My father once told me that when he was a boy in Northern Ontario, he could hear, on very cold, still winter nights, the faint sound of the northern lights, a sort of dry, swishing sound. Since then I’ve talked to many a northerner who’ve said likewise, that the auroras sometimes whisper. I’ve also heard an old trapper say that if you whistle, the lights will respond by changing shapes. He said that if you were lucky you could even “call them down” and they will form above you. I’m not sure about that. A human whistle, even a loud one, can barely be heard more than a mile away, and auroras are more than 40 miles above the Earth’s surface. Besides, there’s not enough atmosphere up there to carry sound waves. Nevertheless, I do like the magical, intimate notion of communicating with these great, cosmic light shows as if they were some sort of entity.
A recent discovery has determined that the auroras have a younger brother called Steve (for strong thermal emission velocity enhancement). Discovered by aurora chasers in Alberta in 2017, Steve appears as a slightly curved vertical ribbon of white light in the sky that sometimes accompanies the northern lights. The ribbon is actually a stream of wildly hot gases (3,000°C), flowing at a speed of 770 miles per hour. Of course, Steve has been there all along; it’s just that new high-resolution night photography has teased him out of the background.
These extravaganzas take place in a surprisingly rarefied atmosphere, like the vacuum inside a fluorescent light. Hundreds of miles above the Earth’s surface, the few remaining atoms make up a gas so fleetingly insubstantial and diaphanous that it evaporates into outer space. This phenomenon is called Jeans escape, after the English astronomer James Jeans who first predicted this process. (The escape of atmosphere, mostly in the form of hydrogen atoms, is a negligible trickle now, but as the sun gets brighter — the sun brightens by 10 percent every billion years — the process will accelerate until the atmosphere disappears.) Once the atoms are in space, they are subject to solar wind, a force that has no effect whatsoever at sea level due to the protection of our atmosphere. Indeed, at such heights and with sparse populations, individual atoms of air get blown into space beyond the reach of Earth’s gravity. They become interplanetary travelers bound for the farther reaches of the solar system.
You’d think the thermosphere would be the final edge of the atmosphere, but you’d be wrong. It seems scientists just don’t know when to stop. They’ve added a final layer, the exosphere, which goes from the top of the thermosphere, 430 miles above Earth, to 6,200 miles into space. There are so few atmospheric particles in this region that they rarely collide. We wouldn’t even know about the exosphere, but some overzealous engineer designed a particle detector so sensitive it could detect gas particles in parts per billion. Scientists found room for his device in a satellite bound for another planet. It detected occasional atmospheric particles up to 6,200 miles away from Earth. So the exosphere came to be. It’s a little like calling the parking lot beside a baseball stadium the outer outfield, but no one’s ever going to hit a ball that far.
3
Cloud Nine
Inside the Misty Giants above Our Heads
“Cloud nine gets all the publicity, but cloud eight is actually cheaper, less crowded and has a better view.”
George Carlin
Clouds are weather. Without clouds, there would be no rain, snow, hail, sleet, tornadoes, hurricanes, typhoons, monsoons, lightning, floods, fog or rainbows. What would be left? Just wind and temperature. In a cloudless world, the evening news forecaster would be reduced to sunrise and sunset times, daily highs and nightly lows capped off with wind speed and direction. If, in a world without clouds, there was any moisture in the air at all, dew would be the sole form of precipitation. The only silver lining for news forecasters with a taste for theater would be dust storms, footage of oversize dust devils and extreme daily temperature variations. A cloudless world would be a dry world. Think the Sahara, where daytime highs of 38°C can be followed by nighttime lows of 0°C.
