A large scale, westward propagating wave that moves due to isentropic gradients of potential vorticity. Rossby waves are also called planetary waves. In particular, in the shallow water approximation, the waves move due to the variation of the Coriolis parameter with latitude, what is known as the ``beta-effect. The dispersion relation in this approximation has frequency omega = -�eta k/(k^2+l^2) in which �eta is the meridional gradient of the Coriolis parameter, and k and l are the zonal and meridional wavenumbers, respectively.