When associated with a large weather system, stratocumulus are often the last cloud to appear before the skies. These layered clouds consisting mostly of liquid water are gray to pale blue in appearance. These clouds form when the middle layers of the atmosphere are moist and gradually lifted. The cloud base does not have much structure. The height of the cloud base is hard to distinguish because of its smooth appearance.
Altostratus clouds are common in the advance of a warm front, preceding the nimbostratus.
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.