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."