Clouds Formation

Cirrus clouds have several additional species unique to the wispy structures of this genus, which include uncinus, filaments with upturned hooks, and spissatus, filaments that merge into dense patches. One exception is the species fibratus which also occurs with cirrostratus that is transitional to or from cirrus. Cirrostratus at its most characteristic tends to be mostly of the species nebulosus which creates a rather diffuse appearance lacking in structural detail. Altostratus and nimbostratus share this physical appearance without significant variation or deviation and are therefore not formally subdivided into species. Low continuous stratus is also of the species nebulosus except when broken up into ragged sheets of stratus fractus. This latter fractus species also occurs with ragged cumulus.
• Varieties
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.