5.1: What is Stability?

Stable vs. Unstable

Figure \(\PageIndex{1}\): A cumulonimbus cloud rises and expands as a product of instability within the atmosphere (Public Domain; NASA via Wikimedia Commons).

When you think of the word “stable,” you typically think of an object that is unlikely to change or something balanced. The opposite is true with something that is “unstable”. An unstable object is likely to fall or change position with time. The same is true with clouds. When you see a fluffy cumulus cloud, you might notice it changing shape from one minute to the next. Such clouds are constantly changing and thus represent the atmosphere in an unstable state.

Figure \(\PageIndex{2}\): A perfect cumulus cloud just west of Magic Island (CC-BY-SA 4.0; Sarah Williamson via Atmospheric Processes and Phenomenon).

Atmospheric instability is a concept closely linked to thunderstorms, cumulus development, and vertical motion. To visualize the concept of stability, imagine a boulder at the bottom of a canyon, surrounded by steep hills, as depicted in Figure \(\PageIndex{3}\) by the blue circle. If you were strong enough to push the boulder from its initial position partway up one of the hills, it would roll back to the bottom once you let go. Despite exerting a force on the boulder and causing an initial displacement, it would return to its initial position, and the net displacement would be zero. In a stable atmosphere, the temperature of a parcel is cooler than that of the atmosphere, so any air (or clouds as depicted in Figure 5.1.3) would be forced to sink. To visualize the concept of instability, imagine the same boulder at the top of a hill (red circle). If you were able to push the boulder just a little bit in any direction, it would begin to roll downward and accelerate away from its initial position. In an unstable atmosphere, the temperature of an air parcel is warmer than that of the surrounding atmosphere, and the parcel (and any associated clouds, like depcited in Figure 5.1.3) would be forced to rise. However, if the same boulder were to be placed on flat ground (green circle) and you were to push it, it would change position, but remain in its new position. This is an example of neutral stability. In a neutral atmosphere, a parcel of air has the same temperature as the atmosphere, and any air (or clouds, as depicted in Figure 5.1.3) would neither rise nor sink.

Figure \(\PageIndex{3}\): Examples of stability and instability in relation to air and parcel temperatures (CC-BY-SA 4.0; Britt Seifert via Atmospheric Processes and Phenomenon).

Each of these concepts can be applied to motions of air parcels in the atmosphere. Stability in atmospheric science is important because cloud formation is closely linked to atmospheric stability or instability. In this chapter, we will connect these concepts to the buoyancy of air parcels and learn to use thermodynamic diagrams to visualize movement.