Among all of the Altocumulus cloud species, Altocumulus castellanus has the greatest impact on the weather forecast. So how to recognize it?

Altocumulus castellanus (Ac cas) is a mid-level cloud with its base from 2 km (6,500 ft) up to 6 km (20,000 ft). They are very similar to Stratocumulus castellanus and the main difference is that they look smaller and there is no precipitation connected to them.

But Altocumulus castellanus is a big help in predictions of incoming heavy showers or thunderstorms. Ac cas is created when a mid-level instability appears. If surface-based convection can connect to the mid-level instability, Ac cas can transform into Cumulonimbus clouds.

Have you ever seen this phenomenon? Send us your photos!