Cloud Varieties - Specific properties of the genera
Intortus is a cloud in which the filaments are very irregularly curved and often seemingly entangled in a capricious manner. It mainly occurs with cirrus.
Undulatus variety is a cloud layer, or the arrangement of its cloudlets, shows an undulating appearance. It usually forms when the air above and below the cloud layer has different speed and/or directions. This wind shearing effect gives rise to cloud billows just like ripples in the sand or sea surface. Undulatus mainly occurs with cirrocumulus, cirrostratus, altocumulus, altostratus, stratocumulus or stratus.
Vertebratus is a cloud whose filaments are arranged in a manner suggestive of vertebrae, ribs or a fish skeleton. It mainly occurs with cirrus.