  Thermal convection in fluid layers heated from below are usually realized
experimentally as well as treated theoretically with fixed boundaries on which
conditions for the temperature and the velocity field are prescribed. The
thermal and velocity boundary layers attached to the upper and lower boundaries
determine to a large extent the properties of turbulent convection at high
Rayleigh numbers. Fixed boundaries are often absent in natural realizations of
thermal convection. This paper studies the properties of convection driven by a
planar heat source below a cooling source of equal size immersed in an
otherwise stably stratified fluid layer are studied in this paper. Unavoidable
boundaries do not influence the convection flow since they are separated from
the active convection layer by nearly motionless stably stratified regions. The
onset of convection occurs in an inner unstably stratified region where the
mean temperature gradient is reversed. But the region of a reversed
horizontally averaged temperature gradient disappears at higher amplitudes of
convection such that the vertical derivative of the mean temperature no longer
changes its sign.
