The natural convection problem formulated abovewas solved in two ways, first on the basis of discretenumerical experiments (sections 3, 4) and, second,based on order-of-magnitude reasoning (section 5).This balanced approach to studying the combinedbuoyancy effects in porous medium natural convectioneliminates the need for exhaustive numerical simulations and, at the same time, highlights the true scalesand ‘physics’ of the phenomenon.