According to collision- coalescence theory, formation of raindrop from cloud droplets occurs when cloud droplets collide and coalesce or stick together.
Explanation:
The only significant difference between a raindrop and a cloud droplet is that a raindrop consist of a velocity that is non-negligible during the fall.
Larger droplets having higher terminal velocities fall faster and collide with smaller droplets. Often the cloud droplets stick together and coalesce to form a larger droplet.
This starts a chain reaction where these bigger droplets fall even rapidly, collide with the other droplets in their path and merge with these droplets.
