Essays. FSF Columns

"Watching the Clouds"

In the simmering depths of a Texas summer, there are few things more soothing than sprawling on a hillside and watching the clouds roll by. Summer clouds are especially bright and impressive in Texas, for reasons we will soon come to understand-- and anyhow, during a Texas summer, any activity more strenuous than lying down, staring at clouds, and chewing a grass-stem may well cause heat-stroke.

By the early nineteenth century, the infant science of meteorology had freed itself from the ancient Aristotelian dogma of vapors, humors, and essences. It was known that the atmosphere was made up of several different gases. The behavior of gases in changing conditions of heat, pressure and density was fairly well understood. Lightning was known to be electricity, and while electricity itself remained enormously mysterious, it was under intense study. Basic weather instruments -- the thermometer, barometer, rain gauge, and weathervane -- were becoming ever more accurate, and were increasingly cheap and available.

And, perhaps most importantly, a network of amateur natural philosophers were watching the clouds, and systematically using instruments to record the weather.

Farmers and sailors owed their lives and livelihoods to their close study of the sky, but their understanding was folkloric, not basic. Their rules of thumb were codified in hundreds of folk weather-proverbs. "When clouds appear like rocks and towers/ the earth's refreshed with frequent showers." "Mackerel skies and mares' tails/ make tall ships carry low sails." This beats drowning at sea, but it can't be called a scientific understanding.

Things changed with the advent of Luke Howard, "the father of British meteorology." Luke Howard was not a farmer or sailor -- he was a Quaker chemist. Luke Howard was born in metropolitan London in 1772, and he seems to have spent most of his life indoors in the big city, conducting the everyday business of his chemist's shop.

Luke Howard wasn't blessed with high birth or a formal education, but he was a man of lively and inquiring mind. While he respected folk weather-wisdom, he also regarded it, correctly, as "a confused mass of simple aphorisms." He made it his life's avocation to set that confusion straight.

Luke Howard belonged to a scientific amateur's club in London known as the Askesian Society. It was thanks to these amateur interests that Howard became acquainted with the Linnaean System. Linnaeus, an eighteenth-century Swedish botanist, had systematically ranked and classified the plants and animals, using the international language of scholarship, Latin. This highly useful act of classification and organization was known as "modification" in the scientific terminology of the time.

Though millions of people had watched, admired, and feared clouds for tens of thousands of years, it was Luke Howard's particular stroke of genius to recognize that clouds might also be classified.

In 1803, the thirty-one-year-old Luke Howard presented a learned paper to his fellow Askesians, entitled "On the Modifications of Clouds, and On the Principles of Their Production, Suspension, and Destruction."

Howard's speculative "principles" have not stood the test of time. Like many intellectuals of his period, Howard was utterly fascinated by "electrical fluid," and considered many cloud shapes to be due to static electricity. Howard's understanding of thermodynamics was similarly halting, since, like his contemporaries, he believed heat to be an elastic fluid called Caloric.

However, Howard's "modifications" -- cirrus, cumulus, and stratus -- have lasted very successfully to the present day and are part of the bedrock of modern meteorology. Howard's scholarly reputation was made by his "modifications," and he was eventually invited to join the prestigious Royal Society. Luke Howard became an author, lecturer, editor, and meteorological instrument- maker, and a learned correspondent with superstars of nineteenth-century scholarship such as Dalton and Goethe. Luke Howard became the world's recognized master of clouds. In order to go on earning a living, though, the father of British meteorology wisely remained a chemist.

Thanks to Linnaeus and his disciple Howard, cloud language abounds in elegant Latin constructions. The "genera" of clouds are cirrus, cirrocumulus, cirrostratus; altocumulus, altostratus, nimbostratus; stratocumulus, cumulus and cumulonimbus.

Clouds can also be classified into "species," by their peculiarities in shape and internal structure. A glance through the World Meteorological Organization's official *International Cloud Atlas* reveals clouds called: fibratus, uncinus, spissatus, castellanus, floccus, stratiformus, nebulosus, lenticularis, fractus, humilis, mediocris, congestus, calvus, and capillatus.

As if that weren't enough, clouds can be further divvied-up into "varieties," by their "special characteristics of arrangement and transparency": intortus, vertebratus, undulatus, radiatus, lacunosis, duplicatus, translucidus, perlucidus and opacis.

And, as a final scholastic fillip, there are the nine supplementary features and appended minor cloud forms: incus, mammatus, virga, praecipitatio, arcus, tuba,pileus, vella, and pannus.

Luke Howard had quite a gift for precise language, and sternly defended his use of scholar's Latin to other amateurs who would have preferred plain English. However elegant his terms, though, Howard's primary insight was simple. He recognized that most clouds come in two basic types: "cumulus" and "stratus," or heaps and layers.

Heaps are commoner than layers. Heaps are created by local rising air, while layers tend to sprawl flatly across large areas.

