Photos by "Bob".

The Natural History of Clouds

Fred Hapgood

As best Jack Borden can recall, he was standing on a hill just outside Boston when he first felt the sky calling for his attention. He had felt the same importuning thousands of times before, as have we all, perhaps on behalf of a bird or a plane or the weather. There was nothing rare or unusual about the sky or his mood or surroundings that day.

Nonetheless, when he looked up, something gathered him, something lifted him up. In that moment his biography opened to a new chapter. "I am never at any time, never at any time, oblivious to the fact that I was once oblivious to the sky," he says now, his voice tense, driving, a bit harsh. "That is forever in my mind. The before and afterness of my awareness of the canopy above."

For the next few weeks Borden, who was a reporter for a local TV station, tried to integrate this epiphany with his career by suggesting sky angles during story conferences. But television has to follow the culture, not lead, and the interest of the news director stayed sluggish. So, incredibly, Borden quit: he walked right out of the glamour and comforts of television work and started a career as press agent to the clouds, or, as he puts it himself, "the Johnny Appleseed of the Sky". He compiled an activity guide for classrooms, published a laminated chart of cloud types, scripted public spots for radio, wrote a stream of articles connecting sky awareness with science, math, religion, music, environmental studies, art, literature, and dance, and began speaking to anyone who would listen. Sixteen years later he is still at it, still laying it out to environmental groups, planetarium directors, homeschoolers, pilot societies, churches, day care administrators, mental health professionals, arts councils, prisons, and huge numbers of schools, to list just the obligations from a few pages of his jampacked dayscheduler. "You.  Gotta.  Hammer. Away.  All.  The.  Time." he says, glowing with intensity.

The day we met the sky was deep and bright. A tangle of gauzy puffs and spun wisps were driving out into the Atlantic high overhead. They looked a bit like sweepings from a makeup table, a collection of white eyebrows and eyelashes: curly and straight, silky and cobbled. At first glance these markings seemed as static as fine strokes of chalk on a blue bowl, but after a minute of quiet looking the sky started swinging like a giant mobile. Curves kinked and twisted; forms stretched and unrolled. After we had watched for a while, I asked Borden to summarize the gospel. He held out his arms, looked up in his Corning Serengetis (in his judgment, the best sunglasses for looking at clouds), and said, "there's a good time to be had by all, everywhere, always, for free".

The citizens of ancient Greece believed that clouds were the thoughts and feelings of Zeus himself, that when they looked up at the sky they were seeing directly into the Divine mind. The priests of Israel taught that the sight of cloud substance, lying as it did halfway between the material and spiritual realms, was as close as mortals could come to seeing the face of God. The great American landscape painters of the last century, Frederic Church, Thomas Cole, and Alfred Bierstadt, were fascinated by the skies and painted them obsessively. "I go forth each afternoon and look into the west a quarter of an hour before sunset, with fresh curiosity ..." Thoreau wrote in his notebooks. "Can Washington Street or Broadway show anything as good? Every day a new picture is painted and framed, held up for half an hour, in such lights the Great Artist chooses, and then withdrawn, and the curtain falls."

Today, as Borden testifies fervently, frequently, and fluently, the status of clouds has fallen. Americans spend millions to go to the Grand Canyon or rain forests in search of exotic, untouched landscapes, when the biggest, least modified, and most varied of them all is right overhead. Communities lay out tax money for fireworks displays when they could celebrate every day with a display more brilliant and imaginative than any they could buy. Schools spend millions on educational materials when the most thematically rich material of them all, boasting natural connections to science, math, art, reading, geography, even music, is free for the price of an upward glance. "99% of the inquiries I get are from teachers looking for a weather unit," Borden says, wincing, slumping, looking downward, the very picture of rejection. "People ask me if I'd do it all over again. I don't take that question lightly. I never dreamed it would be this hard."

The sky activist has not settled on any one theory of how we came to this pass. Perhaps secularism stripped the sky of its spirituality, or perhaps we just lost notice when the bulk of the culture moved off the farms and ocean into built structures. It cannot help either that the environment to which clouds act and react is so transparent. Watching a cloud is like watching an animal in an invisible habitat. We see the creature appear, advance, sniff, pause, twist, and whirl, but we never see the context, the stimulus. Clouds seem to form, fly about, alter, and vanish as spontaneously and freely as thoughts or dreams. Perhaps -- this is just a guess -- the contemporary mind prefers its visual narratives to be more linear, organized, and step-by-step than any story it is likely to see in the sky.

