Windswept: The Story of Wind and Weather - Marq de Villiers (2006)
Chapter 2. Wind's Great Theater
Ivan's story: Air isn't yet wind. The atmosphere is only the theater in which wind presents itself, and air is only the stuff of which wind is made, and at first this thing that was taking place in the southern Sahara, this not-yet-even-pre-Ivan, was merely a matter of moving air masses, a ponderous vertical circulation of the unseen and unknown, in a place still far, far away.
Far away from us, at least. It was late summer, and we had visitors from Paris at our house. One balmy afternoon we went down to Beach Meadows Beach, as our local white sand crescent is called, and strolled along the tide line, looking for shells and signs of clams. Our friend Philippe called it uvotre petite paradis" (your little piece of paradise), and so it was, because the beach was empty and warm underfoot, and the sea glittered in the sun. There was no surf to speak of only small ripples in the water. There was a gentle breeze aloft, in which the gulls played.
But on that same day, half a world away in the Sahara Desert not far from Darfur and in a landscape alien and hostile even by Saharan standards, something altogether different was happening.
This was in the massif called Tibesti—"Tu," or rock, to its inhabitants, the Tubu. It is no small thing, covering about 300 miles northeast to southwest, and about 173 across, smaller than the more famous Saharan Ahaggar Mountains but rivaling those in the ferocity of its demeanor. The massif lies mostly in Chad, but it stretches north into Libya and even west into Niger. It is really less a mountain range than it is a rupture in the earth, formed when lava streams forced their way through the planetary skin, leaving a stratum of crystalline rock covered with sandstone. This sandstone was then forced violently upward again as two massive tectonic plates collided, deep within the mantle; it now rises abruptly from the surrounding plains, reaching to 11,204 feet at the summit of Emi Koussi, the Sahara's highest point. Some of the volcanoes are still active and emit a sulfurous smoke from the roiling magma far below. The whole place is deeply eroded, cracked with ravines and awful wells in the rock, unplumbed and of unknown depth. In places the lava resembles the waves of a petrified sea, albeit a sea with swells reachingfive hundred feet or more. Once, in the Sahara's distant past, great rivers roared through these ravines, leaving scars that are still clearly visible. Three of these ancient river courses provided the secretive Tubu their way into the massif, and there they built a few scattered towns, among them the remote Bardai.
On that summer day, so benign on America's eastern seaboard, the hot breath of the desert flungitself against Emi Koussi and scaled rapidly into the upper troposphere. The superheated air rushing up the slopes (in what is called anabatic, or up-mountain, flow) at some point met the cooler, denser air flowing downward (katabaticflow), creating pulses of turbulence, rapid mixing, and the formation of massive thunderheads and ominous, towering black clouds. The sulfurous volcanic air was riven by lightning, and thunder rolled across the ravines, echoing off the boulders that were scattered like a giant's abandoned toy box across the landscape. The system spun off tornadoes, their evil spirals twisting among the ravines. For the next few days the thunder cells drifted slowly westward, driven by the prevailing easterlies of the season. At high altitudes there were violent showers and localized mountain floods; on the superheated plains west of the salt mining center of Bilma the rains fell, but the air was so hot itflash-evaporated the water before it could reach the ground, and the Tuareg nomads could see the cooling water falling without ever feeling it. A day or so later the system passed south of Agadez in Niger, where it was reinforced by yet more turbulent air generated by the lower but nevertheless forbidding black peaks of the Air massif of the central Sahara. Then the set of thunder cells crossed the desert north of Timbuktu, and the small weather station there recorded its passage in a handwritten logbook. It was one of many that passed by that month. The records say there were violent downdrafts and gusts of 60 miles an hour.
Air, in a gesture of atmospheric alchemy, had by now transformed itself into wind. It was the last week of August 2004.
I
The first time I really thought about air, the stuff that makes the wind, was on a beach on the Indian Ocean side of South Africa, near the tidy little town of George. Our family was lying on the white sand drying out after an early morning swim, and my cousin Colin, who had the day before caught a clawless rock lobster by the simple expedient of diving down through six feet or so of clear water and picking it off the sand, asked a typically unexpected question.
"If the crayfish can't see the water," he said, "and they look up, do they think the fish are flying?"
There was a gentle onshore breeze that morning, I'm sure— there was always a gentle onshore breeze in the morning—but I didn't notice it. I remember looking up at the puffy cumulus clouds scudding overhead, but I paid them no mind either. I found Colin's question curiously vertiginous. It was similar to the question asked in many a school geography class ("If you couldn't see the water, would you not think ships were flying?") but in a way much more unsettling. Did that mean we, like the lobsters, were merely bottom-feeders? Living at the bottom of a towering pile of … something . . . a great weight of something unseen and unfelt, roiling around above our heads, and above that the clouds, and above that, what? Where was the "surface" of the air? Was there some atmospheric equivalent to the ground?
