From SKYbrary Wiki
Detached cloud with significant vertical development relative to horizontal extent.
Associated with instability, cumulus clouds are generally dense with sharp outlines, developing vertically in the form of rising mounds, domes, or towers. The bulging upper part of the cloud may resemble a cauliflower.
Aircraft entering cumulus clouds may expect to encounter turbulence and icing. The operational relevance of these effects will depend upon the aircraft type and will be insignificant in safety terms for larger transport aircraft. Flying beneath a cumulus cloud, aircraft may experience turbulence associated with downdrafts and wind shear. These phenomena are particularly likely to be encountered if Towering Cumulus is penetrated. The severity of turbulence and icing will depend on the extent of the instability (often indicated by the size of the cloud) and the outside air temperature.