When considering the
Oberbeck-Boussinesq equations in an infinite layer it is mostly assumed that the
pressure π is periodic in the plane, whereas the equations only require ∇π to be
periodic. We study here the influence the general admissible form of the
pressure may have on the velocity field u below the onset of convection, a
question which is closely connected with the mean flow. This is a vector field
which depends on z only and which is given by the mean values of ux, uy, uz
over the plane periodicity cell. — The mean value of u over the layer is
constant under stress-free boundary conditions and periodic pressure. If this
constant c is not 0 there is in most cases no longer an exchange of stability
on the onset for the linearization around c. We study its spectrum on the
onset.