Internal gravity waves in a stratified ocean — dispersion ω² = N²sin²θ
Internal waves in a stratified fluid obey ω² = N²sin²θ where N is the Brunt-Väisälä frequency and θ is the angle of the wave vector to the vertical. Remarkably, the group velocity (energy transport) is perpendicular to the phase velocity — energy travels along wave beams at a fixed angle determined by ω/N. This governs mixing in the deep ocean and atmosphere.