Why does rain fall?
The short answer
Rain falls when droplets or ice particles in a cloud grow heavy enough that rising air can no longer keep them up.
How it works
Inside clouds, droplets collide and ice particles collect more water vapor. Large particles fall through the cloud and may collect more droplets. If they melt before reaching the ground, we call them rain.
A familiar example
The same cloud can produce snow, sleet, or rain depending on the temperature between the cloud and the ground.