Abstract
A system of nonlinear equations describing the single-mode dynamics of both the Rayleigh-Bénard instability (RBI) and the Soret-driven instability (SDI) is derived. The system predicts saturation effects on the steady-state vertical temperature (concentration) gradient for the RBI (SDI), and transient relaxation oscillations above a given threshold. Good quantitative agreement is found with the experimental results. The possibility of observing giant pulses analogous to those observed in solid-state-laser transients is suggested.