smooth capping clouds
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
Therefore, there is little tendency for any
significant vertical growth in pileus clouds, which is why
the cloud takes on a smoother horizontal shape.