds. Probably no one was there to see, though. The nomads know better than to be caught on a high dune in a windstorm; they would have taken shelter, such as it was, in the lee of a smaller dune or in a wadi (though mindful of flash floods).

The cell that began at Emi Koussi passed by Timbuktu in the afternoon of August 27. It was one of those whose coherence persisted, and it drifted slowly south of west, passing unrecorded over the ancient capital of the Malian empire, the ruins now known as Koumbi Saleh, and was picked up again by weathermen on the 2Qth, somewhere between the Mauritanian capital, the arid desert town of Nouakchott, and the sprawling and violent slums of Dakar, in Senegal. Airports in both cities kept a wary eye on its passage.

By the end of the day, it had reached the coast. Ahead in its path: the Cape Verde Islands.

This was a complicated place for any weather system to find itself. Behind is the immense furnace of the desert. To the north, aridity. To the south, the sodden hills of Senegal, and beyond them the rain forests beginning at the Gambia River. To the west, the still cold but rapidly warming southbound Canary current, heading for the tropics. The prevailing windsnortheasterlies. Sometimes these factors simply kill thunder cells. At other times, whose conditions look apparently similar, they are energized as they are caught up in the southbound winds.

More than a hundred such systems drift out of the Sahara into the ocean each year.

When they reach tropical waters, they either stall, or they don't. Again, if the conditions are rightthe water temperature just right, the upper atmosphere still so no shear is found at the high altitudes to cut off their topsthe low-pressure systems they have become begin slowly to spin as the Coriolis force takes hold.

The Emi Koussi cell was one of these. In the ocean south of the Cape Verde Islands the Canary current had already reached 28.8° Celsius. The high-altitude winds were steady at low velocities. It was a formidable combination. The effect was like taking the lid off a pot of steaming watermoist air began to ascend rapidly, moisture and energy were dumped at high altitudes, surface winds rushed into the vacuum thus created and in turn forced the winds into a tighter circular motion.

By this time the National Hurricane Center in Miami had taken notice.

The cell was now formally a tropical depression, and as such it was assigned a number. It was the ninth of the season. Tropical Depression Nine, with sustained winds still under the tropical storm threshold of 39 miles an hour, was located 555 miles southwest of the Cape Verde Islands. It was September 2.


I was once pursued by a windstorm across the arid Great Karoo of South Africa. I'd been spending a few days with a friend at his aunt's house in the little town of De Aar—I mostly recall being stuffed by that hospitable woman with all the traditional delicacies of the Afrikaner heartland, grilled baby lamb, roast springbok, pan-nekoek and moskonfyt, babootie, and the rest—and we had headed back to Cape Town on Willie's knockabout motorcycle. The first we knew about the storm, the first inkling that something was amiss, was when our own dust overtook us; a following wind had gotten up, and it was strengthening fast. I looked over my shoulder and the horizon was already a luridly dirty purple lit up with sheets and jagged spears of lightning. For a moment I thought I saw a yellow-tinged funnel shape reaching toward the ground, and if it were true, that would mean really bad news. "Get on with it, Willie!" I said, and he opened the throttle and we made a run for it.

The machine juddered over the ruts. I could feel the storm behind me, chasing … This was stupid, I knew even then. It was just a storm, the basso rumbling that followed us just thunder, the flashing just lightning, the hail just ice, the wind just wind, and the massive clouds of debris—thorn bushes, small birds, dust and grit, tumble-weeds— was aimed at nothing at all. I knew that. I was no longer a child, to be battered by every passing gale. But phobias seldom pay court to rationality, and trump volition every time.

Wliy was the damn thing following me?

For three hours the storm nipped at our heels. The bike wasn't very good and the gravel road was very bad; we were making no better than 40 miles an hour, and the storm was keeping pace, traveling at furious speed across the veld. Even Willie, a structural engineer by training and temperament, felt its malice, or so I fancied. Finally, after hours of bone-jarring flight, the arrow-straight road veered left, and when we could see we were no longer in the direct path, we stopped the bike and watched the monster pass.

