The turbulent motion of the Earth's atmosphere and oceans is hugely influenced by the effects of rotation and stratification. These effects alter the nature of turbulence profoundly from that of a homogeneous fluid. In particular, motions are dominantly horizontal, with vertical motions some three to four orders of magnitude smaller than horizontal motions. Moreover, coherent structures - vortices - are highly anisotropic, with vertical scales one to two orders of magnitude smaller than horizontal scales. And, fluid particle motions are doubly constrained: they must remain on (nearly flat) density surfaces and must retain their scalar value of 'potential vorticity'. These constraints are shown to be powerful, even in flow regimes for which rotation and stratification are not dominant effects.</p
