Clouds Formation

Genus and species types are further subdivided into varieties some of which are determined by the opacities of particular low and middle cloud structures (translucidus, opacus, and perlucidus; the last of which is opaque with translucent breaks). By implication rather than formal designation, all family A high clouds are 'translucidus'. Conversely, all clouds with at least some significant vertical extent, including low to middle family D1 nimbostratus and cumulus, are 'opacus', as are the tall vertical family D2 clouds, cumulonimbus and cumulus congestus.
Other varieties are determined by the arrangements of the cloud structures into particular patterns that are discernable by a surface based observer (cloud fields usually being visible only from a significant altitude above the formations). The variety undulatus (having a wavy undulating base) is common to all high, middle, and low genera except those with significant vertical extent. Another common variety, duplicatus (closely spaced layers of the same genus, one above the other) is found with all the same genera except cirrocumulus. The variety radiatus is associated with cloud rows of a particular genus that appear to converge at the horizon and is seen mostly with cirrus, altocumulus, altostratus, stratocumulus, and cumulus.
Intortus and vertebratus varieties occur only with the genus cirrus and are respectively filaments twisted into irregular shapes and those that are arranged in fishbone patterns. Probably the most uncommonly seen is the variety lacunosus caused by localized downdrafts that punch circular holes into high, middle, and/or low cloud layers of limited convection.