We study the dynamics of small particles suspended in a turbulent flow (e.g. sand grains in an accretion disk around a growing star, water droplets in clouds).We describe how the particles cluster together and collide. The aim is to understand the time it takes to rain from a vigorously turbulent rain cloud.

Downpours Demystified?, Science Now:
As anyone who's been caught without an umbrella knows, even a fluffy, innocuous-looking cloud can unleash a sudden torrent of rain. A new theory may explain why.

Caustic Purge, Nature Physics:
A summer afternoon: the air is hot, and a flock of cumulus clouds hover in the blue sky. Suddenly it pours down. Such an abrupt onset of rainfall from these clouds might be due to the formation of so-called caustics in the velocity field of the raindrops.