Insubstantial factories of infinite forms, clouds are both ephemeral and powerful, and though conjured out of practically nothing, out of ungraspable mist, they can shake the earth with thunder if they have a mind to. Like Joni Mitchell, those of us in the developed world who have flown have seen clouds from both sides, but unlike Joni claims in her song we do know what they are. For one thing, we know they’re made of water vapor. But not many people are aware that the water vapor that makes up clouds is not like the mist from a sprayer nozzle or steam from a teakettle. Each droplet of water in a cloud is much, much smaller. It is only a millionth of a millimeter in diameter. Millions would fit into the period at the end of this sentence, and when billions upon billions of them congregate, they build the huge, fantastic shapes we call clouds. That’s also why a typical cloud — a puffy, small fair-weather cumulus like the one William Wordsworth wrote about in his poem “I Wandered Lonely as a Cloud” — measuring a few hundred yards cubed, contains only a bathtub’s worth of water.
Yet cloud droplets aren’t the smallest form of aerosolized water. They coalesce from even smaller, airborne molecules of evaporated water much less than a millionth of a millimeter in diameter. The warmer the air, the more evaporated water it can hold; the cooler the air, the less evaporated water it can hold. As a matter of fact, there comes a limit, the dew point, when relative humidity levels reach 100 percent, or complete saturation. Only then can cloud droplets form. (Well, actually, there’s one more necessity: cloud droplets need microscopic particles of dust to seed them — without dust, there would be no clouds.) The dew point can be hundreds to thousands of feet above the ground. Or it can be just a few inches. Fog is a cloud creeping on its belly.
You can see the dew point and relative humidity in action when you watch the contrail of a high-flying jet. They are literally seeding clouds, providing the microscopic particles necessary for water vapor to cling to. Some days, if the relative humidity is low, the contrails evaporate instantly. On other days, when the relative humidity is higher, they linger in long, tubular clouds. They form at that magic juncture of water vapor saturation and temperature, and in the constantly changing atmosphere every layer, every region, has a distinctive level of humidity. Cumulus clouds illustrate all of this.
Anatomy of a Cloud
Here at the surface of the Earth, we live in a narrow layer of warmth, like minnows in the shallows of a summer lake. Passengers in hot air balloons know this well — the higher they rise, the colder it is. Even on a hot summer day, balloonists with just a few thousand feet under their belts begin to feel the distinct chill of altitude. Ten miles up, the temperature never rises above -40°C; that’s why blankets are stowed aboard ballons and airliners..
This vertical decrease of temperature is called the lapse rate, and it’s caused by two factors: thinner air holds less heat, and the higher you rise, the farther you are from the radiant heat of the ground, which is warmed by sunlight. On average, the temperature drops 3°C for every 1,000 feet of altitude. There’s a difference of approximately 18°C between the base and peak of a 9,000-foot mountain. A warm spring afternoon in the valley translates into a freezing afternoon at the summit, which is why high mountains have snow on them all year round. It follows that most cloud bases, which start at 3,280 feet, must already be fairly cool.
Yet there’s a sleight of hand, a sort of cover-up going on with the clouds. A cumulonimbus cloud, like the ideal one in the animated logo at the beginning of United Artists films, is an excellent example. Stretching from just above the Earth all the way to the stratosphere, a single cumulonimbus spans three critical thresholds. The dew point is the flat base of the cloud. Below that line, the air is saturated but not yet to 100 percent. Above that line, called the lifting condensation level, it is. You could take a seven-year-old by the hand and point and say, “See where the bottom of the cloud forms? That’s the dew point.” It’s so clear.
What’s not obvious is the freezing line. Let’s imagine
that our cumulonimbus cloud has formed on a hot summer day with a ground temperature of 30°C. If we apply the lapse-rate formula, then the elevation at which the temperature drops below freezing is approximately 16,400 feet above the ground. And here’s the riddle: why isn’t there any demarcation that indicates where a cloud goes from a liquid to a solid? Seeing a whole eight-mile-high cumulonimbus from several miles away, you’d think you would see a line, like the dew point, or perhaps a color difference, but there’s nothing. And that’s because there is none. There’s certainly a threshold above and below which precipitation — rain or snow or hail — is either frozen or liquid, but not the fabric of the cloud itself. Why? It all boils down to surface tension, the “skin” that forms between water and air. The smaller the body of water, the greater the surface tension. A dime-sized puddle of water on the floor has rounded edges, while a bead of dew on a leaf is almost spherical. The tiny water droplets that form clouds are so small that their high surface tension prevents them from freezing. Cloud droplets remain liquid right down to -40°C. Only then do they freeze. (Unless they’re sprinkled with a little silver iodide . . . but more on that later.)
