Granules on the solar surface are the tops of convection cells. The regions above the curve correspond to instability, since a Rayleigh number lying there is higher than the Rayleigh number for marginal instability. If R is sufficiently small, we see from Fig. On raising R further, we see from Figure 10.4 that a range of wavenumbers becomes unstable. If , then the system is stable against all perturbations.
Small perturbations grow into fingers with characteristic mushroom caps.
Most experiments find the critical Rayleigh number to be around 1700. If the experiment is performed with the Rayleigh number close to its critical value, then we expect only perturbations with one horizontal wavenumber to be unstable. This is borne out by experiments. Figure 10.3 shows the pattern of marginal convection in a laboratory experiment. What we see are indeed convection cells on the solar surface known as granules.