From SKYbrary Wiki
A pileus, also called scarf cloud or cap cloud, is a small, horizontal, lenticular-like cloud appearing above a cumulus or cumulonimbus cloud. Pileus clouds are often short-lived, with the main cloud beneath them rising through convection to absorb them.
Pileus cloud formations are created when the air surrounding a cumuliform tower is rising so quickly that it condenses into a smooth umbrella or hood-like shape once it hits its dew point. As such, they are usually indicators of severe weather, and a pileus found atop a cumulus cloud often foreshadows transformation into a cumulonimbus cloud, as it indicates a strong updraft within the cloud.
Pilei can also form above mountains, ash clouds, pyrocumulus clouds from erupting volcanoes, and some mushroom clouds of high-yield nuclear detonations.
Occasionally, bright iridescent colours are seen in pileus clouds as sunlight is diffracted through water droplets in the cloud, which are all the stronger because diffracting droplets are small and similar in size.
Sometimes several pileus clouds are observed above each other.