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.