From AMS Glossary
The protuberances of its upper portion have begun to lose their cumuliform outline; they loom and usually flatten, then transform into a whitish mass with more or less diffuse outlines and vertical striations. Cirriform cloud is not present, but the transformation into ice crystals often proceeds with great rapidity. Most often, this cloud is accompanied by showers. By convention, the name cumulonimbus calvus is given to any highly developed cumuliform cloud that produces lightning, thunder, or hail, although the top shows no obvious trace of transformation into ice. The species calvus is unique to the genus cumulonimbus.