These were a small boy's ponderings and soon faded. But behind Colin's simple question, I know now, was something much more profound. That we are indeed living at the bottom of a towering, restless sea of air goes some way to explaining the nature and persistence (but also the fragility) of life, the biosphere that inhabits the troposphere, here in the unsettlingly narrow layer between the bleakness of outer space and the unrelenting pressure of the boiling rock deep beneath the mantle.
We experience little of this directly, except as wind and weather. But where does wind begin? With the pressure differentials between highs and lows? With the solar energy that causes those differentials? With the nuclear fires in the sun that cause the solar energy that causes the pressure differentials? Or merely with that substance that was so invisible—and then later so mysterious—to our ancestors, the thing called "air"?
Wind, after all, is just air in motion.
Was there ever a time when there was no air? Very likely not. It may have been poisonous to us—but there was something … What exactly it was is still subject to speculation, which is the scientific word for guesswork. However, it is probably true that Earth has had three successive atmospheres with varying degrees of stability.
The most common assumption is that Earth itself is around 4.6 to 5 billion years old, formed by gravity from cosmic junk, clouds of ionized particles around the sun, and debris left over from the somewhere-sometime explosion called the big bang. This cosmic tip-heap coalesced to form a protoplanet, which grew by the gravitational attraction of even more junk, what the cosmologists call particulates. This was the so-called Hadean Eon: a sort-of Earth existed, but there were no continents, and no oceans—and most definitely no life. Just boiling clouds of gas.
At first, this not-quite planet was almost certainly too hot to retain much of the primitive atmosphere it was born with. Based on what we know of free gases in the universe, this first atmosphere would probably have consisted of helium and hydrogen. Until recently, the scientific consensus was that both these gases would have boiled off into space, to be replaced as the planet cooled with the products of volcanic outgassing—water vapor, carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen, ammonia, methane, hydrogen sulfite, and chlorine. A new study, however, has found evidence that hydrogen persisted in the atmosphere, escaping into space much more slowly than earlier thought. It's possible this secondary atmosphere would have contained almost 30 percent hydrogen. There would have been no oxygen at all.1
The massive amounts of water vapor expelled from the condensing Earth would have formed a dense cloud layer, which then precipitated out as pure water.
Which raises the questions: Where did the water vapor in the atmosphere and in the condensing Earth come from in the first place?
What was it in the volcanism that caused our first weather to produce H2O? Very likely, it was already present in all that cosmic junk—comets are sometimes little more than frozen lakes of water— and it came from space, an alien and infinitely curious little molecule. For water is curious, much more curious than it might at first appear, and is actually little understood. Why is it, for instance, that water is the only substance whose solid form is less dense than its liquid one, a phenomenon that has profound implications for aquatic life? "As a liquid, water has special thermal features that minimize temperature fluctuations. First among these features is its high specific heat—that is, a relatively large amount of heat is required to raise the temperature of water. This capacity to absorb heat has several important consequences for the biosphere."2 Water and wind intersect in important ways to regulate our planet.
Over a period of thousands of years, the rain accumulated as rivers and lakes and ocean basins. This left carbon dioxide, CO2, as the main component of air. About half a billion years after its birth, give or take an eon or two, things had settled down enough for this carbon dioxide to react with water and other compounds to form rocks and minerals. The oldest known rocks are in Greenland, and have just celebrated their 3.9 billionth birthday. Earth was still aflame with volcanoes and bombarded by asteroids, meteorites, and whatever else was floating in the interstellar void and intersecting with our nascent planet, but those oldest rocks show signs of having been deposited in an environment already containing water.
At about the same time, methane began to increase. By our planet's billionth birthday, the atmosphere would have been 80 percent methane and related carbon compounds and less than 10 percent nitrogen. Nitrogen, which is not very chemically active, continued slowly to accumulate.
Nothing about this was unique. In the solar system the only body other than ours with an atmosphere is Saturn's largest moon, Titan. NASA's probes have found that Titan's "air" consists largely of nitrogen, hydrogen sulfide, and methane, with trace amounts of other gases—in effect, Titan's air is what we on Earth would call thick smog. Titans might thrive on it, but to us it would smell vile. Breathing it would quickly kill us. You can still get the sense of it by leaning (carefully) into a volcanic vent, if you do so between eruptions.
II
It can plausibly be argued that the most extraordinary event in Earth's long life, at least from the point of view of species now living, occurred when the planet was an adolescent, about three and a half billion years ago. This was the sudden, and still quite mysterious, appearance of photosynthetic bacteria, phytoplankton and algae.
