Lifting of a large air mass followed by condensation combined with atmospheric instability
May signal development of thunderstorms. On rare occasions, may grow into cumulonimbus. This was observed from Ontario, California, on Wed. Aug. 29, 2012. Altocumulus castellanus clouds, based at about 15,000 feet, grew into mature thunderstorms with tops of 35,000 feet, within the space of an hour.