  If the temperature of a cell containing two partially miscible liquids is
changed very slowly, so that the miscibility is decreased, microscopic droplets
nucleate, grow and migrate to the interface due to their buoyancy. The system
may show an approximately periodic variation of the turbidity of the mixture,
as the mean droplet size fluctuates. These precipitation events are analogous
to rainfall from warm clouds. This paper considers a theoretical model for
these experiments. After nucleation the initial growth is by Ostwald ripening,
followed by a finite-time runaway growth of droplet sizes due to larger
droplets sweeping up smaller ones. The model predicts that the period $\Delta
t$ and the temperature sweep rate $\xi$ are related by $\Delta t\sim C
\xi^{-3/7}$, and is in good agreement with experiments. The coefficient $C$ has
a power-law divergence approaching the critical point of the miscibility
transition: $C\sim (T-T_{\rm c})^{-\eta}$, and the critical exponent $\eta$ is
determined.