Suppose it was possible to give the contemporary mind a little help, perhaps with a pair of glasses that visualized the thermal structure of the sky directly. (They would have to be magic glasses, since no such instrument exists in real life.) Suppose one morning, when the sun was just beginning to warm the ground after a cool night, we throw open the door, step out onto the lawn, and put these on. We would see multiple streams of long, thin, shimmering, bubbles rising everywhere around us, floating upwards in wavy strings or chains, like seaweed rising from the ocean bottom. (A more physically precise analogy would be to the strings of bubbles that rise from the bottom of a pot of water just before it starts to boil.) Each string is tied to a patch of surface that happens to be warmer than its immediate neighbor. As these warmer patches heat the air directly over them, the warmed volume expands, grows less dense, and lifts. As it rises it sucks cooler air in from the immediate environs, dousing the rising plume at its base. This turns the stream off until the patch warms the air over it up again, creating a series of bubbles instead of a continuous flow. These are called thermals, or convective thermals. Thermals are invisible but not insensible; they can make flying in a light plane feel like a speed run down a rocky hillside. (A big thermal can drop a passenger jet a thousand feet in a few seconds.)

Some patches will be warmer than their neighbors because they absorb sunlight better, like a dark green bush in a light green lawn, but our stroll will probably leave us more impressed with the importance of height differences. The atmosphere is warmed from the bottom, by contact with the sun-warmed surface, like a pot of water on a stove; the higher you go the cooler the air gets (for the first few miles). When something solid pokes up into the air, like a building or a tree, its solar-radiated top is almost as warm as it would be on the surface, but the air around it is cooler. This greater temperature difference makes the thermal rising from the object relatively lighter and gives it more buoyancy, more vigor. If the air is cool and the surface warm even the smallest height differences will make and shed thermals. Rocks and bushes might breathe out streams of bubbles with dimensions measured in inches; cars and trees, in feet; and skyscrapers and mountains, majestic exhalations the size of blimps or larger. As these bubbles rise vertically they interact with a second set of features visible only through magic glasses: horizontal layers of air, stacked up and thrown over each other like rugs or blankets in a salesroom. These layers (also defined by temperature differences) are constantly moving: flowing under, around, and over each other, heaving some layers up and others down and beating on their neighbors with waves like those we see brushing over wheat fields or rippling through flags. They form rivers and pools and channels and valleys and hills, into and around which other layers ebb and flow and drain and flood, from horizon to horizon, all up and down the height of the sky.

This is cloud country, the niche to which clouds are adapted and on which they leave their mark. They certainly thrive here: at any given moment half the globe's surface, 100 million square miles, is covered with them: sheet-like, rounded, quilted, filamentous, lacy, puffy, rippled, striated, tessellated, ragged, sharp, and hazy, in all degrees of luminescence and opacity, blossoming, spreading, sailing, and fading all around the world. Not one cubic centimeter of the atmosphere is motionless; half is always rising and half sinking. When a volume of air lifts it expands, and that expansion cools it. If it rises high enough, expands enough, and cools enough, sooner or later whatever water vapor it might have contained will condense into droplets or crystals and begin to appear as cloud-stuff.

Clouds are a sign that work is being done, a mark of the atmosphere exerting itself to lift millions of tons of air against gravity. The lifting impulses might come from air currents flowing up a slope of land or air, like an ocean wave sliding up a beach or snow curling up over the face of a roadplow, or from horizontal layers being boosted straight up by other layers swelling or rising under them, or from thermals. Air rivers rotate as they flow through the atmosphere like shafts or screws, and sometimes clouds appear on their rising sides. Sometimes a layer of air that is pushed up and over the top of a mountain falls into a region of relatively warm air, pushing it down like a spring. Eventually this compressed air pushes back, throwing the cool parcel up again, like a person bouncing on a trampoline. Each arc marks its zenith with a little cloud cut into an almond shape, like a scuff on the ceiling. If we ever do get real magic glasses this last drama might prove a tourist attraction for mountain resorts, like the kangaroo is for Australia.

Clouds start when a volume of rising air cools down past dewpoint; they top out when that air hits a layer that is either warmer or as warm as itself. (A mass entering a layer cooler than it is will lift right on through.) If the layer is warmer it will act like a cap or cover, forcing the cooler, heavier, mass to spread out under it into a sheet or slab. A string of thermal balloons hitting one of these would behave like dollops of whipped cream falling onto a floor, only upside down. A mass entering a layer with the same temperature as itself will stop rising, float in place, and puff out, as if something was blowing it up from within.