Water vapor is an invisible gas. It's only when the vapor condenses, and begins to intercept and scatter sunlight as liquid droplets or solid ice crystals, that we can see and recognize a "cloud." Great columns and gushes of invisible vapor continue to enter and leave the cloud throughout its lifetime, condensing within it and evaporating at its edges. This is one reason why clouds are so mutable -- clouds are something like flames, wicking along from candles we can't see.

Who can see the wind? But even when we can't feel wind, the air is always in motion. The Earth spins ponderously beneath its thin skin of atmosphere, dragging air with it by gravity, and arcing wind across its surface with powerful Coriolis force. The strength of sunlight varies between pole and equator, powering gigantic Hadley Cells that try to equalize the difference. Mountain ranges heave air upward, and then drop it like bobsleds down their far slopes. The sunstruck continents simmer like frying pans, and the tropical seas spawn giant whirlpools of airborne damp.

Water vapor moves and mixes freely with all of these planetary surges, just like the atmosphere's other trace constituents. Water vapor, however, has a unique quality -- at Earth's temperatures, water can become solid, liquid or gas. These changes in form can store, or release, enormous amounts of heat. Clouds can power themselves by steam.

A Texas summer cumulus cloud is the child of a rising thermal, from the sun-blistered Texan earth. Heated air expands. Expanding air becomes buoyant, and rises. If no overlying layer of stable air stops it from rising, the invisible thermal will continue to rise, and cool, until it reaches the condensation level. The condensation level is what gives cumulus clouds their flat bases -- to Luke Howard, the condensation level was colorfully known as "the Vapour Plane." Depending on local heat and humidity, the condensation level may vary widely in height, but it's always up there somewhere.

At this point, the cloud's internal steam-engine kicks in. Billions of vapor molecules begin to cling to the enormous variety of trash that blesses our atmosphere: bits of ash and smoke from volcanoes and forest-fires, floating spores and pollen-grains, chips of sand and dirt kicked up by wind-gusts, airborne salt from bubbles bursting in the ocean, meteoric dust sifting down from space. As the vapor clings to these "condensation nuclei," it condenses, and liquefies, and it gives off heat.

This new gush of heat causes the air to expand once again, and propels it upward in a rising tower, topped by the trademark cauliflower bubbles of the summer cumulus.

If it's not disturbed by wind, hot dry air will cool about ten degrees centigrade for every kilometer that it rises above the earth. This rate of cooling is known to Luke Howard's modern-day colleagues as the Dry Adiabatic Lapse Rate. Hot *damp* air, however, cools in the *Wet* Adiabatic Lapse Rate, only about six degrees per kilometer of height. This four-degree difference in energy -- caused by the "latent heat" of the wet air -- is known in storm-chasing circles as "the juice."

When bodies of wet and dry air collide along what is known as "the dryline," the juice kicks in with a vengeance, and things can get intense. Every spring, in the High Plains of Texas and Oklahoma, dry air from the center of the continent tackles damp surging warm fronts from the soupy Gulf of Mexico. The sprawling plains that lie beneath the dryline are aptly known as "Tornado Alley."

A gram of condensing water-vapor has about 600 calories of latent heat in it. One cubic meter of hot damp air can carry up to three grams of water vapor. Three grams may not seem like much, but there are plenty of cubic meters in a cumulonimbus thunderhead, which tends to be about ten thousand meters across and can rise eleven thousand meters into the sky, forming an angry, menacing anvil hammered flat across the bottom of the stratosphere.

The resulting high winds, savage downbursts, lashing hail and the occasional city-wrecking tornado can be wonderfully dramatic and quite often fatal. However, in terms of the Earth's total heat-budget, these local cumulonimbus fireworks don't compare in total power to the gentle but truly vast stratus clouds. Stratus tends to be the product of air gently rising across great expanses of the earth, air that is often merely nudged upward, at a few centimeters per second, over a period of hours. Vast weather systems can slowly pump up stratus clouds in huge sheets, layer after layer of flat overcast that sometimes covers a quarter of North America.

Fog is also a stratus cloud, usually created by warm air's contact with the cold night earth. Sometimes a gentle uplift of moving air, oozing up the long slope from the Great Plains to the foot of the Rockies, can produce vast blanketing sheets of ground-level stratus fog that cover entire states.

As it grows older, stratus cloud tends to break up into dapples or billows. The top of the stratus layer cools by radiation into space, while the bottom of the cloud tends to warm by intercepting the radiated heat from the earth. This gentle radiant heat creates a mild, slow turbulence that breaks the solid stratus into thousands of leopard-spots, or with the aid of a little wind, perhaps into long billows and parallel rolls. Thicker, lowlying stratus may not break-up enough to show clear sky, but simply become a dispiriting mass of gloomy gray knobs and lumps that can last for days on end, during a quiet winter.

When vapor condenses into droplets, it gives off latent heat and rises. The cooler air from the heights, shoved aside by the ascending warm air, tends to fall. If the falling air drags some captured droplets of water with it, those droplets will evaporate on the way down. This makes the downdraft cooler and denser, and speeds its descent. It's "the juice" again, but in reverse. If there's enough of this steam-power set-loose, it will create vertically circulating masses of air, or "convection cells."

