An Ocean of Air

by Gabrielle Walker

  But gentle William Ferrel had no intention of pitching into anyone. He decided instead to use Maury's data to come up with his own ideas of how the winds work.

  This had two odd consequences. By accidentally turning Ferrel's gaze onto meteorology, Maury's contribution to the subject would indeed turn out to be seminal—though not in the way he had hoped. And one of the most important papers ever written in the history of the subject was about to be published in an obscure Nashville medical journal.

  Ferrel decided to ignore Maury's theories completely and look for the facts that he had gleaned from shipboard measurements and reports. It seemed that the winds in the two hemispheres moved in mirror image. On each side of the equator were the steady trade winds, which blew consistently from the east. Beyond these was another pair of wind belts, which were much stormier than the trades and usually blew from the west. Between these two sets of winds lay a mighty mountain range made not of rock, but of air. According to Maury's data, for some reason, air piled up in two giant ridges of high pressure, which circled the entire planet north and south of the equator, and separated the trades from the westerlies. This was the puzzle that Ferrel determined to solve: What drove these giant belts of wind to form, and why did the air pile up in between them?

  Ferrel began by considering how moving air would be affected by passing over a surface (the planet beneath) that was itself moving. He picked up his pencil and began his calculations.

  The math was fiendishly complicated, but the answer surprised Ferrel with its sheer simplicity. In response to the turning Earth below, air itself felt the urge to turn, and always in a certain direction. Put plainly, "in whatever direction a body moves on the Earth's surface there is a force arising from the Earth's rotation which deflects it towards the right in the northern hemisphere, but the contrary in the southern."

  Air descends at the tropics to create two "mountain ranges" of high pressure.

  Equatorwards of this, air moves to the west, while polewards it moves to the east.

  In other words, Ferrel had discovered the precise nature of the giddying effect that our planet's spin has on the air above it. Unscrupulous locals crouching over buckets at the equator might have tried to con money from you by convincing you that water goes down plugholes counterclockwise in the north and clockwise in the south because of it. This isn't true—in something as small as a bucket, any tiny perturbation in the water will be big enough to swamp the effect. But on a larger scale, the effect of the planet's spin certainly does make storms circle in opposite directions in the different hemispheres. Chances are you think of this as the "Coriolis effect."

  Nobody really understands why the effect was called this. About 1930, forty years after Ferrel's death, textbooks began inexplicably to refer to it this way, after French mathematician Gustave Gaspard Coriolis, who in 1836 had published a set of equations explaining how objects behaved in theoretical rotating systems. His math was flawless. But Coriolis had never applied his research to the atmosphere, nor even imagined using it to explain the winds.

  Even in Ferrel's time, meteorologists called the "north hemisphere turns right, southern hemisphere turns left" rule after someone else. A year after Ferrel came up with his rule, Dutch scientist Christophe Buys Ballot from the Royal Netherlands Meteorological Institute published a paper simply pointing out the observation that northern air tends to move right. He had made no attempt to derive it mathematically, nor did he take it any further. However, because nobody had heard of William Ferrel, researchers began calling this "Buys Ballot's Law," and the name stuck.

  As soon as Buys Ballot heard of Ferrel's work, he was thoroughly embarrassed. Obviously Ferrel, not he, should have had the credit. He even wrote to Ferrel offering that their names be used jointly. Poor shy Ferrel made no attempt to hide his horror at this proposal. He wrote back immediately, concluding his letter with these words: "Although I would esteem it a great honor to have my name in any way connected with yours, yet I will never encourage the change which you so generously propose."

  But to my mind (and to that of many others), the name of this effect belongs much more properly to this farm-boy genius. Though he was self-taught, and is still—thanks to his own diffidence—unsung, Ferrel both discovered the Coriolis effect in its fullest form and then went on to apply it. Using it, he was to become the first person in the world who would truly comprehend the winds.

  To understand what I will now obstinately—and unilaterally—call the "Ferrel effect," think of the shape of Earth: a sphere spinning around an axis that goes through its center. Although every part of the planet rotates precisely once a day, some parts have farther to travel than others. The equator has the hardest task. Being the broadest part of Earth, it has much the farthest distance to cover in its twenty-four hours, and every point on its surface is perpetually hurtling through space at more than one thousand miles per hour. Farther north or south, the planet is narrower, and the speed of travel much slower; by the time you reach the poles, the surface doesn't move at all.

  Air is affected by this because it is in contact with the spinning ground, and yet is free to move relative to it. The Ferrel (Coriolis) effect isn't a force so much as an optical illusion, brought about because we forget that we, too, are spinning with the ground beneath our feet.

  Usually, this effect is explained in terms of motion north or south. One way is to take an orange to represent the spinning Earth and a black pen to show the motion of air traveling south. Start with the pen at the orange's "north pole" and then move it directly south, while making the orange spin west to east. You will find that the black line you draw curls off to the west, to the pen's "right."

