Planetary (Rossby) waves are large-scale atmospheric (and oceanic) wave motions that arise from departures from geostrophic equilibrium, driven by the restoring effect of the variation of the Coriolis parameter with latitude (the beta effect); these waves are characterized by relatively slow evolution and long periods (on the order of several hours to days for the atmospheric case), distinguishing them from faster inertia-gravity waves, which arise from departures from geostrophic balance with much shorter periods (minutes), and from the concept of geostrophic balance itself, which is the equilibrium state (Coriolis force balancing pressure gradient force) from which these waves represent a departure, not the wave-generating mechanism itself.