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Clouds: Signposts of the Sky

With things the way they are, a lot of pilots will either choose to (or be otherwise 'convinced' to) stay on the ground and, if that's your fate, there are still ways you can become a better pilot.With things the way they are, a lot of pilots will either choose to (or be otherwise 'convinced' to) stay on the ground and, if that's your fate, there are still ways you can become a better pilot. Clouds have always fascinated me, for reasons both aesthetic and meteorological. I don’t feel qualified to address the former, but I can explore what they mean to those who fly -- or wonder. Besides helping to regulate the earth's energy balance by reflecting sunlight or absorbing re-radiated infrared energy, clouds are also important visual indicators of physical processes, which shape the weather that we fly through. How important? Come see...

Clouds form when air rises, expands, and cools. When unsaturated air does that without any energy being added or removed, it cools at what is known as the dry adiabatic lapse rate -- 9.76 Centigrade degrees per Km (or 5.33 Fahrenheit degrees per thousand feet). Eventually, it will cool to its dew point temperature -- where air becomes saturated and water begins to condense in it. As water condenses in air, the process releases copious amounts of heat, because water has a very high 'latent heat.' It takes large amounts of energy to boil water, and large amounts are given up when it returns to the liquid state: from about 540 calories/gram at 100 degrees C up to almost 600 at zero degrees C. Because of this, rising air actually cools off at a lower rate, the moist adiabatic rate -- where some of the cooling due to ascent is offset by the latent heat released by the condensing water vapor. (This rate varies with temperature, and is roughly 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 climatic effects wherever there is a large body of water nearby!

Several things lift air, like surface heating, movement over higher terrain, or uplift along fronts. But even if none of these things occurred, the fact that a parcel of air might pick up moisture will itself make it rise. See, the same total number of water molecules (in vapor form) are about one-third lighter than the identical combined number of air molecules. Generally, since even super-steamy jungle air will only hold about 6% of water vapor by weight, 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. Next to solar heating, air picking up moisture is the biggest driving mechanism behind the hydrologic cycle!

The air isn't always rising, though. 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 -- and results in unstable air -- until it cools down (note the choice of words). When rising air is colder than its surrounding 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.

Translation: 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 extend horizontally if it can't get back down. Any clouds will be thin, spread out, and have flat tops and bases -- stratus clouds. The atmosphere becomes more stable if air aloft warms by sinking and compressing, warmer air moving in or if surface air cools (like nighttime cooling, arrival of colder air, or contact with a cold surface). Incidentally, that’s why you see more hot air balloons early in the morning, when the lowest surface temperatures are recorded.

When the atmosphere's temperature profile shows a rapid drop -- say 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 cumulus 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). While it is much cooler and less humid in an airplane at altitude during these conditions, this is also 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 unstable air -- which depends on how humid the air is and at what point it becomes saturated.

BOTTOM LINE: Atmospheric stability is an important part of weather. You’ve probably 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. But this should at least give you a better intuitive understanding of the physical processes behind cloud formation.

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