When B nard's experiment was repeated it was found that the cells could also be produced when the heating plate was cooled, whereas according to Rayleigh's ideas the fluid should remain at rest. The instability has also been observed for a layer of liquid beneath a plate being heated from above and in space, where gravity and hence buoyancy forces are zero. In the late 1950s a new model for B nard convection was developed in which variations of surface tension caused by temperature variations on the surface of the liquid drove the motion. This model also predicted a depressed surface above rising fluid. In reality both B nard and Rayleigh effects must be present.
Conditions determine which predominates.
For thin layers the unstable flow takes the form of a regular array of hexagonal cells regardless of the shape of the container. For thicker layers the basic unstable flow is a series of rolls parallel to the container's sides with the direction of flow adjacent to its rim and determined by its temperature relative to its base.