A pileus, also called scarf cloud or cap cloud, is a small, horizontal, altostratus cloud that can appear above a cumulus or cumulonimbus cloud, giving the parent cloud a characteristic “hoodlike” appearance. They are formed by strong updrafts acting upon moist air at lower altitudes, causing the air to cool to its dew point. As such, they are usually indicators of severeweather, and a pileus found atop a cumulus cloud often foreshadows transformation into a cumulonimbus cloud, as it indicates a strong updraft within the cloud.
Clouds that are attached to pilei are often given the suffix “pileus” or “with pileus”. For example, a cumulonimbus cloud with a pileus attached to it would be called “cumulonimbus with pileus”. Pilei can also form above ash clouds and pyrocumulus clouds from erupting volcanoes (see the image above). They can also form above some mushroom clouds of high-yield nuclear detonations; in that context they are called ice caps.