Cumulo- means "heap" or "pile" in Latin. Cumulus clouds are often described as "puffy", "cotton-like" or "fluffy" in appearance, and have flat bases. Cumulus clouds, being low-level clouds, are generally less than 1,000 m (3,300 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.