Essays. FSF Columns

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. Ante

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.