static stability
A fluid, such as air, tending to become or remain turbulent is said to be statically unstable; one tending to become or remain laminar is statically stable; and one on the borderline between the two (which might remain laminar or turbulent depending on its history) is statically neutral. The concept of static stability can also be applied to air not at rest by considering only the buoyant effects and neglecting all other shear and inertial effects of motion. However, if any of these other dynamic stability effects would indicate that the flow is dynamically unstable, then the flow will become turbulent regardless of the static stability. That is, turbulence has physical priority, when considering all possible measures of flow stability (e.g., the air is turbulent if any one or more of static, dynamic, inertial, barotropic, etc., effects indicates instability). Turbulence that forms in statically unstable air will act to reduce or eliminate the instability that caused it by moving less dense fluid up and more dense fluid down, and by creating a neutrally buoyant mixture. Thus, turbulence will tend to decay with time as static instabilities are eliminated, unless some outside forcing (such as heating of the bottom of a layer of air by contact with the warm ground during a sunny day) continually acts to destabilize the air. This latter mechanism is one of the reasons why the atmospheric boundary layer can be turbulent all day.
Compare dynamic stability, lapse rate, Brunt–Väisälä frequency, nonlocal static stability, adiabatic equilibrium;
see also slice method, buoyant instability
Stull, R. B. 1991. Static stability—An update. Bull. Amer. Meteor. Soc.. 72. 1521–1529.
