Over the central United States, they also form from outflow of severe thunderstorms. The appearance of altocumulus varies considerably. They can be thin or thick, white or gray, occur in organized lines or random distributions. They occur in the middle levels of the atmosphere when the air is moist. They are similar in appearance to stratocumulus, though with a higher cloud base and generally do not fill as large a part of the sky.
Altocumulus clouds appear in the advance of a warm front, preceding the altostratus.
If the altocumulus clouds appear with other cloud types, an approaching storm is likely. Altocumulus occur frequently and are common in many places of the world. They are a common cloud type that occurs in vertical motions ahead of large scale weather patterns. Cirrus clouds are associated with all weather systems. They can also be generated by flow over mountains or in regions with strong winds, such as the jet stream.