These were not the planet's first life forms; yet more primitive sulfur-eating bacteria already existed, and indeed still do exist in miasmic sulfur vents in obscure places around the globe—bacteria that, for lack of a better theory, are thought to have emerged in a pool of soupy, chemical- and nutrientrich water, although a small but influential scientific subset believes life too might have come to us from space, ready-formed, cosmic nuggets among the infinite dross. "Runoff collected in a small volume is the most likely means of achieving the necessary concentration of ingredients," Gustaf Arrhenius, a geochemist at Scripps Institution of Oceanography, told National Geographic in March 1998. The presence of hydrogen in substantial concentrations would help to explain how these concentrated compounds attracted other molecules that then acted as catalysts in subsequent reactions. Hydrogen is a volatile and active gas, and in its presence sugar phosphates, possible precursors to organic life, would have been produced.
In any case, the new photosynthetic organisms began the laborious job of converting carbon dioxide to oxygen and of splitting water vapor into its component parts, and free oxygen began to trickle, molecule by molecule, into the air. As Richard Fortey put it in his book The Earth: An Intimate History, "Three billion years of photosynthesis, much of it achieved by very simple organisms like blue-green bacteria … the minute rod- or thread-like remains of the bacteria that changed the world have been found among the earliest fossils. They formed sticky mats, which left their record in finely layered, crimped or cushion-like fossils called stromatolites, looking like piles of petrified flaky pastry" If you were alive at this period, Fortey wrote, "[you would see] a hot spring bubbling and hissing at your feet, surrounded by brilliant orange and purple livid stains. They feel slick to the touch. The colors are painted by bacterial life that forms a slimy film; the lively hues are pigments that shield the tiny cells from harmful rays in the harsh sunlight. From here, you can see the tacky green mats in the sea that are slowly transforming the earth's atmosphere."3 At the same time, the days were getting longer and longer as the Precambrian progressed and the planet's rotation slowed, allowing ever more photosynthesis to take place.
Then, a mere billion years ago, the first eukaryotic organisms, the earliest ancestors of all of us, appeared. These were more "organized" cells than their predecessors, with DNA coding segregated into a nucleus instead of being jumbled up with everything else, and they were far more efficient at converting carbon dioxide to oxygen. The percentage of oxygen consequently began to climb, going up from mere traces about 600 million years ago to its present level of about 20 percent.
These processes, acting sequentially and then simultaneously, produced the delicate balance preserved in modern air, a combination of permanent gases (nitrogen at 78.084 percent of all air, and oxygen at 20.947 percent) and gases considered to be variable (gases that have changing concentrations over a finite period of time). Variable gases are essentially water vapor and aerosols (tiny liquid droplets that include ice crystals, smoke, sea salt crystals, dust, and volcanic emissions, suspended in the air), plus carbon dioxide, methane, and ozone, all of which are critical to maintaining planetary temperatures and therefore life (see Appendix 1).
This combination is not at all magical, except for the one simple fact that our lives are entirely dependent on it, and even a small deviation in its proportions would kill us and profoundly alter the planet's climate. But as Richard Fortey pointed out, nothing in evolutionary history made the photosynthetic bacterium that resulted in us "a dead cert" to emerge at all—the evolutionary winner could just as easily have been a bacterium that depends on sulfur, in which case we would never have evolved. Something else would have, but not us: No wonder evolution makes creationists nervous. It is not so much that we are descended from primeval slime that is disturbing, but that our whole existence is such a fluke.
Before the coming of life, then, Earth was a bleak place, a rocky globe with shallow seas and a thin band of gases—largely carbon dioxide, carbon monoxide, molecular nitrogen, hydrogen sulfide, and water vapor. It was a hostile and barren planet. This strictly inorganic state of Earth is called the geosphere, which is itself made up of the lithosphere (the rock and soil), the hydrosphere (the water), and the atmosphere (the air). Energy from the sun relentlessly bombarded the surface of the primitive Earth, and in time—millions of years—chemical and physical actions somehow produced the first evidence of life: formless, jellylike blobs that could collect energy from the environment and produce more of their own kind. Heritability was the real key to evolution, the ability to reproduce and therefore evolve. Thus was created the biosphere, the zone of life, an energy-diverting, entropy-fighting, rapidly evolving but perilously thin skin on Earth's surface that uses the matter of the earth to make living substance. It happened in water, and it happened in air. Air was the necessary predisposing factor, for all life depends on it.
III
So much we understand now; we can parse air to a fare-thee-well and measure particulate matter to a part per billion. But for millennia (from the beginning of recorded thought) and until very recently, air was the most mysterious of, well, substances, if that's what it can be called. In fact, there was considerable doubt that air existed at all; and when philosophers imagined it into being, there were no instruments able to measure it or prove its existence.
The first conceptual breakthrough came when the Greek philosophers grasped that air was something rather than nothing. It was, after all, far from self-evident. You couldn't see air, taste it, or measure it. True, there were clues—odors were a puzzle, and so was wind. Fog also—clouds were too far out of reach to worry about, but you could walk through fog, its appearance and its evident moisture obvious, though its nature was still obscure. Before that, air was just … nothing. We were like the crayfish of the Indian Ocean in my cousin Colin's question—you couldn't think about it because you didn't know there was anything there to think about.
