Buoyancy-driven instability: conduction → convection rolls at Ra > Ra_c ≈ 1708
Rayleigh-Bénard convection occurs when a fluid layer heated from below reaches the critical Rayleigh number Ra_c ≈ 1708 (for rigid boundaries). Hot fluid rises (red), cool fluid sinks (blue), forming convection rolls. The Nusselt number Nu = 1 + convective heat flux / conductive heat flux increases with Ra.