Large water droplets or ice crystals fall faster than smaller ones causing them to run into each other as they fall through the cloud.
Can be a cold or warm cloud process.
Water droplets stick to each other/merge together as they run into each other.
Requires liquid water, therefore can only occur in warm clouds.
Falling ice crystals may freeze supercooled droplets on contact, producing larger ice particles.
Requires ice and supercooled liquid water, so it is only a cold cloud process.
Falling ice crystals may run into each other and stick to other ice crystals producing snowflakes.
Only occurs in cold clouds.