The man who understood that things might be a little more complicated was Anaximander, a philosopher of the sixth century B.C. The sky and the earth and everything on it, he taught, were conjured into being when the great primordial sea was evaporated by celestial fires; everything left over is the consequence of basic oppositional forces—light and dark, dryness and wetness, heat and cold. He said nothing about air specifically, but his pupil Anaximenes took the discussion one significant step further. He suggested that air was itself an element, and the most important one at that: "Air is the first principle of things, for from this all things arise and into this all are resolved again." Its properties were intermediate between fire and water, and air was therefore the basic matter from which these two were formed. Some centuries later Plutarch, who was a great admirer of Anaximenes, quoted him as saying, "all things are [thus] generated by a certain condensation or rarefaction of it."4
The notion was obviously current in the schools of philosophy. Among the earliest surviving texts, dating from about the same period as Anaximander, is a theory contained in a document called the Derveni papyrus, discovered in 1962 half burned on top of a coffin; it had been part of the dead man's funeral pyre. Derveni's tale was an allegory about Orpheus, but in the text the author's worldview was clearly stated. The world had two unequal basic principles, fire and air, and air is, at the same time, divine and also called "mind." Cold air has the property of checking hot fire. Originally, fire was dispersed through the universe, created havoc, and prevented the formation of order, or kosmos. To set creation in motion, mind-air acted to concentrate fire in the stars and the sun: This made the world as we know it.
Anaximander
This was, even by ancient standards, pretty hit and miss, and there it rested until it was taken up by Aristotle (384-322 B.C.), the great synthesizer of Greek thought. The meteorological and chemical ideas codified by Aristotle remained "true" for more than 1,500 years. They can be summarized as two overweening concepts: the four elements of matter and the atomistic view of matter. These overlapped and reinforced each other, but were also to some degree in conflict—Aristotle himself agreed with the former but not the latter, and the debate he set off was the source of more than a thousand years of sometimes acrimonious philosophical and alchemical wrangling.
The first complete description of the four-elements theory dates back to a philosopher called Empedocles (490-430 B.C.), but his notes were restated more elaborately and concisely by Aristotle. All matter, in this view, was made up of four basic, irreducible elements: earth, air, fire, and water. In turn, these four basic pillars of the universe are derived from the four "properties," hotness and its opposite, coldness; and dryness and its opposite, wetness.
Fire and water are obvious opposites, according to Aristotle, and so are earth and air. They have nothing in common and share no properties. Each element existed somewhere in an ideal, or pure form, not found on Earth. Real or earthly things were impure mixtures of the ideal elements. Smoke, for example, was a mixture of the air and earth with some of the element of fire added. The elements could be changed into one another by removal of one property and addition of another—an idea later seized on by the medieval alchemists, the precursors of modern chemistry. Another point that seemed obvious was that the four elements had a natural tendency to separate in space; fire moved upward, away from the earth, and the earth moved inward, away from the air. Air and water, for lack of any other conceptual framework, were described as intermediate. Matter, in whatever form, could be subdivided indefinitely in theory, although Aristotle acknowledged that this would not always be practical or easy to do.
It was, in fact, an early theory of everything, still the holy grail of science. It explained almost all the actions of nature. A fire, for example, was on earth merely impure ideal fire. When a pot was placed over a fire, the bottom of the pot became black; this happened because the real fire was a mixture of ideal fire and ideal earth, and therefore when the fire entered the pot to give it more of the property of hotness, some or all of the earth mixed with it was left behind as a soot. When sea water was heated, it absorbed the hotness of fire and moved away from water, becoming air; the impurity in real water, earth, then was left behind on the bottom of the pot as dry salt. And air, when cooled, would condense droplets of water, as when cold metal was placed in contact with the air above a kettle of boiling water or in moist air. This occurred because the property of coldness, taken from the metal or earth, moved the air toward wetness and therefore partially toward water.
Hmmm … it worked too. Mostly. But as a refinement, Aristotle, an honest skeptic, was obliged to add another element to the mix proposed by Empedocles. He called it quintaessentia, and the very name is a giveaway that he wasn't completely satisfied that the Big Four were a complete set. Quintaessentia was an eternal and unchangeable element otherwise known as ether, or space; it was the framework in which the other four existed, and it haunted physics until after 1850, when meticulous measurements finally made its retention impossible.
