Pileus Cloud Accessory: Cap, Hood Over A Cumuliform

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Definition: A cloud accessory describing a cap cloud that sits above a growing cloud tower

Description & Characteristics. The ‘pileus’ cloud accessory can be found amongst two cloud types: cumulonimbus and cumulus. Translated from latin, meaning cap, the two types of pileus clouds are respectively abbreviated as ‘Cb pil’, and ‘Cu pil’. Pileus clouds also go by the name cap clouds and can be easily identified by their smooth, almost lenticular-like appearance on top of a growing cumuliform tower.

Pileus cloud formations are created when the air surrounding a cumuliform tower is rising so quickly that it condenses into a smooth umbrella or hoodlike shape once it hits its dew point. Because pileus clouds are made from ice crystals, on the rare occasion you might see iridescence in the cloud, giving a rainbow-colored appearance. Seeing a pileus clouds is a good sign that the cloud is growing quickly, indicating that storms could be on the way. They’re more often found above cumulus congestus and cumulonimbus calvus clouds, but on the more rare occasion can also be seen on smaller cumulus and cumulus mediocris clouds.

Pileus clouds are a cloudspotter favorite. They’re fairly uncommon, though you can see them if you know where to look on days where you might expect cumulus clouds to grow into thunderstorms. Pileus clouds typically don’t persist for more than a few minutes, but if you’re lucky enough to catch one, good examples of pileus cloud formations can be quite picturesque. Don’t confuse them with their close relative, the lenticular cloud (lenticularis cloud species). Pileus clouds accessorize cumuliform clouds, where lenticular clouds are standalone.

Cumulus congestus pileus (Cu con pil)
Cumulonimbus calvus pileus (Cb cal pil)
Cumulus congestus pileus (Cu con pil)
A photograph of a cumulus congestus pileus cloud (Cu con pil) over some trees

Pileus Cloud Types

The pileus cloud accessory is associated with only two cloud types: cumulonimbus and cumulus.

A graphical illustration of a cumulonimbus pileus cloud

Cumulonimbus (Cb)


A graphical illustration of a cumulus pileus cloud

Cumulus (Cu)

Low, puffy, fair-weather