 In the case of a Rayleigh–Bénard cell, heat at the base of the liquid produces an uneven distribution of high energy molecules which will tend to diffuse towards the surface.As the temperature of the heat source increases, density effects come into play.Simple diffusion can no longer dissipate energy as fast as it is added and so the bottom of the liquid becomes hot and more buoyant than the cooler, denser liquid at the top.