The cumulus principle cloud type describes all cloud forms which have the dominant characteristic of vertical development. Cumulus feature rising mounds, domes or towers ranging from small to very large in size found in cumulus through various sub-forms such as cumulus mediocris and cumulus congestus. Cumulus reveal significant upward motion in the atmosphere. Condensation of water vapor releases heat which causes upward motion to increase until the air reaches a stable layer which prevents additional upward motion.