The term CUMULUS PREFIX refers to words that are related to cumulus clouds, which are large, fluffy, white clouds. The crossword solutions for this term are ALTO (4 letters), STRATO (6 letters), CLOUD (5 letters), HEAP (4 letters), FLEECY (6 letters), and FLEECIEST (9 letters). ALTO is a prefix meaning 'HIGH, ' STRATO is a prefix meaning 'LAYER, ' CLOUD is a noun referring to a cumulus CLOUD, HEAP is a noun meaning 'a large pile, ' FLEECY is an adjective meaning 'covered with soft, fine wool or fur, ' and FLEECIEST is an adjective meaning 'having the most wool or fur.'