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.
“Spires on the Skyline”
Broadcast towers are perhaps the single most obvious technological artifact of modern life. At a naive glance, they seem to exist entirely for their own sake. Nobody lives in them. There’s nothing stored in them, and they don’t offer shelter to anyone or anything. They’re skeletal, forbidding structures that are extremely tall and look quite dangerous. They stand, usually, on the highest ground available, so they’re pretty hard not to notice. What’s more, they’re brightly painted and/or covered with flashing lights.
And then there are those things attached to them. Antennas of some kind, presumably, but they’re nothing like the normal, everyday receiving antennas you might have at home: a simple telescoping rod for a radio, a pair of rabbit ears for a TV. These elaborate, otherworldly appurtenances resemble big drums, or sea urchin spines, or antlers.
In this column, we’re going to demystify broadcast towers, and talk about what they do, and why they look that way, and how they’ve earned their peculiar right to loom eerily on the skyline of every urban center in America.
We begin with the electromagnetic spectrum. Towers have everything to do with the electromagnetic spectrum. Basically, they colonize the spectrum. They legally settle various patches of it, and they use their homestead in the spectrum to make money for their owners and users.
The electromagnetic spectrum is an important natural resource. Unlike most things we think of as “resources,” the spectrum is immaterial and intangible. Odder still, it is limited, and yet, it is not exhaustible. Usage of the spectrum is controlled worldwide by an international body known as the International Telecommunications Union (ITU), and controlled within the United States by an agency called the Federal Communications Commission (FCC).
Electromagnetic radiation comes in a wide variety of flavors. It’s usually discussed in terms of frequency and wavelength, which are interchangeable terms. All electromagnetic radiation moves at one uniform speed, the speed of light. If the frequency of the wave is higher, then the length of the wave must by necessity become shorter.
Waves are measured in hertz. One hertz is one cycle of frequency per second, named after Heinrich Hertz, a nineteenth-century German physicist who was the first in history to deliberately send a radio signal.
The International Telecommunications Union determines the legally possible uses of the spectrum from 9,000 hertz (9 kilohertz) to 400,000,000,000 hertz (400 gigahertz). This vast legal domain extends from extremely low frequency radio waves up to extremely high frequency microwaves. The behavior of electromagnetic radiation varies considerably along this great expanse of frequency. As frequency rises, the reach of the signal deteriorates; the signal travels less easily, and is more easily absorbed and scattered by rain, clouds, and foliage.
After electromagnetic radiation leaves the legal domain of the ITU, its behavior becomes even more remarkable, as it segues into infrared, then visible light, then ultraviolet, Xrays, gamma rays and cosmic rays.
From the point of view of physics, there’s a strangely arbitrary quality to the political decisions of the ITU. For instance, it would seem very odd if there were an international regulatory body deciding who could license and use the color red. Visible colors are a form of electromagnetism, just like radio and microwaves. “Red” is a small piece of the electromagnetic spectrum which happens to be perceivable by the human eye, and yet it would seem shocking if somebody claimed exclusive use of that frequency. The spectrum really isn’t a “territory” at all, and can’t really be “owned,” even though it can be, and is, literally auctioned off to private bidders by national governments for very large sums. Politics and commerce don’t matter to the photons. But they matter plenty to the people who build and use towers.
The ITU holds regular international meetings, the World Administrative Radio Conferences, in which various national players jostle over spectrum usage. This is an odd and little-recognized species of diplomacy, but the United States takes it with utter seriousness, as do other countries. The resultant official protocols of global spectrum usage closely resemble international trade documents, or maybe income-tax law. They are very arcane, very specific, and absolutely riddled with archaisms, loopholes, local exceptions and complex wheeler-dealings that go back decades. Everybody and his brother has some toehold in the spectrum: ship navigation, aircraft navigation, standard time signals, various amateur ham radio bands, industrial remote-control radio bands, ship-to-shore telephony, microwave telephone relays, military and civilian radars, police radio dispatch, radio astronomy, satellite frequencies, kids’ radio-controlled toys, garage-door openers, and on and on.
The spectrum has been getting steadily more crowded for
decades. Once a broad and lonely frontier, inhabited mostly by nutty entrepreneurs and kids with crystal sets, it is now a thriving, uncomfortably crowded metropolis. In the past twenty years especially, there has been phenomenal growth in the number of machines spewing radio and microwave signals into space. New services keep springing up: telephones in airplanes, wireless electronic mail, mobile telephones, “personal communication systems,” all of them fiercely demanding elbow-room.
AM radio, FM radio, and television all have slices of the spectrum. They stake and hold their claim with towers. Towers have evolved to fit their specialized environment: a complex interplay of financial necessity, the laws of physics, and government regulation.
Towers could easily be a lot bigger than they are. They’re made of sturdy galvanized steel, and the principles of their construction are well-understood. Given four million dollars, it would be a fairly simple matter to build a broadcast tower 4,000 feet high. In practice, however, you won’t see towers much over 2,100 feet in the United States, because the FCC deliberately stunts them. A broadcast antenna atop a 4000-ft tower would hog the spectrum over too large a geographical area.
Almost every large urban antenna-tower, the kind you might see in everyday life, belongs to some commercial entity. Military and scientific-research antennas are more discreet, usually located in remote enclaves. Furthermore, they just don’t look like commercial antennas. Military communication equipment is not subject to commercial restraints and has a characteristic appearance: rugged, heavy-duty, clunky, serial-numbered, basically Soviet-looking. Scientific instruments are designed to gather data with an accuracy to the last possible decimal point. They may look frazzled, but they rarely look simple. Broadcast tower equipment by contrast is designed to make money, so it looks cheerfully slimmed-down and mass-produced and gimcrack.
Of course, a commercial antenna must obey the laws of physics like other antennas, and has been designed to do that, but its true primary function is generating optimal revenue on capital investment. Towers and their antennas cost as little as possible, consonant with optimal coverage of the market area, and the likelihood of avoiding federal prosecution for sloppy practices. Modern antennas are becoming steadily more elaborate, so as to use thinner slices of spectrum and waste less radiative power. More elaborate design also reduces the annoyance of stray, unwanted signals, so-called “electromagnetic pollution.”
Essays. FSF Columns Page 11
