Pileus (Latin for "skullcap") is a smooth cloud found either at the top of or attached to a mountain or a growing cumulus tower. Pileus clouds form when air at higher levels is forced to rise. In the photograph below, a pileus cloud is attached to the top of a cumuliform cloud.

The air that is lifted to form the pileus clouds is drier and consequently has a much higher lifting condensation level than does the moist air from lower levels often responsible for cumulus clouds. Therefore, there is little tendency for any significant vertical growth in pileus clouds, which is why the cloud takes on a smoother horizontal shape.