  This paper discusses the evolution of the droplet size distribution for a
liquid-in-gas aerosol contained in a Rayleigh-B\'enard cell. It introduces a
non-collisional model for broadening the droplet size distribution, termed
'convective ripening'. The paper also considers the initiation of rainfall from
ice-free cumulus clouds. It is argued that while collisional mechanisms cannot
explain the production of rain from clouds with water droplet diameters of $20\
\mu {\rm m}$, the non-collisional convective ripening mechanism gives a much
faster route to increasing the size of the small fraction of droplets that grow
into raindrops.
