An Etymological Dictionary of Astronomy and Astrophysics

The frequency at which an air parcel will oscillate when subjected to an infinitesimal perturbation in a stably stratified atmosphere.
For a medium with a continuous density gradient, it is expressed by the formula:

N² = -(g/ρ)∂ρ/∂z , where

g is the → gravitational acceleration, ρ is density, and z geometric height.
The stability condition is N > 0. It is also sometimes referred to as the buoyancy frequency. The higher the value of N the more stable the flow.

See also: Named aster David Brunt (1886-1965), British meteorologist (1927, Q.J.R.Met.Soc. 53, 30)
and Vilho Väisälä (1889-1969), Finnish meteorologist (1925, Soc. Sci. Fenn. Commental. Phys. Math. 2 (19), 19);
→ frequency.