Abstract
There is much evidence that both large-scale circulation and turbulence are important transport mechanisms in the upper atmosphere. There is considerable evidence both from direct observation of tracers and from the thermal end composition structure that the lower thermosphere is normally turbulent. A necessary but not sufficient condition for the growth of infinitesimal disturbances in laminar inviscid incompressible flow is that the Richardson number be less than 0.25. This critical value also appears to apply well in the troposphere and stratosphere for the initiation of shear-generated turbulence. However, there is no theoretical basis for the use of 0.25 as a critical value for the maintenance of turbulence once it has started. So long as the work done against buoyancy forces exceeds the viscous dissipation by a substantial factor, as should be expected when the static stability is high, the critical value for. maintenance of turbulence should be near unity; however, it must be much smaller when the stability is very small, in which case the usefulness of the Richardson criterion vanishes.