Downdraft winds are invisible, but they are a vital part of the cloud system. In a patchy summer sky, downdrafts fill the patches between the clouds -- downdrafts *are* the patches. They tear droplets from the edges of clouds and consume them.

Most clouds never manage to rain or snow. They simply use the vapor-water cycle as a mechanism to carry and dissipate excess heat, doing the Earth's quiet business of entropy.

Clouds also scour the sky; they are the atmosphere's cleaning agents. A good rain always makes the air seem fresh and clean, but even clouds that never rain can nevertheless clean up billions of dust particles. Tiny droplets carry their dust nuclei with them as they collide with one another inside the cloud, and combine into large drops of water. Even if this drop then evaporates and never falls as rain, the many dust particles inside it will congeal thorough adhesion into a good-sized speck, which will eventually settle to earth on its own.

For a drop of water to fall successfully to earth, it has to increase in size by about one million times, from the micron width of a damp condensation nucleus, to the hefty three millimeters of an honest raindrop. A raindrop can grow by condensation about to a tenth of a millimeter, but after this scale is reached, condensation alone will no longer do the job, and the drop has to rely on collision and capture.

Warm damp air rising within a typical rainstorm generally moves upward at about a meter per second. Drizzle falls about one centimeter per second and so is carried up with the wind, but as drops grow, their rate of descent increases. Eventually the larger drops are poised in midair, struggling to fall, as tiny droplets are swept up past them and against them. The drop will collide and fuse with some of the droplets in its path, until it grows too large for the draft to support. If it is then caught in a cool downdraft, it may survive to reach the earth as rain. Sometimes the sheer mass of rain can overpower the updraft, through accumulating weight and the cooling power of its own evaporation.

Raindrops can also grow as ice particles at the frigid tops of tall clouds. "Sublimation" is the process of water vapor directly changing from water to ice. If the air is cold enough, ice crystals grow much faster in saturated air than a water droplet does. An ice crystal in damp supercooled air can grow to raindrop size in only ten minutes. An upper-air snowflake, if it melts during its long descent, falls as rain.

Truly violent updrafts to great heights can create hail. Violent storms can create updrafts as fast as thirty meters a second, fast enough to buoy up the kind of grapefruit-sized hail that sometimes kills livestock and punches holes right through roofs. Some theorists believe that the abnormally fat raindrops, often the first signs of an approaching thundershower, are thin scatterings of thoroughly molten hail.

Rain is generally fatal to a cumulonimbus cloud, causing the vital loss of its "juice." The sharp, clear outlines of its cauliflower top become smudgy and sunken. The bulges flatten, and the crevasses fill in. If there are strong winds at the heights, the top of the cloud can be flattened into an anvil, which, after rain sets in, can be torn apart into the long fibrous streaks of anvil cirrus. The lower part of the cloud subsides and dissolves away with the rain, and the upper part drifts away with the prevailing wind, slowly evaporating into broken ragged fragments, "fractocumulus."

However, if there is juice in plenty elsewhere, then a new storm tower may spring up on the old storm's flank. Systems of storm will therefore often propagate at an angle across the prevailing wind, bubbling up to the right or left edge of an advancing mass of clouds. There may be a whole line of such storms, bursting into life at one end, and collapsing into senescence at the other. The youngest tower, at the far edge of the storm-line, usually has the advantage of the strongest supply of juice, and is therefore often the most violent. Storm-chasers tend to cluster at the storm's trailing edge to keep a wary eye on "Tail-End Charlie."

Because of the energy it carries, water vapor is the most influential trace gas in the atmosphere. It's the only gas in the atmosphere that can vary so drastically, plentiful at some times and places, vanishing at others. Water vapor is also the most dramatic gas, because liquid water, cloud, is the only trace constituent in our atmosphere that we can actually see.

The air is mostly nitrogen -- about 78 percent. Oxygen is about 21 percent, argon one percent. The rest is neon, helium, krypton, hydrogen, xenon, ozone and just a bit of methane and carbon dioxide. Carbon dioxide, though vital to plant life, is a vanishingly small 0.03 percent of our atmosphere.

However, thanks to decades of hard work by billions of intelligent and determined human beings, the carbon dioxide in our atmosphere has increased by twenty percent in the last hundred years. During the next fifty years, the level of carbon dioxide in the atmosphere will probably double.

It's possible that global society might take coherent steps to stop this process. But if this process actually does take place, then we will have about as much chance to influence the subsequent course of events as the late Luke Howard.

Carbon dioxide traps heat. Since clouds are our atmosphere's primary heat-engines, doubling the carbon dioxide will likely do something remarkably interesting to our clouds. Despite the best efforts of whirring supercomputers at global atmospheric models around the world, nobody really knows what this might be. There are so many unknown factors in global climatology that our best speculations on the topic are probably not much more advanced, comparatively speaking, than the bold but mistaken theorizing of Luke Howard.

One thing seems pretty likely, though. Whatever our clouds may do, quite a few of the readers of this column will be around in fifty years to watch them.