Marine Glossary

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stability

A general term that describes how a physical (or emotional!) system will respond to a small perturbation. A ball in the bottom of a bowl is in stable equilibrium — disturb it slightly and it will return to where it was, even though it may roll around a bit while returning to the old system. A ball resting on the top of an inverted bowl is in unstable equilibrium — disturb it slightly and it will roll off and be gone, the old system has changed and won't return on its own.

Stability of the air is a state in which the vertical distribution of the atmosphere is in hydrostatic equilibrium (like the ball in the bottom of the bowl) so that an air parcel resists displacement from its original level. This type of atmospheric stability is called static stability. If stable air is forced up it cools more than the surrounding air, so it sinks back again. If it is forced down, it warms more than the surrounding, so it floats back up. Hence stability of air is determined by how it responds to changes in elevation, which in turn depends on how much water vapor is in the air and at what temperature. The stability of the air is crucial to the development of squalls and Lows in general. See lapse rate and related discussion in ART-13 on humidity.