That the storm was moving was evident. We could see it coming after us, we could see it pass by, we could see it vanish over the horizon. So why, I wondered in later years, did no one figure out, in the centuries past, that weather traveled? The Tuareg in the desert could watch storms pass, mariners at sea could surely see how they moved. And yet, in two thousand years of musings about winds, by some of the brightest intellects in history, the notion that storms were self-contained "systems" that moved from one place to another was never mooted. It wasn't until the nineteenth century, when data collection was fully developed, that it was finally understood.

Why wasn't it evident to the ancients?

I suppose the answer is to be found in context. The natural philosophers of the past just didn't—yet—have the conceptual framework to understand how wind worked. This is the familiar story of the scientific method, of course. It is the way scientific understanding advances. First come the framing theories, almost always wrong, then the painstaking accumulation of data, then corrected theories, then more data, and only afterward the brilliant insight. After which, the pattern repeats itself …

The history of thinking about wind paralleled the thinking about air itself. It was similar, but not the same.

The first of the framing theories, the first quasi-scientific definition of wind, was that of Anaximander, the same Anaximander whose considered view of air (that every thing, earth and the heavens above, came into existence when the primordial sea was dried by celestial fires, source of said fires unspecified) was recounted in the previous chapter. All nature, in this view, was the product of a few simple properties—hotness and coldness, wetness and dryness, lightness and darkness. So far, so familiar. But then he made a big conceptual breakthrough: Wind, he suggested, was a current formed when mists were burned off by the sun. Later, he simplified the notion: Wind, he said, was "a flowing of air."1 Two hundred or so years later Empedocles, the inventor of the four-elements theory of matter, conducted the first experiment to demonstrate what wind really was. He used a simple flow tank to show that air and wind were really the same thing.2

Aristotle, writing three hundred years later, agreed with much of what Anaximander had to say about air—indeed, he codified and improved the earlier man's musings—but on wind he couldn't go along. He rejected the whole idea, just as he had the notion of atoms. He thought it self-evidently false, and that the people who adhered to the theory were bringing the whole idea of philosophical thought once more into disrepute—obviously, scholarly dignity was a sensitive subject in the Athens academies. If winds were indeed a "flowing of air," he wrote, this must mean that all winds were one wind because all air was one air. To him, this was self-evidently false. "This is just like saying all rivers are one and the same river, and an ordinary man's view is better than a [mediocre] philosopher's view like this one," he declared, with rather lumbering irony.3 Still, he wasn't altogether off the mark when he wrote that the sun pushes the winds and checks their speeds; in a way, that is indeed what happens. You can say with some truth that wind is, as Aristotle wrote, an exhalation arising from the earth.

While this philosophical wrangling was going on in the Athenian schools, practical Greeks were making the first attempts to blend legend and fact. In this they were the global forerunners, as they were in much else besides. Homer and his contemporaries had only identified the four cardinal winds, but this was too crude a measure for decent navigation, and the increasingly skilled sailors of later generations began to parse the directions ever more finely into increasingly useful and accurate segments, generally using the sun's movement as a guide, since magnetic north was still unknown. This new precision was graphically depicted in the Tower of the Winds, an eight-sided building erected in the market place of Athens, now the Plaka district, by Andronicus of Cyrrhus, somewhere between ioo and 50 B.C. The tower still stands, in decent though not pristine shape, and the eight winged gods representing the eight important winds can still be seen as a marble frieze, the figures in relief. The tower is only about forty feet tall. Sundials protrude from all sides (it was also called the Horologium, or timepiece) and a water-powered clock was built into the south wall. This clock was said to have included a diagram of the night sky, but when the tower was turned into a Christian baptistry and later a place of worship for the dervishes, the mechanism disappeared.4

The eight winds, shown flying clockwise around the building, are Boreas, the north, the personification of a strong wind; Skiron, the northwest; Zephyros, the west, carrying a heap of flowers to denote his newfound benignity; Lips, the southwest; Notos, the south, representing the wet winds of winter; Euros, the southeast, shown as a winter gale; Apeliotes, the east, carrying a basket of fruit; and Kaikias, the northeast, a strong wind; he is stocky, bearded, and looks none too friendly.

