Cap-shaped, like hats, and not belonging to a cloud, pileus clouds usually appear on top of cumulus or cumulonimbus. They are thin, small, horizontal, usually, although they also have different sizes and there are some larger than normal. As oddities that they are, they tend to be very dynamic, they change shape quickly.
Also calls "Pileus", a word derived from Latin meaning "cup“In honor of their likeness, from time to time they leave something more than a simple drink. From time to time, with the rays of light passing through and from a good perspective, we can see the full range of colors of the rainbow. Also, adopt the reflections or colors of a sunset, seeing yourself "there" up in the cloud, a spectacle of color that is difficult to stop looking at.
How are pileus clouds formed?
They are formed from strong updrafts of moist air coming from low latitudes. When the air is cooled below its dew point, that is when they appear. They are good indicators of severe weather ahead. For example, when they appear on top of a cluster. This usually ends up transforming into a cumulonimbus, just because of the strong currents that are existing inside.
The fact that these types of clouds appear, are usually identified more as a descriptive adjective of what the cloud itself in which it has formed is. Thus, one might say, "a cumulus cloud with a pileus." No "cumulus cloud with pileus cloud", because the pileus exists as a result of the first.
Another way to define the pileus would be with lenticular clouds. Also called circular or orographic, due to the similarity of nature. They are some of those that appear in the shape of a cup, surrounding the top of some mountains, for example.