  Dependence of boundary layer thickness on layer height in laminar and
transient regimes is studied for convection from localised heat source with an
open surface. The measurements of Nusselt number and characteristic frequency
of thermal plume formation are done for a wide range of Rayleigh number,
different Prandtl numbers and different aspect ratios. The obtained results
prove that for the developed (but not turbulent) convective flow the boundary
layer thickness is almost independent of the layer height and it is mainly
defined by applied temperature difference and physical properties of the fluid.
