ALTOCUMULUS CASTELLANUS

Altocumulus castellanus (ACCAS) is named for its tower-like projections that billow upwards from the base of the cloud. The base of the cloud can form as low as 2,000 metres (6,500 feet), or as high as 6,000 metres (20,000 feet).Castellanus clouds are evidence of mid-atmospheric instability and a high mid-altitude lapse rate. They may be a harbinger of bad weather and, if surface-based convection can connect to the mid-tropospheric unstable layer, continued development of castellanus clouds can produce cumulonimbus clouds and thunderstorms.
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