Aristotle also bought into the notion proposed by his master, Plato, that each of the four elements existed in a particular geometric form and the properties of the element were therefore related to that form. So fire particles were tetrahedrons, four-faced figures whose sharp points gave speed and burning sensations like arrows striking the flesh. Earth particles had the shapes of cubes, which accounted for their solidity; water particles had the smoother shape of twenty-sided icosahedrons, while those of air had the shape of an octahedron, a figure with eight sides. Ether, being the highest of the elements, had the most complex geometry, that of a pentagonal dodecahedron, a solid figure with twelve equal pentagonal faces. This was all wrong, of course, but not so wrong that its central notion, that each of the elements was made up of particles having a single definite shape, didn't resonate strongly with the modern theories of chemistry being developed in the seventeenth and eighteenth centuries.
The other prevailing Greek theory of the universe, and therefore of air, was the curiously modern-sounding theory of atoms. The first sighting of this interesting notion was in the writings of Leucippus, of whom otherwise nothing is known, and his student Democritus, somewhere around 400 B.C. They theorized, contrary to the four-elements school, that matter was not capable of infinite subdivision but contained ultimate and extremely small particles they called "atomus." These tiny particles, like matter, are eternal. Differences between substances are therefore due to the atoms of which they are composed, which are of different shapes and arrangements. Differences in the properties of substances, moreover, are not due to the atoms themselves but to the way in which they are arranged. The second point was that these atoms were in constant motion. The third was that they were separated by voids, or vacuums, in which they moved.
Aristotle disagreed vehemently with this whole idea of atoms. There was no such thing as a vacuum, he believed. Air has mass, yes— he showed that a container filled with air could not also be filled with water—but it couldn't possibly have weight; he flattened an airtight bag and weighed it, then filled it with air and weighed it again, and found no difference. And how could atoms be in constant motion? The whole theory, in his opinion, brought philosophy into disrepute.
Philo of Byzantium, in the third century B.C. performed a more sophisticated version of Aristotle's mass experiment, and was thus the first to really prove that air had substance. He attached a tube to a glass globe, then inserted the open end of the tube into a dish of water. When he placed the globe in shadow, the water rose within the tube. When he exposed the globe to sunlight, the level fell. "The same effect," he wrote, "is produced if one heats the globe with fire."
He had—though he didn't know it—stumbled on the true cause of wind.5 He was the first meteorologist.
IV
And there the science of air rested, for another two thousand years or so. Inquiry into the physical world went on, but at a desultory pace, and philosophers turned their attention to other matters, such as the transmutation of base metals into gold, and to increasingly odd notions of cosmology; alchemy and astrology dominated the physical sciences until the Middle Ages. The first comprehensive theory after Aristotle's that sought to explain air and combustion wasn't formulated until the late seventeenth century. This phlogiston theory was entirely plausible in the light of current knowledge, though it, too, was entirely wrong. Its ascendancy lasted about one hundred years, and when it was finally put to rest, the way was opened for the birth of a modern, measurement-based, technologically oriented, practical discipline that we now know as atmospheric science, of which meteorology is an important part.
The theory was German in origin, and the two scientists most identified with it were Johann Joachim Becher and Georg Ernst Stahl, who first used the word phlogiston in 1700. Following the ancient Greek codification practice, the theory sought to reduce everything to three essences that make up all matter: sulfa, or terra pinguis, the essence of inflammability; mercury, or terra mercurialis, the essence of fluidity; and salt, or terra lapida. Terra pinguis was later renamed phlogiston. Under the pressure of the church, living matter was excluded from this taxonomy, because of course it contained the potential for a soul that differed in composition from nonliving matter, and since it was divinely inspired, was necessarily outside earthly classification. This was Stahl's vitalism theory, outlined in his The True Theory of Medicine.
The governing idea of phlogiston was that all metals were made of a calx, or residue, combined with phlogiston, the fiery principle, which was liberated during combustion, leaving only the calx. Air, according to this theory, was merely a receptacle for phlogiston. Combustibles, or calcinable substances, were not elements at all, but merely compounds containing phlogiston. For example, rusting iron was believed to be losing its phlogiston and thereby returning to its elemental state.
Similarly, flames extinguish because the air becomes saturated with phlogiston. Charcoal leaves little residue after burning because it is nearly pure phlogiston. Mice die in confined spaces because the air becomes saturated with phlogiston.
The theory had many advantages. It explained, for example, how air at first supports combustion and then after a while does not. It also addressed some of the obvious drawbacks of Aristotle's theory, particularly his hazy notions of chemical change. But phlogiston nevertheless came under increasing attack. At first, the results of hundreds of practical experiments in dozens of laboratories around Europe were shoehorned into the theory, but too many just didn't seem to fit. Why, for example, did some metals, such as magnesium, actually gain mass when burned? Phlogistonists finessed the discrepancy by assigning phlogiston a negative mass, or by asserting that air entered the metal to fill the vacuum after phlogiston left, but this satisfied hardly anyone.
