Cumulus Clouds Clouds that like fluffy,rounded piles of cotton are called cumulus (KYOO myuh lus)clouds. The word cumulus means "heap" or "mass" in Latin Cumulus clouds form less than 2 kilometers above the ground, but they may grow in size and height until they extend opward as much as 18 kilometers. Cumulus clouds that are not very tall usually indicate fair weather. These clouds, which are common on sunny days, are called "fair weather cumulus." Towering clouds with flat tops, called comulonimdus clouds, often produce thun-derstorms. The suffix-nimbus means "rain