Brunt–Väisälä frequency

Summary

The Brunt–Väisälä frequency, or buoyancy frequency, is the frequency at which a small parcel of fluid oscillates vertically when displaced in a stably stratified fluid. It measures the strength of the stratification. It is the largest frequency of internal waves, and it enters the Richardson number and the quasi-geostrophic equations. A negative square signals an unstable stratification, which leads to convection.