The tower was very precisely oriented. A thousand years before the compass, Andronicus plotted the directions exactly. Modern engineers wouldn't move the tower a third of a degree.

At about this time, the Romans began to weigh into the debate. Pliny the Elder, the Roman naturalist who was active a few decades after the start of the Christian era, got the origin of winds more or less right when he wrote in book 1 of his Natural History of the World that "the sun's rays scorch and strike everywhere on earth in the middle of the universe and, broken, bounce back and take with them all they have drunk. Steam falls from on high and again returns on high, empty winds violently swoop down and go back with their plunder … the earth pours breath back to the sky as if it were a vacuum." He wasn't very consistent (he asserted in book 2 that winds can just as easily "issue from certain caves in Dalmatia") but then he added, somewhat redeeming his naturalist's credentials, "neither is it impossible, but that they do arise out of waters, breathing and sending out an air, which neither can thicken into a mist, nor gather into clouds: also they may be driven by the lugitation and impulsion of the Sun, because the wind is conceived to be nought else but the fluctuation and waving of the air."

The fluctuation and waving of the air! Precisely. A modern meteorologist could hardly have put it more succinctly.

More winds exist than the four Homer described, Pliny wrote, but "the Age ensuing, added eight more; and they were on the other side in their conceit too subtle and concise. The modern sailors of late days found out a mean between both: and they put unto that short number of the first, four winds and no more, which they took out of the later. Therefore every quarter of the heaven hath [just] two winds apiece," which pretty much restated what the Horologium had been saying for a hundred years already. This assertion came in the middle of a rant against the sad materialism of modern times, in which the naturalist lamented that "men's manners are waxen old and decay; now, all good customs are in the wane: and notwithstanding that the fruit of learning be as great as ever it was, and the recompence as liberal, yet men are become idle in this behalf. The seas are open to all, an infinite multitude of Sailors have discovered all coasts whatsoever, they sail through and arrive familiarly at every shore: all for gain and lucre, but none for knowledge and cunning."5

That's pretty much where it rested for the next thousand years or so. Greek and Roman sailors plied the Mediterranean, Arab sailors the Indian Ocean, Chinese mariners the Yellow Sea, and the Phoenicians ventured out into the Atlantic and Indian oceans. Sailors are practical men, and they learned how winds can become a network of conduits taking them across the seas and back again; they understood that a typical journey might come to use a number of different winds to proceed in different directions—if one chooses carefully, one can always come home. As Sebastian Smith put it in Southern Winds, this was wind hopping, similar to changing buses several times to cross a city.6

Not just directions but also seasons were plotted, and dangerous winds were known to arrive at certain times of the year. Prudent rulers prohibited travel during those seasons to minimize their losses. In early Christian times the Coptic calendar listed precise dates for the beginning of bad winds. March 20 you could expect a two-day easterly gale, April 29 another. The khamsin was supposed to blow from the day after Easter to Pentecost. Ocean travel in the east Mediterranean was discouraged from St. Dmitri's Day (October 26) to St. George's Day (May 5).7

Over the centuries, the wind rose was developed, first appearing on Portuguese and Spanish charts by the thirteenth century. Pliny notwithstanding, early wind roses denoted the direction of thirty-two winds, eight major winds, eight half winds, and sixteen quarter winds. Diagrammed into a circle, these thirty-two points resembled the European wild rose, with its thirty-two petals, hence the name. In after years, the symbol of the rose itself became a beacon for the lost, and when the compass was invented, the wind rose mutated into the equally ornate compass rose, which, often embellished with puffing wind gods, was still shown on maps in the nineteenth century and is still occasionally added by cartographers to give a satisfactorily antique tone to their products.

A typical wind rose. Sometimes these have fanciful illustrations of puffing wind gods, often not. Sometimes the four principal Greek winds are named; in most cases they are taken for granted.