This subthermal property of cloud droplets explains why the dew point has the same effect under subzero conditions. Cirrus clouds, at 18,000 feet and above, undergo their whole life cycle, from formation to evaporation, in subzero temperatures. In fact, when a grand cumulonimbus turns into a storm cloud and builds up until it reaches the bottom edge of the stratosphere, some 42,240 to 52,800 feet up, it smears out sideways, like smoke trapped under a glass ceiling. This is where the top of the troposphere meets the lower stratosphere, neatly delineated for us. Here the shearing winds of the lower stratosphere flatten and stretch the top of the cumulonimbus into a long cirrus cloud of frozen vapor. From a distance, the profile of the entire cloud resembles an anvil, hence the name cumulonimbus incus (incus is Latin for anvil). It’s the ultimate storm cloud. Inside, it’s pure bedlam.
Getting a Handle on Clouds
The ancient Greeks always seem to be the first to propose anything like a rational explanation for natural phenomena, and, as always, without scientific instruments, their speculations were guided by logic and observation. One of the earliest Greeks to wrestle with the enigma of clouds was Anaximander (610–546 BCE). He proposed that lightning was a product of friction within clouds (which, in a way, it is, if you think of it as analogous to the accumulation of static electricity on the surface of a balloon rubbed over someone’s hair) and that wind was “a flowing of air,” being moved by unknown forces. Two for two.
Later, early in the fourth century BCE, the philosopher Theophrastus from the island of Lesbos thought that the shape of clouds could be used to forecast weather. He observed, “If in fair weather a thin cloud appears stretched in length and feathery, the winter will not end yet,” and, “Fear not as much a cloud from the land as from ocean in winter; but in summer a cloud from a darkling coast is a warning.”
It was Democritus (460–370 BCE) who, along with his prescient theory of atomic particles, first speculated on how clouds were formed. He wrote that the vapors from melting snow and ice (he had traveled to northern Europe) were carried aloft by the wind southward until they hovered over the lakes that fed the Nile, then they fell as rain. This extraordinary speculation not only correctly described the water cycle of evaporation, recondensation and rainfall but also hinted at the vaster systems of prevailing winds and seasonal weather patterns.
Aristotle (384–322 BCE) was the first philosopher-scientist to write a meteorological treatise. It was based on his broader theories that the world was in constant flux and everything we see, everything around us, boiled down to the interplay between earth, air, fire and water — with earth at the center surrounded by water, then air, and then the outer sphere of fire. This is why, at the beginning of his meteorological musings, he wrote that the key question about clouds is why do they “not form in the upper air as one might on the face of it expect?” He answered the question by pointing out that the sun’s heat reflecting off the ground prevents clouds from forming there, and that they do not form too high in the atmosphere because there is too much fire there. Thus, clouds form in the happy medium zone — the middle atmosphere — between two hot spots. It’s a logical speculation, though if he’d known about the lapse rate he’d have had to change his theory.
The Romans tackled clouds as well. One of the earliest was Lucretius (99–55 BCE) who, with startling insight, suggested that clouds began as a “sudden coalescence, in the upper reaches of the sky, of many flying atoms of relatively rough material, such that even a slight entanglement clasps them firmly together.” It was as close to the concept of the dew point, where water vapor in the form of humidity condenses into water droplets around particles of dust, as any thinker in antiquity got.
Seneca (4 BCE–65 CE), a great observer of Roman society, also wrote about weather and clouds. In his Natural Questions, a 10-volume treatise on natural history, Seneca focused on the changeability of the atmosphere and weather: “now rainy, now clear, now a varied mixture between the two. Clouds — which are closely associated with the atmosphere, into which the atmosphere congeals and from which it is dissolved — sometimes gather, sometimes disperse, and sometimes remain motionless.” He correctly viewed clouds as indivisible from the weather.
