Pileus (meteorology)

A pileus (/ˈpaɪliː.əs/; Latin for cap), also called scarf cloud or cap cloud, is a small, horizontal, lenticular 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 updraft at lower altitudes, acting upon moist air above, 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.