Eight Cloud Forms: A Gallery of Structure and Light

Clouds can resemble lenses, breaking waves or a rough sea seen from underneath. Their shapes offer clues to atmospheric processes, but a dramatic photograph still needs careful identification.

Using the WMO International Cloud Atlas

The World Meteorological Organization’s International Cloud Atlas is the reference for cloud classification. Names such as asperitas, fluctus and cavum replace some informal or older labels commonly repeated in viral galleries. They do not all describe cloud genera; the classification has several levels.

What a cloud photograph can tell you

A smooth mountain cloud and a turbulent-looking cloud base arise in different settings. A photograph may omit the motion and wider sky that would help identify them. Use the gallery to notice layers, edges and texture, then compare those observations with the Atlas rather than treating every unusual cloud as a sign of an approaching disaster.

Further information: WMO International Cloud Atlas

Explore more: an asperitas-like Nebraska sky; what distinguishes a supercell.

Fluctus, cavum and asperitas in the gallery

For the curled formations often labelled Kelvin–Helmholtz clouds, the Atlas uses the supplementary-feature name fluctus. These are relatively short-lived billows that resemble breaking waves, usually along a cloud’s upper surface. The gallery also includes smooth lenticular forms, anvils, roll clouds, polar stratospheric clouds and pouch-like mammatus. Their familiar descriptive names are useful starting points, but they do not all occupy the same level in the classification system. ^[1]

A fallstreak hole is classified as cavum: a distinct opening in a thin cloud layer, often with wisps descending through its centre. Asperitas describes the irregular, wave-like underside that can make the sky look like a rough sea viewed from below. Comparing those two features is a good first exercise. One is an opening within a layer, while the other is a pattern in the cloud base. Noticing where the feature sits makes the terminology much easier to remember. ^[2]