Cumulonimbus clouds are a type of cloud that looks like puffy cotton balls, and they can become very tall. They can become thunderclouds in the right conditions. The clouds are no more than 3,000 feet above the ground, but the top often varies in height. “Small, separate cumulus are associated with fair weather cumulus humilis.”
Cumulonimbus clouds are also known as thunderheads due to their unique mushroom shape. Some instances, a thunderhead with enough energy can develop into a supercell which can produce strong wings, flash floods, and a lot of lightning. Some can even become tornadoes given the right conditions. They are also a variation of nimbus or precipitation bearing clouds. They are formed beneath 20,000 ft .
(photo(s) reference: random unattributed cloud photos found on-line.)