Even Joseph Priestley, the towering and cantankerous intellect who discovered the existence of oxygen, was a devout believer in the phlogiston theory (he simply called oxygen "dephlogisticated air") and late in his life issued a ringing denunciation of the antiphlogistons, whose activities he actually compared to Robespierre and the Terror, a gross libel and a calumny. (This was from his new home in America, whence he had been driven by a lynch mob sent by King Charles, angry at his presumed anticlerical activities; Priestley was never one to suffer fools lightly—it was perhaps why he became such a good friend to Thomas Paine.)
It was left to the ever-practical Frenchman Antoine-Laurent Lavoisier, to demolish the phlogiston theory entirely. He was the first to understand the significance of Priestley's writing on oxygen, and he disproved phlogiston by showing that oxygen is required for combustion, as well as for rusting and respiration. Lavoisier, who began his career in the Royal Gunpowder Administration in Paris, is best known for his synthesis of chemical knowledge in his Traite elementaire de chimie, in which for the first time the modern notion of elements was laid out systematically.
In the revolutionary spirit of the time, Lavoisier made a symbolic break with the theory by burning all the books on the subject he could lay his hands on.
And in the other spirit of the time, though a political moderate, he died on the guillotine during the Terror, in 1794.6
Lavoisier was influenced by Robert Boyle (1627-1691), who had made precise measurements in studying the relationship between volume and pressure of gases. In the Sceptical Chymist Boyle questioned Aristotle's view of the four elements and proposed matter was composed of tiny particles, therefore becoming the first modern to synthesize the two Greek notions of the universe. He was followed by Joseph Black (1728—1799), who discovered carbon dioxide in 1750 and showed that it was produced in combustion, human breath, and fermentation; and by Henry Cavendish (1731-1810), who found that common air was made up of nitrogen and oxygen in a 4:1 ratio. Carbon dioxide might still be called fixed air, hydrogen inflammable air, and nitrogen dead air, but oxygen was no longer called dephlogisticated air, and Cavendish's mix was about right.
So now we knew what air was.
V
But we didn't yet know what the atmosphere was. That had to wait for modern science, for the atmosphere is much more complex than was thought even a hundred years ago. Air, with its patented mix of gases and in its various degrees of thinness and density, is one thing, but the atmosphere is also made up of bands of radiation, clouds of ionized gases, zone layers, and swirling magnetic fields, all of which impinge on our weather and our winds, and thus on us, down here at our little intersection of atmosphere and lithosphere where living things make their home.
The first complication is that the atmosphere has layers, according to the density of the air. In the conventional taxonomy, weather occurs and we live in the troposphere, a layer of air ranging from the earth's surface, where it is densest, to somewhere between 5 miles at the poles and a little more than 10 miles at the equator. Within the troposphere, the air gets colder the farther away from the earth it is, which is why airplanes flying at around 20,000 feet usually push through air whose temperature is well below zero, even at equatorial latitudes. (The range is generally thought of as "earth-normal" to —6o° Celsius). Despite the commonsensical observation that winds are largely horizontal, which is how we perceive them, the prevailing movement of air within the troposphere is actually vertical, in what are now called Hadley and Ferrel cells.
The atmosphere, showing the approximate ranges of the major layers, the aurora regions (in the thermosphere, at left), and the orbiting altitude of the space shuttle.
Above the troposphere is a thin layer of stable temperatures generally called the tropopause, to distinguish it from the next layer up, the stratosphere, which itself reaches to around 31 miles from the earth's surface. Temperatures within the stratosphere actually rise with altitude, reaching the freezing mark of wrater (o° Celsius) at the highest elevations. Stratospheric winds are almost all stable, persistent, and horizontal. This is where the now notorious ozone layer is to be found.
Above the stratosphere is—big surprise, this—the stratopause, and above that is the mesosphere, reaching up to about 53 miles. The air in the mesosphere is too thin to circulate much at all, and winds hardly exist there. Not too thin, though, to burn up most of the meteors that enter our atmosphere. Temperatures in the mesosphere deline, from about o° Celsius to somewhere around — 100° Celsius.
Then, of course, there's the mesopause.
Beyond that is the thermosphere, which stretches out as far as around 370 miles and represents the outer limits of earth's thermal reach. Temperatures in the thermosphere range from around —8o° Celsius to as low as — i,ooo° Celsius, though the number of air molecules at that altitude is so small, and the consequent heat transfer so meager, that it would not feel at all cold to the human skin (provided that skin's owner could somehow deal with the absence of oxygen and the vacuumlike pressure).
Just to complete the set, however, atmospheric scientists generally include one more layer, which they call the exosphere. As its name implies, the exosphere is simply that part of space beyond any influence from Earth; the exosphere, therefore, is a near vacuum, containing not much at all, and yet includes pretty much all the remaining universe—the human species is a parochial one, measuring influence by its own small corner of its own small galaxy. The satellites that beam TV signals into your living room circulate on the lower fringes of the exosphere.
