Why Are There Clouds?

Aside from framing sunsets, or forming an idle afternoon's aerial topiary, clouds serve as signposts of the weather we fly through. Indeed, these manifestations of the weather have interesting origins.

Clouds form when air rises and cools. When air ascends into an area of lower pressure, it cools. Unsaturated air cools on ascent, without any energy being added or removed, following what is known as the dry adiabatic lapse rate - approximately 9.76?C per kilometer, or about 5.33?F per 1,000 feet.

Once it reaches its dew point, the rising air is no longer unsaturated. Water begins to condense, releasing copious amounts of heat. Water has a very high "latent heat." Just as it takes large amounts of energy to boil water, water vapor gives up large amounts of heat when it condenses - from 541 calories per gram at 100?C to 597 calories per gram at 0?C.

Some of the cooling that occurs when air ascends is offset by the latent heat water vapor releases when it condenses. In other words, the air cools at a lower rate - called the moist adiabatic rate. This rate varies with temperature, and is on the order of 6?C per kilometer, or about 3.3?F per 1,000 feet. (Incidentally, water also has a high specific heat. 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.)

Several mechanisms cause air to rise and clouds to form. They are surface convective heating (the sun heats the surface, which heats the air and makes it rise); orographic lifting (the wind blows against a surface feature, such as mountains, which causes the air to rise); convergence of air masses; and uplift along fronts. Surprisingly, even if none of these things occur, the fact that a parcel of air might pick up more moisture than the surrounding air contains, will itself make it rise.

This happens because the water molecules (as vapor) are about one-third lighter than the identical combined number of molecules of nitrogen, oxygen, and other gases that we call air. Even super-steamy jungle air will only hold about six percent of water vapor by weight, so very humid air can only be about two percent lighter (one-third times 0.06) than absolutely dry air - but it's enough to make it rise. This is one of the biggest cogs (the biggest of course being solar heating) in the mechanism that drives the hydrologic cycle!

The air isn't always rising, though - or if it is, the amount of lifting changes. Whether or not the amount of lifting changes hinges on the concept of atmospheric stability. If a volume of air is colder than surrounding air, it will sink. The air is called "stable" because it resists displacement. If a volume of air is warmer than the surrounding air, it will rise until the temperatures of the rising and surrounding air equalize. This air is called "unstable" because it's on the move.

When rising air is (or would be) colder than its environment at all levels, it is considered absolutely stable. This happens when the environmental lapse rate is less than either the dry or moist adiabatic lapse rates (ALR). Picture a nearly-homogeneous air mass, where temperature drops little with altitude.

In this case, if something causes a parcel to rise, it will always be cooler and heavier than the air around it (left arrow at 2,000 meters), and it will spread out horizontally if it can't "get back down." Any clouds that form 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 by advection of neighboring warmer air) - or if surface air cools (by nighttime radiation cooling, advection of colder air, or contact with a cold surface). Incidentally, that's 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.)

When the atmosphere's temperature profile shows a rapid drop - for example, anything greater than even the dry adiabatic lapse rate - all air parcels (even dry ones) won't cool as quickly as the ambient air. Once they "get the chance" to rise, they will always be hotter and lighter than the air around them (right arrow at 2,000 meters), and they keep going up. This is an absolutely unstable atmosphere. It is characterized by cumuliform clouds.

The steeper angle of the environmental lapse rate occurs when air aloft gets colder, or when the surface becomes warmer (by daytime radiation 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 (2,000 meters in this example). Here the air is 100 percent saturated (at its dewpoint).

Above this altitude 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. How unstable depends on how humid the air is and at what point it becomes saturated.

So you see, atmospheric stability figures prominently in our study of weather. In fact, glider pilots use something called the "Composite Moisture Stability Chart," issued twice daily by the National Weather Service, which shows a "lifted index" that reflects the stability of the air over the continental United States. Even if you don't follow the rising air in a sailplane, you should have a better intuitive understanding of the physical processes behind cloud formation.