  Perhaps a better way to understand the effect, rather than simply to see it in action, is to think about what happens if a piece of tropical air (with fairly slow spin, because Earth there is fairly narrow) starts to move south toward the equator (where the local spin is much faster because the planet is broader). When the air arrives at the equator, it finds itself in the fast lane, where the ground is zipping eastward under its feet. Here at the equator, our tropical air parcel lags behind so much it seems to be going backward. In other words, it seems to turn to the west. To anyone standing on the surface, oblivious to Earth's spin, this would look exactly like air that was turning right.

  The same principle applies to an air parcel moving north. Starting at the equator, it is already spinning very rapidly eastward. But as it moves north, it finds itself hovering over a surface that is much more sluggish. Our air packet now finds itself in the slow lane, with its own foot still on the accelerator. And so it seems to turn eastward, which is once again to its right.

  This much had already been hinted at by a British scientist named George Hadley, who had been trying to explain the trade winds. But because this explanation rests on the difference in east–west motion between air and ground, until Ferrel came along everybody believed that it could only apply to motion north or south. A parcel of air moving eastward or westward relative to the ground should feel no effect at all.

  And this was Ferrel's genius: Through a combination of mathematical reasoning and brilliant intuition, he discovered that air is forced to curve even if it is moving east or west. In other words, whatever direction air is moving relative to the surface beneath, whether north, south, east, or west, Earth's spin will always make the air appear to turn.

  The explanation for why air moving east or west still turns is more complicated than for north or south, and much harder to picture. Think of how fast a particular piece of air is spinning west-to-east compared with the surface. Since Earth itself spins from west to east, any air moving toward the east will have all the spin of the planet beneath plus a bit extra. The faster anything spins, the more it wants to fly outward. (You can see this same effect if you attach a weight to a piece of elastic and then swing it in a circle. As the weight spins more quickly, the elastic stretches so the weight can move outward. It also explains why spinning salad in a c
entrifuge squashes the leaves up against the walls.) However, gravity keeps the air too snug to the surface to allow it to move far enough upward to accommodate its extra spin.

  The only other option left to the air is to move to a point on the surface that is farther from the central axis of spin, the line that runs from north to south right through the middle of Earth. In other words, the air needs to find a place where the planet is naturally wider. The closer you go toward the equator, the wider the Earth gets, so eastward-moving winds turn toward the equator—to the right in the northern hemisphere and to the left in the southern.

  The same principle applies for winds moving to the west relative to the surface. Now the air is spinning slightly slower than Earth beneath, which means it needs to move closer to the axis. Since the surface itself is in the way, preventing the air from burrowing in where it stands, it needs to move to a point where the planet is naturally thinner. The farther away from the equator you go, the thinner Earth gets, so westward-moving winds turn away from the equator—which means once again that they turn right in the northern hemisphere and left in the southern.

  Ferrel found that this simple additional effect of living on a spinning world—the inexorable urge for northern hemisphere air to turn right and southern hemisphere air to turn left—was exactly what he needed to explain the mysterious patterns that he had read about in Maury's book.

  To make Ferrel's argument simpler, think of the winds in the northern hemisphere. (Exactly the same arguments will apply to the southern, but in mirror image.) At the equator, the sun's heating is intense. Hot equatorial air will rise vertically upward. This equatorial air can't go on rising forever. Instead it is drawn northward toward the cold pole. However, moving air is always tugged sideways so this northward air will begin to curve toward the east. Because this is happening at high altitude, it goes unnoticed at the surface.

  However, the displaced high altitude air leaves a low-pressure gap beneath. Surface air will rush south to fill the gap; and since moving air always turns, this air will slew to the right to create the easterly trade winds.

  Meanwhile, the high altitude air is now moving eastward, and also cooling. Sooner or later it will begin to fall again, and this happens more or less over the tropics. As it's falling, the air is still being tugged to its right, which means it begins to curve south again. This is the air that closes the loop and turns back into the surface trades.

  At the other side of the tropics, the cold poles are still trying to draw air from the south. But this air also has the perpetual urge to turn right. Before it can arrive at the North Pole it, too, is forced to turn to the east, and then to the south.

  Now Ferrel was left with two perpetually diverted currents of air, one heading south from the North Pole and one north from the equator. These, he realized, would collide at the tropics, piling up to create that mysterious mountain range of high air pressure encircling the globe.

  And because air can only move into a part of the atmosphere where the pressure is lower, this mountain range acts as a barrier. That means the surface beneath is scarcely ever refreshed by damp air, which would bring rain. This is why Earth's deserts occur mainly in two giant rings centered about 30 degrees north and south of the equator, even though the equator itself is hotter. (You can check this on a globe. Trace out the positions of the Sahara and the deserts of Asia and central America in the northern hemisphere, and the deserts of South America, Namibia, and Australia in the southern.)

  Finally, the high-pressure system set up by those two currents of right-turning air explains beautifully the origin, location, and persistence of the two great bands of winds that Columbus discovered: the trades and the westerlies. These two bands form on opposite sides of the high-pressure zone. This mountain range of piled-up air behaves as if it has sloping sides to the north and south. Equatorial air that has risen to great heights and is arriving from the south hits the top of the mountain and is forced to roll back down the slope southward (and hence westward) to form the surface trade winds. High air that arrives from the north slides down the opposite slope, then turns back northward (and hence eastward) to become the westerlies.