The air gets thinner the higher you go. Unsurprisingly, then, atmospheric pressure—and the density of air's life-sustaining gases—diminishes rapidly with altitude. At 9,000 feet the air pressure is already only three quarters of that at sea level, and almost everyone—except hardy Andes dwellers, Sherpas of the Himalayas, and a few Ethiopian and Kenyan marathoners—feels the effects. The reduced pressure causes the brain to swell slightly, resulting in headaches and nausea. At double that height, 18,000 feet, the pressure has dropped to half earth-normal; no permanent settlements exist at these altitudes because the human lungs just can't cope, and expel too much CO2, fatally disrupting the body's balance. At Everest's peak, just over 29,000 feet, pressure is only 30 percent, about 300 millibars instead of the earth-normal of 1,000; at those altitudes, the human body begins an inevitable, and final, breakdown.7 It's possible—at least for a few extrahardy mountaineers—to survive for brief periods at Everest's summit without an oxygen tank, but survival at that altitude is measured in minutes, not hours.
It's interesting to contemplate what a perilously thin layer our sustaining atmosphere really is. Earth's diameter is only a little more than 7,000 miles. The troposphere, in which our weather (and the greenhouse effect) happens, is at most 10 miles deep. Put another way, if the earth was a ball 4 feet or so in diameter, the troposphere would be a fraction of an inch thick, about the thickness of the lead in a common pencil.
But the theater of the wind is more complicated than just stacking decreasingly dense layers one on the next.
In the popular imagination, and in the scientific consensus before the space age, "outside" seemed empty, dark, and frigid, a void through which to view distant planets and stars; the very word space seemed imprecise, lacking in definition, an afterthought, coined only in opposition to nonspace, or Earth. In fact, our planet's hinterland is not empty at all, but filled with magnetic fields, electric fields, matter, energy, ionized gases, and radiation, generally invisible to the eye or the telescope but easily apparent to instrumentation. Some of these are worth noting because they affect our climate and our wind patterns and therefore the weather.
The most curious of these is the ionosphere, a region starting about 50 miles above the surface, where atmospheric gases are so thin that free electrons can exist for brief periods before being captured, or ionized, by free-ranging positive ions, which in turn are produced by solar X-rays and ultraviolet radiation. The ionosphere is organized, more or less, into a series of broad bands, or levels, which for convenience sake but with scant regard for felicity are unimaginatively called D, E, F, and Topside. The D region, the lowest, and Topside, which is pretty obviously the highest, have little impact on human life; both are regions of weak ionization. It is the middle two layers, E and F, that impact us most. E is sometimes known as the Kennelly-Heaviside layer (or just the Heaviside layer), after the American engineer Arthur Kennelly and the British physicist Oliver Heaviside. It is a layer of strongly ionized gas between 54 and 90 miles thick, and because of its frequency (eight hertz), it has the useful effect of reflecting medium-wave radio transmissions, thereby allowing Midwestern country-and-western stations to be heard over the horizon in, say, Maine, to the great benefit of listeners there pining for hurtin' music; I remember in the old Soviet Union how teenagers would use the effect to listen to the Beatles singing "Back in the USSR" on the BBC, to the great irritation of their politically correct elders. Its usefulness varies, though, by time of day, season, and sunspot activity, so radio stations are wise not to make extravagant promises. The F region, above 90 miles, does the same thing.
But apart from its impact on radio broadcasting, why pay it any attention? Because the ionosphere is a flexible, dynamic, and rather fragile system buffeted about by electromagnetic emissions, by variations in the earth's magnetic field, and by the acoustic motion of the atmosphere itself, which means it is acutely sensitive to atmospheric changes. To monitor the ionosphere, then, is a good way of keeping a watch on atmospheric events. But there's more: Many scientists believe that, as NASA puts it on its Web site, "there is persuasive evidence of an ionospheric precursor to large earthquakes," and that it could even be used as a predictor. This is partly because acoustic waves are generated both before and after earthquakes, but also because it is thought that part of the run-up to an earthquake is the generation of electromagnetic emissions, which have been detected in the ionosphere up to six days prior to a large quake. In other words, if we learn how, we might be able to use the ionosphere as an early-warning system, dramatically increasing the time people living in earthquake zones have to react. A few days' warning could have made a massive difference, say, in the death toll of the Asian tsunami the day after Christmas 2004.
Above the ionosphere is the so-called magnetosphere. It is simple to visualize but complicated in its effects (which include, curiously, huge numbers of conspiracy-based Web pages that accuse the scientific community and the U.S. government in particular of massive cover-ups and fraud regarding its existence).