The famous Roman natural historian Pliny the Elder (23–79 CE) wrote about weather around the same time, iterating Democritus’s view of the atmosphere playing host to a great cycle of water vapor rising and falling. “Steam falls from on high and again returns on high,” which, in essence, is another very concise description of the water cycle. He was on the right track, but it took almost two millennia before a more scientific view of clouds, and cloud formation, was put forward during the Renaissance by the French philosopher René Descartes. In his Discourse on Method (1637), Descartes proposed a methodology that, if followed correctly, would lead to the complete understanding of any natural phenomenon. Clouds became an example for him.
He wrote that humankind had always mythologized clouds; that because of their lofty abode, they had come to be associated with deities, and Descartes decided it was time to bring them down to earth. The elusive clouds were a natural target for his rationalism; they were so changeable, so ephemeral, so seemingly unclassifiable.
He got a lot of the science right. He speculated that clouds were composed of water droplets or small particles of ice that coalesced into “little heaps, and these gather’d together compose vast Bulks.” They stay aloft because they are “so loose and spungy, that they cannot by their weight overcome the Resistance of the Air.” But if they keep coalescing, he went on to say, they wouldn’t be able to resist the pull of gravity and would then fall as rain or snow. Except for a few particulars, he was on the money.
Curator of the Clouds
Almost 200 years later, Luke Howard (1772–1864) turned his attention to meteorology. As a 16-year-old living with his Quaker family in Stamford Hill (now a part of North London), he built in his back garden a weather observation station consisting of a thermometer, a rain gauge and a recording barometer. He took readings twice a day and kept records in a journal, listing wind direction, air pressure, rainfall and maximum and minimum temperatures. Four years later, he was working as a chemist and apothecary in partnership with William Allen, the founder of a scientific club called the Askesian Society.
Public interest in science was burgeoning, and England was chockablock with gentlemen’s clubs and learned societies, mostly populated by well-heeled amateurs, many of them natural historians. The Askesian Society was perhaps one of the most eccentric of these societies. Like other clubs of the period, its members attended lectures delivered by fellow members and guests. These usually consisted of the reading of learned papers, as well as popular experiments, often a showy demonstration of a chemical reaction.
Of all the phenomena the scientific world presented to th
e Askesian Society, the members were most curious about psychoactive substances, particularly nitrous oxide. William Allen claimed it had “a remarkably inebriating effect.” One of the popularizers of nitrous oxide (now known to us as laughing gas), Humphry Davy, became so addicted to “the gaseous oxide of azote,” as it was then exotically referred to, he confessed to taking it “three or four times a day.” So it was a jolly group of neophyte scientists. Very unlike a Quaker congregation.
Luke Howard had tried nitrous oxide along with the other members of the Askesian Society, but his central fascination was meteorology. Howard wished to do for clouds what Carl Linnaeus (1707–1778), the Swedish wunderkind naturalist, had done for zoology: namely to develop an overarching classification system for all cloud types.
It was only natural for Howard to emulate Linnaeus. He was universally regarded as one of the greatest scientists of the eighteenth century. Linnaeus’s Systema Naturae, published in 1735, was an astounding advancement in zoological nomenclature. In it, he introduced a binomial, Latinate system of describing species that we still use today. We humans, for example, are famously Homo sapiens, where homo is the genus and sapiens is the subspecies. The Linnaean classification system is so comprehensive that every living thing, and everything that has ever lived, including all fossils, has a place within in it. Even undiscovered species will have a place in the system. Without Linnaeus, Darwin couldn’t even have begun his theory of evolution. Johann Wolfgang von Goethe once said of Linnaeus, “With the exception of Shakespeare and Spinoza, I know no one among the no longer living who has influenced me more strongly.”
18 Miles Page 4
