5.1: Introduction
- Page ID
- 46227
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A cumulonimbus cloud rises and expands as a product of instability within the atmosphere (Public Domain).
When you think of the word “stable,” you typically think of an object that is unlikely to change or something that is 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 them changing shape from one minute to the next. Such clouds are in a constant state of change, and thus represent the atmosphere in an unstable state.
Instability in the atmosphere is a concept that is intimately connected with thunderstorms, cumulus development, and vertical motion. In order to visualize the concept of stability, you might imagine a boulder sitting at the bottom of a canyon surrounded by steep hills, as depicted in the figure below 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. To visualize the concept of instability, imagine the same boulder at the top of a hill (red circle below). 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. However, if the same boulder were to be placed on flat ground (green circle below) and you were to push it, it would change position, but remain in its new position. This is an example of neutral stability.
Each of these concepts can be applied to motions of air parcels in the atmosphere. The topic of stability in atmospheric science is important because the formation of clouds is closely related to stability or instability in the atmosphere. In this chapter we will connect these concepts to the buoyancy of air parcels, and learn to use thermodynamic diagrams to visualize movement.
