Altocumulus clouds are composed of flattened, thick, gray, globular
masses. They are middle level clouds comprised primarily of water droplets.
In the mid-latitudes, their bases are usually found between 8,000 and 18,000
feet. They often appear as a wavy billowy layer of cloud. They are easily
confused with cirrocumulus clouds, but their elements (individual clouds)
have larger mass and cast a shadow on other elements. They may form several
sub-types, such as altocumulus castellanus or altocumulus lenticularis.