Our magic glasses are of course a metaphor for the accumulated wisdom of modern cloud mechanics, cloud physics, meteorology, and atmospheric physics. These sciences and their discoveries are quite recent. Until the opening of the nineteenth century, clouds didn't even have any names: they were just "essences" of the atmosphere. In the winter of 1802/3 a London pharmacist named Luke Howard suggested a descriptive system based on four cloud types: flat (stratus), puffed (cumulus), wispy (cirrus), and precipitating (nimbus). The great virtue of his terms was that they could be plugged into each other to describe more complex cases: "cumulonimbus" described a precipitating puffy; "cirrostratus", a wispy veil covering the sky; "stratocumulus", a layer of rounded or rolled cloud masses, and so on. Howard's system won immediate acceptance. Years later the great German poet Goethe wrote a commemoratory poem to Howard that included the following lines:

That which no hand can reach, no hand can clasp,
He first has gain'd, first held with mental grasp;
Defin'd the doubtful, fix'd its limit-line,
And nam'd it fitly -- Be the honor thine!
As clouds ascend, are folded, scatter, fall,
Let the world think of thee who taught it all.

The term 'nimbus' in Howard's scheme might seem odd in that stratus, cumulus, and cirrus are matters of appearance while nimbus seem to point to behavior, but in practise 'nimbus' has a visual meaning too. The term does not actually refer to the simple act of releasing liquid or frozen water, since all clouds do that (as soon as a droplet forms it starts to fall). It means clouds whose rain or snow travels all the way to the ground.

This is a rare achievement, since every cloud base is separated from the ground by a gauntlet of warm air (warmer than dewpoint, anyway) that reevaporates most drops falling from non-nimbus clouds. In order for a drop to reach the ground it usually has to gang up inside the cloud with lots of other droplets to make a drop so large and fat that a kernal of it will survive to hit the ground. The taller the cloud, the more room there is for droplets to drift into each other, coalesce, and accumulate the reserves required. A cumulonimbus, or thunderstorm, might tower for six miles or even higher. Thus in terms of actual field usage the labels of Howard's typology reduce to puffy, flat, wispy, and tall.

Howard was part of the great rationalist rebellion against the medieval practice of explaining phenomena with "essences" that predisposed them to do this or that. His philosophy was that clouds had physical histories and contexts that could be traced, discovered, and understood. For example (in contemporary terms), the wispiness of cirrus clouds is connected to the high altitudes at which they form. The less water vapor an air mass is carrying the cooler it has to be before that vapor will start to condense. A cloud appearing at a high altitude implies an air mass that had to rise, expand, and cool quite a lot before passing dewpoint, and therefore contained very little water vapor to begin with. Thus one reason why high altitude clouds are so loosely woven, so lacy, is that they don't have as much aqueous material to work with as the thick, plump, opaque, low-altitude cumulus or stratus clouds. Also, the water vapor in high-altitude clouds tends to condense as ice crystals rather than droplets. Crystals resist reevaporation better than droplets, which means that when they fall out of their cloud they fall quite far, leaving long, twisting, trails that look like loose gatherings of hair or fibrous streams. Sometimes these plumes of crystals fall through air currents flowing in different directions, which from the ground gives them a curved or kinked line, like a bit of handwriting. In short, clouds are part of an interacting sequence of physical events that includes them and their neighborhoods. They connect to the world; they make things happen.

Luke Howard's system is still the foundation of cloud systematics, though the tree has of course grown more complex over the last two centuries. (The World Meteorological Organization recognizes ten genera, twenty-five species, and thirty-odd varieties of cloud names, plus intermediate and transitional forms, supplementary features, accessory clouds, mother-clouds, and "special" clouds, clouds that contain non-aqueous material.)

The rationalist perspective has found even richer rewards. When scientists trace back the cascade of interactions and exchanges they find that clouds are connected to more than just their local circumstances: it is possible they govern the energy budget of the planet. The speculation is that as the earth heats up and more water is lifted into the atmosphere, a higher proportion of the planet's cloud cover is changed from thin wispy cirrus clouds into thick, opaque, low-altitude clouds. These reflect more sunlight back into space, cooling the earth homeostatically. While this idea is still unproved, it is historically the case that the average temperature of the Earth has fluctuated within a fairly narrow range, relative to much more dramatic changes in the output of the sun. It certainly looks as though there is a thermostat in the system somewhere.

In the 80's analysts looking at photos of ocean clouds taken from orbit saw straight lines of lightness appearing across them, as though someone was drawing on them with a giant laser. Investigation proved these lines were being made from underneath, by ships. The sulphur in their diesel exhausts was rising into the clouds and introducing chemical reactions that made the cloud droplets smaller and brighter. (The larger a drop, the more energy it absorbs from a light ray passing through it.) This raises the question of whether atmospheric pollution might not be brightening the backs of clouds all around the world, reflecting more light, and thus contributing to a global cooling effect. Other calculations have suggested that when polluted clouds evaporate they leave behind a 'waste' of sulfate particles that stay in the atmosphere for up to a week. These would also have a back-scattering, cooling effect, only in clear air. If global warming exists, these effects might balance it out; it not, maybe they will form the grist for a new generation of alarmist literature.

