Clouds found in the middle levels of the troposphere have the prefix alto attached to their name. Two clouds are described here: altocumulus and altostratus.
Altocumulus form as large masses in patches or rows that may or may not merge with one another. Individuals usually have a sharp outline as they are composed of water and not ice. Altocumulus clouds are easily confused with cirrocumulus and stratocumulus clouds. Cirrocumulus are smaller and less dense than altocumulus. Elements of stratocumulus are larger than altocumulus. If you extend your arm in the direction of the cloud, altocumulus tend to be the size of your thumbnail, while stratocumulus are the size of your fist.
Figure 7.22 Altostratus cloud
Altostratus are a formless layer of grayish cloud that cover most if not all the sky. Altostratus clouds are more dense than the cirrostratus. The sun is barely visible through altostratus clouds giving the appearance of a "watery sun". Where enough light shows through cirrostratus clouds to create shadows, it does not with altostratus. Altostratus thicken into nimbostratus as a warm front approaches your location.