Natural convection in rectangular slots has vital applications in cooling of nuclear reactor insulation, chemical vapor deposition, insulation of double pane windows, environmental processes, and phase change processes. Flow patterns arising out of density differences depend on height to width ratio (aspect ratio (AR)) and temperature difference. These flow patterns have a significant effect on rate of heat transfer. Hence flow patterns in a slender vertical slot with large height to width ratio (AR = 20) have been numerically studied for four fluids (mercury, air, and water and silicon oil) of varying Prandtl numbers. The circulation cell formation and merging has been seen as a quasi-periodic behavior for water and silicon oil at various Rayleigh numbers. The genesis of formation of circulation cells due to the pressure differentials (as a result of thermal gradients) has been satisfactorily explained for mercury, air, and water and silicon oil and a criterion for the same has been developed for the range of Prandtl number between 0.71 and 7 on the basis of numerical predictions. (c) 2007 Elsevier Ltd. All rights reserved.