For the next few hundred years the two great branches of human endeavor—the practical, or artisanal, what we could today call the engineer; and the scholarly, or philosophical—went their separate ways, each developing its own brand of expertise. Practical men didn't bother with theory, the philosophers with experiment or observation. And so the theories of weather and wind that have survived tend to be the fruit either of pure reason innocent of observation or of visions; entirely missing are the thoughts of, say, a miller at his windmill, or a sea captain running before a gale, or a farmer who watched winds destroy or nurture his crops, or a roofer whose rafters collapsed in a storm. Typical of what did survive were the notions of eminent medieval scholar Hildegard of Bingen, a Benedictine sister at a nunnery in the Hunsruche near the Rhine. Sometime around 1145 she had a vision that the winds held all the elements together, each wind a wing of God working to keep the firmament in the right place, and causing it to rotate around the earth from east to west. Onto this lovely notion she then cobbled an impressively abstract view of how nature worked: The air was made of four layers, each governed by one of the cardinal winds. The east wind was closest to the ground; above it was the west wind, then the north, and finally the south. Within these layers everything else is to be found—the sun, the moon, all the constellations, storm clouds, and thunder. Hildegard was later beatified, but not for her work on the weather.

Throughout the medieval period, earlier theories persisted. Ger-vase of Tilbury wrote in Liber de mirabilibus mundi that "mountains and water cause winds." William of Conches maintained that four great ocean currents created the four cardinal winds. Sometime in the twelfth century Adelard of Bath produced his Quaestiones Naturalies, a compilation of seventy-six discussions of nature, including the weather. He ignored the Greeks, relying instead on imported Arab science, then the most mathematically inclined culture on earth. "Wind," Adelard asserted, "is merely a species of air."

Even by the time Columbus sailed the ocean blue the split between philosophy and science was still evident. The effects of winds, and their general direction, were known. The existence of the trade winds was known. Sails and sailing vessels were becoming ever more sophisticated devices for actually employing the winds. But the science of meteorology had scarcely improved, and natural philosophers, as they were coming to be called, were still enjoined in arcane debates about exhalations from the earth or the sea.

Nevertheless, the warning signs of bad weather were understood by those who needed to know them. Sullen swells in an eerie calm meant a storm was due. Red sky in the morning, sailor's warning; red sky at night, sailor's delight—these things were generally known to be true. Columbus himself, who had suffered through a Caribbean hurricane on his first voyage, knew when one was due on a subsequent voyage. But he didn't know why. Or how. Above all, no one understood that weather traveled. Even the most apocalyptic storms were thought to develop, wreak havoc, and then dissipate, in one place.

Reconciliation of the two branches of knowledge was not to come for several more centuries. In 1582 the great astronomer Tycho Brahe began the work of systematization; he kept a meteorological daybook and began defining the winds not only by the direction but also by their force. His gradations included dead calm, two categories of light winds, five categories of strong winds, three of storms; it was the first wind scale, several hundred years before Beaufort's. But Brahe never took it that one step further, by actually measuring the speed of those winds. At the time, he had no way of doing so.8

In 1622 Francis Bacon, given lots of time for thought during a stint in the Tower of London, produced his Historia Ventorum, or History of the Winds. He still believed that wind was generated by vapors expanding their volume suddenly. Where the winds or the vapors actually came from remained opaque: "The places where there are great stores of vapours [are] the native Countrie of the Windes."9 Ironically, Francis Bacon's illustrious ancestor Roger Bacon, the great medievalist and scientific prodigy, was closer to the mark more than four hundred years earlier when he simply noted in a journal that "heat makes air move." At the time, the observation went unheeded, mostly because the earlier Bacon was so prolix with his supply of ideas and inventions that his contemporaries were some what overwhelmed: He was the first person in the west, for example, to describe gunpowder; he invented spectacles for the eyes; and he was the first person anywhere, as far as is known, to propose mechanically propelled ships and carriages and an airplane with flapping wings. He was later imprisoned for his vocal contempt for his superiors in the Orders of Friars Minor and for his sarcastic views on the "puerilities" of other philosophers of his time.

