In the present study the onset of natural convection in an initially quiescent, horizontal fluid layer heated isothermally from below has been analysed, by employing the propagation theory. The stability analysis under the linear theory has been conducted deterministically for the limiting cases of extremely small and infinite Prandtl numbers. A new heat transfer correlation connecting the theoretical critical conditions on thermal instability to tur-bulent thermal convection has been suggested as a function of both the Rayleigh number and the Prandtl number. It is found that the resulting new correlation agrees well with experimental data of mercury, air, water and silicon oil.