A pileus ( /ˈpaɪliːəs/; Latin for cap), also called scarf cloud or cap cloud, 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 to 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.
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 to the right).
Pilei form above some mushroom clouds of high-yield nuclear detonations; in that context they are called ice caps.
Pilei clouds indicate that the parent cloud is growing rapidly, has plenty of moisture, and is highly unstable. This means that the parent cloud could quickly grow to become a cumulonimbus cloud and continue to grow into a cumulonimbus incus cloud. These clouds can cause severe weather and sightings of pelei should be viewed as a warning of an impending storm. Pilei also indicate strong and possibly damaging winds.
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