Those same puerilities were still occasionally being trotted out even after the later Bacon. In 1668 Margaret Lucas Cavendish, the Duchess of Newcastle, published her Grounds of Natural Philosophy in which she asserted that "the strongest winds are made of the grossest vapours. Concerning the Figurative Motions of Vapour and Smoak, they are circles; but of Winds, they are broken Parts of Circular Vapours: for, when the Vaporous Circle is extended beyond its Nature of Vapour, the circumference of the Circle breaks into perturbed Parts; and if the Parts be small, the wind is, in our perception, sharp, pricking and piercing; but, if the Parts are not so small, then the wind is strong and pressing." As much "smoak" as this is, it did contain one new approach: an observation of what the winds actually felt like. Twenty or so years later, in 1684, Dr. Martin Lister, writing in the journal Philosophical Transactions,suggested that trade winds were caused by the constant breath of seaweed. In his view, their very regularity made their origin obvious: "The matter of that [ocean] wind, coming (as we suppose) from the breath of only one Plant it must needs make it constant and uniform: Whereas the great variety of plants and trees at land must needs furnish a confused matter of Wind."10

Despite these eminences, things had begun to change. Galileo helped. A proper lens-grinding technology helped. Precision instruments in the laboratory helped. The slow transformation of alchemy into chemistry helped. Artisans became educated and philosophers descended into the workshop. As early as 1627, the German Joseph Furtenbach fired a cannonball straight into the air to prove that the earth rotated. The ball, when it landed, was a little to the west of where it would have landed on an unmoving earth, no doubt to Furtenbach's relief (he was standing next to the cannon all the time). In 1639 Galileo essayed a variant of Aristotle's leather bag experiment. He manufactured a glass bulb with an airtight valve and sucked the air out. Then he weighed it. Next he forced air back in and weighed it again. There was a measurable difference. Aristotle's idea had been right, but his instruments defective, and Galileo was able to prove that air did, despite Aristotle's claim, have weight. A mere five years later, in 1644, Galileo's apprentice, Evangelista Torricelli, used his master's experiment to construct the first barometer, although the word barometer wasn't used until 1668, when Robert Boyle coined it for his own similar device. Whatever it was called, Torricelli's device was nevertheless the crucial breakthrough. For the first time a way existed to accurately measure a meteorological phenomenon.

The practical men, too, were observing nature ever more accurately. In 1626 Captain John Smith, notorious exaggerator of his own derring-do ("puller of the longbow," as historian Samuel Eliot Morison put it) and best known for his role in the Pocahontas saga, put together his Sea Grammar, with its careful observation of the various winds and of the proper terms therefor: "When there is not a breath of wind stirring, it is a calme or a starke calme. A Breze is a wind blows out of the Sea, and commonly in faire weather beginneth about nine on the morning, and lasteth til neere night; so likewise all night it is from the shore … A fresh Gale is that doth presently blow after a Calme, when the wind beginneth to quicken or blow. A faire Loome Gale is the best to sail in, because the Sea goeth not high, and we beare out all our Sailes. A stiffe Gale is so much wind as our topsailes can endure to beare … It over blowed when we can beare no top-sails. A flaw of wind is a Gust which is very violent upon a sudden, but quickly endeth … A storme is knowne to every one not to bee much lesse than a tempest, that will blow down houses, and trees up by the roots … A Hericano is so violent in the West Indies, it will continue three, foure, or five weekes, but they have it not past once in five, six or seven yeeres; but then it is with such extremity that the Sea flies like raine, and the waves so high, they over flow the low grounds by the Sea, in so much, that ships have been driven over tops of high trees there growing, many leagues into the land, and there left." A little longbow-pulling there in his description of a hericano, but otherwise useful enough.

In 1654 John Winthrop, the first governor of Massachusetts colony, recorded the first use of the current spelling of hurricane, when he wrote in his History of New England of the "great colonial hurricane" that had just passed by.

