cumulo- (n.)

1650s, "a heap," from Latin cumulus "a heap, pile, mass, surplus," from PIE *ku-m-olo-, suffixed shortened form of root *keue- "to swell." Meteorological use for "rounded mass of clouds, snowy white at the top with a darker, horizontal base" is attested by 1803.

in reference to clouds, "of the nature of cumulus clouds," 1815, see cumulo- + -ous.