  By discovering the simple "everything turns right" rule among his equations, Ferrel had managed to explain nearly everything about the wind currents.

  Ferrel published his explanation of the world's winds as "An essay on the winds and currents of the ocean" in his friend Bowling's paper, the Nashville Journal of Medicine and Surgery, in 1856. It wasn't exactly the world's most widely read journal for meteorological research, but nonetheless word of this and Ferrel's other investigations began to trickle out. The following year he received an unexpected invitation to move to Cambridge, Massachusetts, to work on the American Ephemeris and Nautical Almanac, which was published by the U.S. Naval Observatory. Though it wasn't an academic appointment, Ferrel eagerly accepted, and he suddenly found himself among thinkers and scientists who, much to his surprise, seemed to consider him as one of them.

  From there he was poached by the U.S. Signals Service in Washington, D.C., who insisted on republishing his Nashville Journal paper and other obscure essays so that the world's meteorologists could cast away their poor, tatty copies of the originals. Ferrel never sought a single academic position or honor, but people persisted in giving them to him. He became a member of the National Academy of Sciences, an associate fellow of the American Academy of Arts and Sciences, an honorary member of the meteorological societies of Austria, Britain, and Germany, and received the honorary degrees of M.A. and Ph.D.

  Ferrel would produce many other findings in his time, on an impressive variety of topics. He came up with the mathematical equivalent of labor-saving devices: formulas that made very effective shortcuts in long calculations. He worked out a new and better way to calculate pi, the ratio of the circumference to the diameter of a circle. He even calculated the weight of the moon, and explained why a throbbing star called Algol was winking increasingly rapidly at Earth. Though his scientific career didn't begin until he was nearly forty, by the time he retired thirty years later he had produced some three thousand pages of scientific research.

  In all that time, Ferrel never lost his paralyzing shyness—a failing he was well aware of but couldn't seem to shake. Once, he wrote a paper, "Note on the influence of the tides in causing an apparent acceleration of the moon's mean motion," and realized that it contained an original and important finding. He decided to present the paper to the American Academy of Arts and Sciences, but somehow he couldn't bring himself to stand up and read it aloud. "I carried it to the meetings of the Academy time after time with the intention of reading it," he later confessed, "and my courage failed."

  William Ferrel finally retired at age seventy to the Midwest, but he couldn't bear to be so far away from a ready supply of books and soon moved back east. He died five years later, as peacefully as he had lived. One of his greatest friends, meteorologist Cleveland Abbé, who had known him for thirty years, wrote: "We all remember his quiet ways, his indefatigable industry, his shyness, his perpetual absorption in the contemplation of some new and complex problem. He lived in an atmosphere of abstraction; he was with us, yet not of us."

  Another, more formal, obituary, by a meteorological professor who had worked a little with Ferrel, said: "It is a curious commentary on renown to name Ferrel, of whom the great world knows nothing ... as one of the most eminent scientific men that America has produced." America has produced plenty of eminent scientific men and women since that was written, but it still holds true on both counts. Ferrel is still relatively unknown, and he remains one of the best American scientists who has ever lived.

  ***

  Without the great wind belts of the world, our planet would be a very different place, part frozen, part fried. If all the heat that arrived in the tropics from the sun simply remained there, the equatorial region would be a full 25 degrees Fahrenheit warmer than today, and life there would be all but impossible. The poles would b
e in a worse state—they need to gain heat even more than the tropics need to lose it. Not only do the poles receive less direct sunlight than the equator, but their white caps reflect a large proportion of their sun back into space. Because of this, if they received no helping hand from farther south, both of the polar regions would be 45 degrees Fahrenheit colder than today, and this cooling would spill over into the heavily populated middle to high latitudes. In other words, if heat stayed only where it landed, most of Earth's surface would be unbearable.

  Global winds are the agents that perform this redistribution of thermal wealth, and the middle latitudes are the engines of their endeavors. The most important ingredient of this engine is another vital aspect of moving air that Ferrel's new "north turns right" rule managed to explain: not wind currents, but storms.

  Until Ferrel came along, nobody had understood why the winds around storms and weather patterns move in circles. In fact, before satellites could show us all the dramatic images of spiraling hurricanes, many people refused to believe that they were circular. The explanation for the shape of storms lies very simply in the new rule that Ferrel discovered. Every piece of moving air on Earth has the perpetual urge to turn. The sky is never still, nor is it uniform. Air is always on the move from one place to the next, and storms begin with any small patch of sky that has lost a little air to its neighbors. Air from the surrounding regions starts to try to fill this hole, but it can't. As soon as it starts to move toward the low-pressure center, it has to turn. Northern air turns right, which is why northern cyclones are always counterclockwise. Similarly, southern storms move clockwise, because the air there turns left. Every circling storm is a direct manifestation of our constantly spinning planet, and Ferrel was the first to understand that.