In 1663 Robert Hooke suggested "a Method for making a History of the Weather" by observing and recording "the Strength and quarter of the Winds." He recommended a scale from one to four, which included half numbers, so it was really a scale of nine.12

Eminent explorer and notorious pirate William Dampier was even more precise. In 1687 he encountered a major storm in the South China Sea: "Typhoons," he wrote, "are sorts of violent whirlwinds"— the first time this observation had been recorded. "Before these whirlwinds come on … there appears a heavy cloud to the northeast which is very black towards the horizon, but towards the upper part is a dull reddish color. The tempest came with great violence, but after a while the winds ceased all at once and a calm succeeded. This lasted … an hour more or less, then the gales were turned around, flowing with great fury from the southwest."13

In the middle of the eighteenth century, astronomer Edmund Halley, of comet fame, published an article in Philosophical Transactions that declared the cause of winds to be the heating of air by the sun. He wasn't quite right—he suggested that winds blow primarily from the east because the sun rises there, thereby making the classical mistake of generalizing from an untypical particular, the fact that at his home the morning winds were easterlies—but his article did suggest that the rotation of the earth had an effect on weather. And the article was memorable for another innovation: He published the first crude weather map.14

After that, developments came thick and fast. Anders Celsius, who constructed a temperature scale, also put together a wind-force scale. In Germany the Palatine Society, which joined the mania for measuring winds, set up the first weather office by coordinating observations from several cities in the Mannheim region of Germany.I5 The ever-curious Benjamin Franklin added to the storehouse of knowledge in 1743, after a storm blocked a sighting of an eclipse of the moon he had been particularly eager to see. The winds in Philadelphia were from the northeast, but he discovered through correspondence that Boston, which was to his northeast, actually suffered the same storm later than he had. A few years later this curiosity led him to a theory of storms, and he concluded they were disturbances that moved independently. He didn't know yet that they rotated. If he had seen Dampier's observations, they hadn't registered with him.

By the end of the century the practical men understood a good deal about storms, though a more careful calibration and a really comprehensive theory wasn't to be for another few decades. Sea captains understood clearly that pressure affected weather—that low pressure meant a storm—and no ship would leave port without a glass, as barometers were by then called. By the early nineteenth century meteorologists were commonly drawing isobars on maps to denote pressure, which meant winds.17 In 1802 Nathaniel Bowditch produced his American Practical Navigator, which was filled not only with graphs and charts (one of his graphs suggested that a drop of one millibar in an hour indicated a storm center twenty-two miles away; a drop of three millibars a storm center ten miles away) but with hard advice to sailors that made it clear just how storms actually traveled, and what they could do about it. A storm's center could be found by the simple expedient of facing the wind, in which case the center would be to your right. If you did this periodically, you could tell which way the storm was moving. The worst possible place to be was in the direct path of the storm's advance or toward its right—that is, to the north of a storm tracking west, to the east of a storm tracking north. Bowditch called that the dangerous semicircle, where the winds actually pushed vessels back into the center of the storm. The safest place, by contrast, was to the left of the path, where the winds would tend to propel you away from the center—and away from the center was very definitely where you wanted to be. He called this the navigable semicircle.18 All this made sense only in the northern hemisphere, because the storms were rotating counterclockwise.

Later Joseph Conrad had some fun with these rather head-scratching directions, and in Typhoon had his captain mulling the possibilities: "[The skipper] lost himself amongst advancing semi circles, left and right hand quadrants, the curves of the tracks, the probable bearing of the center, the shifts of wind and the readings of the barometer. He tried to bring all these things into a definite relation to himself, and ended by becoming contemptuously angry with such a lot of words and with so much advice, all head work and supposition, without a glimmer of certitude. 'It's the damndest thing, Jukes,' he said. 'If a fellow was to believe all that's in there, he would be running most of his time all over the sea to get behind the weather.' "I9

Finally, in 1831, the true cyclonic nature of storms was persuasively described by the autodidact scientist William Redfield. His conclusions were drawn from careful observation, especially by tracking detailed reports of hurricane damage after a storm had passed through Connecticut. He learned from these reports that trees had been felled in different directions, depending on where they had been in the storm's path. And it became clear that although at certain places the winds had been driving in from the north, those same northerly winds felled trees in southerly counties before they did their damage in the more northerly ones. That was Redfield's major eureka moment. The only explanation possible was that the storm was, in essence, a giant whirlwind. His conclusions, called "On the Prevailing Storms of the Atlantic Coast," were published in the American fournal of Science. He didn't call these storms cyclones—that term, derived from the Greek word meaning a coiled snake, was invented some time later by an Englishman, Henry Piddington, who applied Redfield's data to the massive storms in the Bay of Bengal. Nevertheless, the circular nature of the storms was conclusively established. Redfield's data also showed that cyclonic winds move in spirals, and not in concentric circles, that the velocity of rotation— the sustained winds—increases from the fringes toward the center, and that at the same time the whole body of the storm is itself moving, at a speed far less than that of the rotational winds.20