The magnetosphere is the field of influence of Earth's magnetic force—Earth is the equivalent of a normal, if rather over-size, bar magnet, mostly because of its nickel-iron core. Its most obvious use to us, of course, is in the magnetic compass; sailors have navigated by compass since it was independently invented sometime both in China and in Europe in the twelfth century, when savants in both places noted that a piece of lodestone (magnetic ore) floating on a stick in a jar of water pointed to the polestar. The magnetosphere ranges thousands of miles out into space—many orbiting space probes have as one of their scientific missions to help map this field, a task still incomplete. This simple and orderly system is complicated by the massive amounts of solar plasma making up the solar wind, which is itself strongly magnetized and pulses off the sun as a stream of high-energy electrons and protons at the speedy rate of about 240 miles a second, or about a million miles an hour (London to Paris in about three quarters of a second), bleeding off 10 million tons of hydrogen and helium from our sun every year—a trivial amount, considering its mass. This solar wind reaches Earth in about two to three days, before passing on through the solar system. It hits us in a spiral shape, mostly because of the sun's own rotation. This spiral has been given the homely moniker "the sprinkler" because it rather resembles the spray pattern of an ordinary lawn sprinkler. The sun's magnetic influence also pushes Earth's own magnetic field into a cylindrical shape, facing away from the sun— the solar wind compresses the sunward side of the magnetosphere downward close to Earth, to within six to ten times the radius of the earth, and conversely drags out the night-side magnetosphere to perhaps one thousand times Earth's radius. This end of the spiral is called the magneto tail.
The sun's magnetic energy doesn't penetrate all the way to the planet's surface, because one magnetic field cannot easily penetrate another. Earth's own magnetism deflects that of the sun, thereby helping to keep us alive. If our planet's core was made of, say, aluminum, we'd all be dead.
Within the magnetosphere are two doughnut-shaped radiation belts, sometimes known as the Van Allen belts after NASA scientist James A. Van Allen, whose Geiger counter on the probe Explorer I discovered them in the 1950s.
The inner belt, which circles Earth above the equator, is populated, in the scientific jargon, by high-energy electrons, mostly caused by cosmic rays, that readily penetrate spacecraft and that can, on prolonged exposure, damage both instruments and the people who use them. This belt is not quite a neat circle; the offset between Earth's true north and magnetic poles causes it to reach downward to about 150 miles above the Atlantic near Brazil, causing the South Atlantic Anomaly, a kind of Bermuda Triangle of near space. Low-orbiting satellites frequently pass through it, and are frequently damaged and sometimes fatally zapped as a consequence.
The outer belt is less hazardous but also less stable, and subject to electronic and magnetic storms. It tends to wax and wane with the sun's eleven-year sunspot cycle.
These Van Allen belts, it seems obvious to the conspiracy buffs who populate the Web's wilder shores, make it perfectly clear that the penetration of space has never happened, that man has never been to the moon, and that the Russian space station is really just a set built in the Nevada desert by Russian Communists and their American stooges. If gamma rays lurk in the belts and can kill astronauts, then obviously you would need tons of lead shielding to survive them, and so just as obviously no one has ever been through them alive—QED.
The facts, as they usually are in these cases, are rather less dramatic. It's true that both manned and unmanned spacecraft tend to stay out of the Van Allen belts if they can, but it is equally true that traveling at speed through the belts would yield a radiation exposure of about i rem (at 25 rem you start to show symptoms; at 100 you are dead). The principal hazard in the belt is not cancer-inducing gamma or X-rays, which readily penetrate most substances, as Superman could have told you, but high-energy electrons and protons, which are not difficult to shield against. In addition, the belts span only about 40 degrees of Earth's latitude, 20 each side of the equator, and so if the path of a spacecraft were inclined 30 degrees to Earth's equator, it would bypass all but the edges of the belts.
So, as with the ionosphere, why should we care? Astronauts need to care, but do we? How do the radiation belts affect the air, and therefore the winds, and therefore the climate, if at all?
We know that magnetic storms have caused current surges in power lines, causing blackouts. They also cause static interference that interrupts radio and television signals and cause dead zones for cell phones, to the annoyance of teenagers everywhere. More seriously, they cause air and marine navigation instruments to behave erratically. They can also change—and damage—the ozone layer that absorbs harmful ultraviolet radiation from the sun.
But it goes beyond this. It is becoming clearer that the sun's activity affects short-term weather patterns and perhaps long-term climate trends. Current thinking is that the solar wind has only a minor long-term effect on climate, although no one really knows, and a great deal of earnest activity is underway to uncover the truth. But with the wind and the weather, the evidence is rather different. As NASA's Dr. James Green puts it, "Changes in the magnetosphere seem to be transmitted to the lower atmosphere, where they may influence the circulation of air masses. If we can discover the physical links between these two regions of our environment that trigger weather and climate changes, we can better predict and prepare for our weather."8
Thus the theater of the air, the stage setting for wind. Wind itself is a product of solar radiation, but it is shaped and affected by planetary rotation, pressure differentials, and by air in all its multiple movements, patterns and paths. And air in turn is affected by ionization, radiation, magnetism, and the cosmic wind. No wonder the ancients found air ineffable and wind mysterious.