From Glossary of Meteorology


(Also called cap cloud, scarf cloud.) An accessory cloud of small horizontal extent, often cirriform, in the form of a cap, hood, or scarf, which occurs above or attached to the top of a cumulus or cumulonimbus (less often stratocumulus) cloud that often pierces it.

Sometimes several pileus clouds are observed above each other. Pileus is formed as a moist layer locally lifted due to rising cloud below.
See cloud classification.