Many other scientists, such as Elias Loomis of Cleveland and James Pollard Espy of Philadelphia, later confirmed Redfield's data. Espy, who in 1842 was a professor at the Franklin Institute, became the first official U.S. meteorologist. It was Espy who contributed the final piece of the puzzle of great storms: the notion of rising air and latent heat. He proved that when moist, warm, rising air cools and precipitates out, it releases heat, for the curious but simple reason that molecules of water contain less energy than molecules of vapor. It is this latent heat that reheats the cooling air, causing it to rise farther, thereby drawing more air up after it; it is the key to the self-sustaining nature of tropical cyclones, and an explanation of their awesome power—they are self-generating furnaces and continue to exist as long as a supply of fuel, warm and moist air, can be found at the surface.

The practical men and the natural philosophers had at last come together, being described for the first time in the nineteenth century by the new word scientist. One of these new scientists was Matthew Fontaine Maury, born in 1806 in Virginia, who joined the navy and within a few years had made three voyages, to Europe, around the world, and to the Pacific coast of South America. He then spent the years 1834 to 1841 producing voluminous works on sea navigation and plotting the best paths for sea voyages. His best known work, Explanations and Sailing Directions to Accompany the Wind and Current Charts, contained chapters on the atmosphere, on "red fogs and sea dust," on the winds, and on matters as diverse as the equatorial cloud ring, the salts of the sea, the ocean currents, the Gulf Stream, the influence of the currents on climate, the depths of the ocean, the Atlantic Basin, and on gales, typhoons, and tornadoes. In 1842 he was appointed superintendent of the Depot of Charts and Instruments for the U.S. Navy, where he developed a system for recording the oceanographic and atmospheric data provided by both naval and merchant marine captains, and published the results in 1855, in his The Physical Geography of the Sea, the first real textbook of modern oceanography. One sea captain reported that he had followed Maury's instructions and had cut the duration of a voyage from New York to Rio from forty-one to twenty-four days, and it wasn't long before marine merchants insisted that their skippers use the new scientific navigation techniques too. Maury resigned from the US. Navy when Virginia seceded from the Union, and became commander of the navy of the Confederate States.

In a curious byway of science history, Maury has more recently been adopted by the woollier fringes of the Christian far right, who have come to believe, erroneously, that he was prompted to discover the Gulf Stream and other ocean currents through interpretation of a biblical passage on the "paths of the sea." In fact Ponce de Leon had written about the Florida current in the early 1500s, and a chart by Benjamin Franklin, published in 1786, well before Maury's birth, clearly shows the Gulf Stream.

Contemporaneous with Maury was the work of William Ferrel, whose Essay on the Winds and Currents of the Ocean rediscovered the forgotten work of Gustave-Gaspard Coriolis. It was in this essay that Ferrel, a self-taught farm boy from what is now West Virginia, proposed his famous model for the midlatitude circulation of the earth's atmosphere that was late to be called the Ferrel cell. His theory was that air flows toward the pole and eastward near the earth's surface, and toward the equator and westward at higher altitudes. His theory doesn't match precisely what actually happens, but it was still the first real explanation for the westerly winds in the middle latitudes of both hemispheres.

Only after manned flight in the twentieth century were the overall patterns of air circulation finally plotted. The work was given some urgency in the First World War, because the commanders of the new air forces desperately needed data they could use to protect their lethal but nonetheless fragile little bombers. By the late 1920s it was understood that winds were the continuing collision of huge air masses in waves, fronts, ridges, and troughs, all caused by solar radiation and the rotation of the planet. The final piece of the puzzle—the discovery of the high-altitude stratospheric winds and the jet streams—had to wait until aircraft could fly higher still. By the Second World War a real understanding of winds was, finally, in place.