concept

Rossby number

A dimensionless number, U divided by f times L — flow speed over the Coriolis parameter times the horizontal scale — that tests whether planetary rotation or inertia dominates a flow. When Ro is much less than 1, rotation rules and geostrophic balance holds; when Ro is of order 1 or greater, rotation is a minor player and the flow simply goes where it is pushed. That makes the number the arbiter of something much larger: whether a world gets organised zonal jets at all, or whether its atmosphere merely convects in unbanded disorder.