Altocumulus castellanus
Abbreviation ACCAS
Family B
Type Altocumulus
Occurrence Worldwide
Altitude 2000-5000m
Formation Lifting of a large air mass followed by condensation combined with atmospheric instability
Precipitation None
Potential Hazards 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.

High Clouds (Family A): Cirrus (Ci) • Cirrus aviaticus • Cirrus intortus • Cirrus radiatus • Cirrus uncinus • Cirrus Kelvin-Helmholtz • Cirrostratus (Cs) • Cirrocumulus (Cc) • Pileus • Contrail
Middle Clouds (Family B): Altostratus (As) • Altostratus undulatus • Altocumulus (Ac) • Altocumulus undulatus • Altocumulus mackerel sky • Altocumulus castellanus • Altocumulus lenticularis
Low Clouds (Family C): Stratus (St) • Orographic stratus • Fog • Nimbostratus (Ns) • Cumulus humilis (Cu) • Cumulus mediocris (Cu) • Stratocumulus (Sc)
Vertical Clouds (Family D): Cumulonimbus (Cb) • Cumulonimbus incus • Cumulonimbus calvus • Cumulonimbus with mammatus • Cumulus congestus • Cumulus castellanus • Pyrocumulus  • Pyrocumulonimbus