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. 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. A sheet of altostratus cloud often is seen lower down in a cumulonimbus cloud, this is known as a velum cloud.