Clouds…
Beautiful, fluffy ones-dark, roiling, malevolent ones; they addimmeasurably to our natural landscape. For non-aesthetic reasonsthey are important, too. They help regulate the earth’s energybalance, by reflecting and scattering solar radiation or absorbingthe earth’s radiated infrared energy. But they also visually indicatephysical processes which shape the weather that we fly through-notthe least of which is this thing called atmospheric stability.It’s part of what makes those clouds that perhaps some small personhas already asked you about-or that perhaps you yourself havewondered 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 anyenergy being added or removed, it follows what is known as thedry adiabatic lapse rate-which is approximately 9.76 Centigradedegrees per Km (or about 5.33 Fahrenheit degrees per thousandfeet). When it reaches its dew point temperature, the rising parcelis no longer unsaturated. Water begins to condense. As it doesso, it releases copious amounts of heat.
Water in fact has a veryhigh “latent heat.” Just as it takes large amounts ofenergy to boil water, large amounts are given up when it returnsto the liquid state-from about 541 calories/gram at 100 degreesC up to about 597 calories per gram at zero degrees C. So someof that cooling off due to its ascent is offset by the latentheat released by the condensing water vapor. The air parcel thencools off at a lower rate-called the moist adiabatic rate. Thisrate varies with temperature, and is on the order of 6 Centigradedegrees per Km, or about 3.3 Fahrenheit degrees/1000 feet.
(Incidentally,in addition to having a high latent heat, water also has a highspecific heat. Just within the liquid phase, it takes plenty ofenergy to raise or lower the temperature of a given amount ofwater-witness the pronounced climatological effects whereverthere is a large body of water nearby!)
Lifting
You may also know that several mechanisms can cause air to riseand clouds to form: surface convective heating; orographic lifting;convergence of air masses; and uplift along fronts. What you maynot know is that even if none of these things occurred, the factthat a parcel of air might pick up a greater amount of moisturethan the surrounding air contains will itself make it rise. Thisis because the same total number of water molecules (in vaporform) are about one-third lighter than the identical combinednumber of molecules of nitrogen, oxygen, and other gases in theirproper proportions; i.e., air. In reality, even super-steamy jungleair will only hold about 6% of water vapor by weight, so evenvery humid air will only be about 2% lighter (one-third times0.06) than absolutely dry air-but it’s enough to do the trick.This is one of the larger cogs (the biggest of course being solarheating) in the driving mechanism behind the hydrologic cycle!
And temperature…
The air isn’t always rising, though-or if it is, the amount oflifting changes. The reason why hinges on the concept of atmosphericstability. If rising air is colder than surrounding air, it willsink 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 atall levels, it is considered absolutely stable. Such is the casewhen the environmental lapse rate is less than either the dryor moist adiabatic lapse rates. Picture a nearly-homogeneous airmass where temperature drops little with altitude: In this case,if a parcel is forced to rise, it will always be cooler and heavierthan the air around it, and it will spread out horizontally ifit can’t “get” back down. Any clouds will be thin, spreadout, and have flat tops and bases-in other words, stratus clouds.
The atmosphere becomes more stable if air aloft warms (eitherby sinking and compressing, or via advection of neighboring warmerair)-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 extremecase of this of course is the inversion-when temperatures risewith altitude.)
Don’t forget lapse rate!
When the atmosphere’s temperature profile shows instead a rapiddrop-for example anything greater than even the dry adiabaticlapse rate-all air parcels (even dry ones) will not cool as quicklyas the ambient air, and once they “get the chance” togo up, they will always be hotter and lighter than the air aroundthem, and they keep going up. This is an absolutely unstable atmosphere.It is characterized, of course, by cumuliform clouds. This steepeningof 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). Doesthis 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 whateverreason) and cools, first at the dry adiabatic rate. This is greaterthan ambient, which therefore makes it colder, heavier, and thus,stable-that is, until it reaches its condensation level. Herethe air is 100% saturated (i.e., at its dew point). Now abovethis the air cools at the moist adiabatic rate. Due to the releaseof latent heat, it cools more slowly than the air around it, whichmakes it warmer, lighter, and thus unstable. This is conditionallyunstable air-which depends on how humid the air is and at whatpoint it becomes saturated.
Wasn’t that easy?
This is enough for Meteorology-On-Daddy’s-Lap 101. Atmosphericstability does figure prominently in our considerations of weather.You have most likely heard of the National Weather Service’s CompositeMoisture Stability Chart, issued twice daily: It features itsown panel, showing the “lifted index”, which reflectsthe stability of the air over the continental US. (Those gliderpilots among you will no doubt have heard of it!) I hope thishas given you-or that it will allow you to give someone you know-abetter intuitive understanding of the physical processes behindcloud formation.
