In order to grow into rain drops, cloud droplets must grow by coalescence
Larger droplets fall faster than small ones, and can collide with them
The process by which a sufficient number of large enough droplets is generated remains a topic of active research
Ice crystals provide a more efficient process
Saturation vapour pressure over ice is less than that over water ice crystals grow at expense of water droplets
If ice crystal touches a droplet, the droplet freezes
Once large enough, ice crystals – or clumps of crystals – fall past droplets and collect them. Rapid growth of soft hail pellet (graupel) by riming.
Graupel falls from cloud, melting before reaching the surface as rain