They tend to appear on these five cloud types: cirrus, altocumulus, cumulus, stratus, and stratocumulus. But it's equally rare to find fluctus clouds in each one, according to the International ...

Asperitas clouds are a distinctive, but relatively rare cloud formation that takes the appearance of rippling waves.

Because of their rarity and unusual appearance, fallstreak holes, or punch hole clouds, are often attributed to UFOs because they form as large circular gaps in cirrocumulus or altocumulus clouds.

or a series of lenticular clouds, is capable of forming. These are often mistaken for UFOs because of the saucer-like shape. They can separate into altocumulus standing lenticular, stratocumulus ...