Rain will produce from very thick cumulus clouds, but not from thinner cumulus clouds.why?

Question

Rain will produce from very thick cumulus clouds, but not from thinner cumulus clouds.why?

Answer 1

Thick cumulus clouds have deeper up/down air motion.  As a result, a water particle caught up in the airflow is going to have more collisions with other water particles, and this will allow drops to grow bigger till they are large enough to escape the air currents within the cloud and fall out.  When the up/down currents are very strong as in thick/tall cumulus, the drops will be very large when they finally escape the currents and fall out of the cloud.  

Thin cumulus tend to have much smaller up/down velocities.  While collisions will still take place between droplets in a smaller cumulus, the drops won't have to be very big to escape the weaker air currents and fall out of the cloud.  When these smallish drops fall out of the cloud, they tend to evaporate before they hit the ground.

Answer 2

Those cumulous clouds with no real thickness don't hold enough moisture to cause rain,  while the towering cumulous clouds and cumulonimbus clouds have a lot of moisture.