Daddy, Why Are There Clouds?
The question is deceptively simple, but as you'll see, the answer isn't. The author is an instrument-rated pilot who flies out of Gaithersburg, Maryland. He also holds a degree in meteorology.
Beautiful, fluffy ones—dark, roiling, malevolent ones; they add immeasurably to our natural landscape. For non-aesthetic reasons they are important, too. They help regulate the earth's energy balance, by reflecting and scattering solar radiation or absorbing the earth's radiated infrared energy. But they also visually indicate physical processes which shape the weather that we fly through—not the least of which is this thing called atmospheric stability. It's part of what makes those clouds that perhaps some small person has already asked you about—or that perhaps you yourself have wondered about!
How they're formed
You undoubtedly know that clouds form when air rises and cools. When a blob of air goes up into an area of less pressure, it cools. When an unsaturated volume of air cools on ascent, without any energy being added or removed, it follows what is known as the dry adiabatic lapse rate—which is approximately 9.76 Centigrade degrees per Km (or about 5.33 Fahrenheit degrees per thousand feet). When it reaches its dew point temperature, the rising parcel is no longer unsaturated. Water begins to condense. As it does so, it releases copious amounts of heat.
Water in fact has a very high "latent heat." Just as it takes large amounts of energy to boil water, large amounts are given up when it returns to the liquid state—from about 541 calories/gram at 100 degrees C up to about 597 calories per gram at zero degrees C. So some of that cooling off due to its ascent is offset by the latent heat released by the condensing water vapor. The air parcel then cools off at a lower rate—called the moist adiabatic rate. This rate varies with temperature, and is on the order of 6 Centigrade degrees per Km, or about 3.3 Fahrenheit degrees/1000 feet.
(Incidentally, in addition to having a high latent heat, water also has a high specific heat. Just within the liquid phase, it takes plenty of energy to raise or lower the temperature of a given amount of water—witness the pronounced climatological effects wherever there is a large body of water nearby!)
You may also know that several mechanisms can cause air to rise and clouds to form: surface convective heating; orographic lifting; convergence of air masses; and uplift along fronts. What you may not know is that even if none of these things occurred, the fact that a parcel of air might pick up a greater amount of moisture than the surrounding air contains will itself make it rise. This is because the same total number of water molecules (in vapor form) are about one-third lighter than the identical combined number of molecules of nitrogen, oxygen, and other gases in their proper proportions; i.e., air. In reality, even super-steamy jungle air will only hold about 6% of water vapor by weight, so even very humid air will only be about 2% lighter (one-third times 0.06) than absolutely dry air—but it's enough to do the trick. This is one of the larger cogs (the biggest of course being solar heating) in the driving mechanism behind the hydrologic cycle!
The air isn't always rising, though—or if it is, the amount of lifting changes. The reason why hinges on the concept of atmospheric stability. If rising air is colder than surrounding air, it will sink back down: the air is said to be stable because it "resists" displacement. If rising air is warmer than the surrounding air, it will continue to rise (unstable air) until it's not.
When rising air is (or would be) colder than its environment at all levels, it is considered absolutely stable. Such is the case when the environmental lapse rate is less than either the dry or moist adiabatic lapse rates. Picture a nearly-homogeneous air mass where temperature drops little with altitude: In this case, if a parcel is forced to rise, it will always be cooler and heavier than the air around it, and it will spread out horizontally if it can't "get" back down. Any clouds will be thin, spread out, and have flat tops and bases—in other words, stratus clouds.
The atmosphere becomes more stable if air aloft warms (either by sinking and compressing, or via advection of neighboring warmer air)—or if surface air cools (by nighttime radiational cooling, advection of colder air, or contact with a cold surface). Incidentally, that is why you see the most hot air balloons early in the morning, when the lowest surface temperatures are recorded. (The extreme case of this of course is the inversion—when temperatures rise with altitude.)
Don't forget lapse rate!
When the atmosphere's temperature profile shows instead a rapid drop—for example anything greater than even the dry adiabatic lapse rate—all air parcels (even dry ones) will not cool as quickly as the ambient air, and once they "get the chance" to go up, they will always be hotter and lighter than the air around them, and they keep going up. This is an absolutely unstable atmosphere. It is characterized, of course, by cumuliform clouds. This steepening of the environmental lapse rate occurs when air aloft gets colder, or when the surface becomes warmer (by daytime radiative heating, advection of warm air, or conductive heating from below). Does this sound like the familiar summer afternoon thunderstorm scenario?
When the lapse rate is between the moist and dry adiabatic rates, things get interesting. An unsaturated parcel rises (for whatever reason) and cools, first at the dry adiabatic rate. This is greater than ambient, which therefore makes it colder, heavier, and thus, stable—that is, until it reaches its condensation level. Here the air is 100% saturated (i.e., at its dew point). Now above this the air cools at the moist adiabatic rate. Due to the release of latent heat, it cools more slowly than the air around it, which makes it warmer, lighter, and thus unstable. This is conditionally unstable air—which depends on how humid the air is and at what point it becomes saturated.
Wasn't that easy?
This is enough for Meteorology-On-Daddy's-Lap 101. Atmospheric stability does figure prominently in our considerations of weather. You have most likely heard of the National Weather Service's Composite Moisture Stability Chart, issued twice daily: It features its own panel, showing the "lifted index", which reflects the stability of the air over the continental US. (Those glider pilots among you will no doubt have heard of it!) I hope this has given you—or that it will allow you to give someone you know—a better intuitive understanding of the physical processes behind cloud formation.