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A 'pileus' (Latin for ''cap'') is a small, horizontal cloud that can appear above a cumulus or cumulonimbus cloud, giving the parent cloud a characteristic "hoodlike" appearance. Pilei tend to change shape rapidly. They are formed by strong updrafts acting upon moist air at lower altitudes, causing the air to cool beneath its dewpoint. 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.
Clouds that are attached to pileus 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".

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★ Factbytes article on cumulonimbus clouds - An interesting, if sparse, resource on the various types of cumulonimbus clouds.