Abstract
Turbulent shear flow generally contains large eddies which appear to be due to a shear instability of the profile of the mean flow. By analogy with laboratory experimental results, it is inferred that large eddies in the planetary boundary layer of the atmosphere should take the form of horizontal roll vortices with an orientation between that of the surface wind and that of the geostrophic flow. The vertical extent and intensity of the vertical motions associated with these roll vortices often may be sufficient to give rise to bands of clouds in the lower atmosphere. For an adiabatic boundary layer the spacing of cloud bands formed by this mechanism is tentatively predicted to be given by the relation L=200U/sinϕ, where L is measured in meters, U is the geostrophic speed near the ground in meters per second and ϕ is latitude.