So perhaps, bit by bit, the civilization is finding its way into a new relationship with the sky. In the old days we looked up and "saw" the mind of the Divine; maybe in twenty years we will "see" the infrastructure of the planet, a great engineering wonder, a huge solar-powered, supremely efficient machine regulating the support systems of Earth and which it is our responsibility to maintain. There will be tours, perhaps even theme parks floating in the sky, illuminating this elegant and sophisticated technology, which will be held out as an inspiration for terrestial engineers.

One windy morning the sky was full of cumulus flying along, and I decide to inspect this prospect first-hand. I drove to a small airport, rented a plane, and pointed to the clouds I wanted to visit. The pilot pointed the nose of plane up. We kept underestimating their altitude, which meant we were also underestimating their size. By the time the plane arrived at the right altitude, six thousand feet, the clouds seemed enormous, a hundred feet high by thousands wide and long, like the tops of a mountain range that had been lopped off and sent sailing. They were moving at a brisk 35 knots, but an even brisker wind was blowing over their tops in the same direction, pushing them forward, so their bows were raked like that of a fast motorboat. As we pulled closer we could see faint parallel bars of gold and black slanting down from the bottom of each cloud, as the sun shone down through the clouds, into a shallow beard of rain falling underneath. Complex surfaces of mist tendrils branched out into the air, like milky nerves growing through transparent tissues.

One of the great questions in the early history of cloud science was how clouds managed to stay up, given the obvious weight of the water deposited in rain barrels after a shower. Some workers speculated that the drops in clouds were bubbles with fires inside, like warm air balloons. (The astronomer Halley, discoverer of the comet, even thought some anti-gravity principle might be at work.) Eventually physicists learned there was no problem after all, because clouds didn't stay up the way people thought; as soon as water vapor condenses into a droplet that droplet starts to fall out of the cloud. A cloud that does not replace the water carried away by condensing drops with new supplies of water vapor will rain itself out and vanish in minutes. Still, the theory that clouds stay up because they contain bits of fire is not completely wrong. When water vapor condenses into droplets it is the slowest, coolest water molecules that are gathered up first. When the droplets fall out, into that beard of rain we saw under the cloud, it is the cold, low-energy, molecules that are being dumped overboard. This leaves the molecules with the most energy buzzing around inside the cloud. Clouds do keep themselves hot, not by adding heat, but by subtracting cold.

We found an open space, like a harbor, and flew about, trying to keep track of the landscape. A hanging valley dropped and then folded up; a dome blew up on the horizon and toppled. A cloud fissioned in front of us, like a microorganism. No single feature survived for long. The clouds were mounding and kneading themselves, smoothly and evenly. These changes were driven not by the wind -- the clouds were moving at air speed and therefore felt no wind -- but by the flood of energy being released by the condensation motors running inside, reworking all the surfaces and plumping out lobes like spinnaker sails. If you looked to the west you could see these formations appearing in the sky about fifty miles off, fed by huge thermals rising from a chain of hills.

The thermals rose, clouds formed, and the condensation motors started to run. The motors heated the air inside the cloud (by dumping out the cold molecules) and that air lifted in reaction, sucking up more air from underneath and outside. When the prevailing winds blew the clouds east, away from the hills, this updraft gave the cloud independence, self-sufficiency, by allowing it to graze on the reservoirs of water vapor floating above the eastern half of the state. The condensation motor had turned the cloud into an "acquavore".

Towards the end of my tour the pilot lifted up and circled over the tops of these brilliant air lilies. (Clouds are much brighter on their tops because the uppermost droplets tend to be smaller.) The upwelling in the interiors of clouds is balanced by downdrafts falling around their outside edges, and this current regulates the distance between cumulus clouds, like a kind of social spacing. Clouds not only organize and sustain an internal metabolism, by sucking up vapor, they organize themselves socially as well, into flocks. (The essence of flocking is flying at a constant distance from your neighbor.)

These self-organizing societies had been crossing over continents long before the dinosaurs, long before biological life evolved. For billions of years around the globe they had been sailing out into the world, grazing on the atmospheric vapor, running before the wind, looking for adventure. Perhaps someday we will ride this ancient cycle ourselves, but at this moment our primitive technology was starting to run out of gas, and we had to land.