Internal waves in the atmosphere and ocean propagate in a strongly turbulent flow. As a result, their energy is continuously redistributed by scattering. I will explain how a kinetic equation describing this scattering can be derived and approximated as a diffusion in wavenumber space. For slowly evolving turbulence, this diffusion is highly anisotropic and leads to energy confined close to a constant-frequency cone in wavenumber space. The diffusion equation predicts an equilibrium energy spectrum for internal waves that is close to the energy spectrum observed in the atmosphere and ocean at (sub)mesocales, suggesting that internal waves dominates the dynamics at these scales.