Enhancement of natural convection heat transfer in a horizontal fluid layer is an important problem pertinent to suppression of thermal stratification inside low-temperature liquid storage tanks. The experiments are carried out with an air layer between horizontal plates of aspect ratio 8.4, of which the bottom surface is isothermally heated, the upper surface is cooled at a constant room temperature and the vertical side wall are adiabatic. Some visualized results are reported of flow and temperature fields induced by natural convection in a horizontal fluid layer of finite extent; the former is measured with the laser light sheet methods and the latter with the holographic interferometry. The effects of partition intervals attached to the bottom surface and the Rayleigh number on local Nusselt numbers are discussed. It is disclosed that (1) increases in the Rayleigh number result in enhancement of the vortex convection and the heat transfer performance improves, (2) upward flows are created at the fin positions, and the heat transfer is enhanced to the greatest extent when downward flows occur between the fins, and (3) there is the optimum pitch of the partitions to enhance the heat transfer effects.