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.