concept

Rossby wave

A planetary-scale meander in a jet, held and restored by the way planetary vorticity changes with latitude — the beta effect — together with the flow's conservation of potential vorticity. A parcel pushed poleward gains planetary vorticity and must spin the other way to compensate, and that restoring tendency turns a small nudge into a wave thousands of kilometres long. Rossby waves propagate westward relative to the mean flow; when that westward propagation exactly matches the eastward flow carrying them, the two cancel and the wave pattern stands nearly still over the ground.