Clouds Formation

Genus types are subdivided into species that indicate specific structural details. However, because these latter types are not always restricted by height range, some species can be common to several genera that are differentiated mainly by altitude. The best examples of these are the species stratiformis, lenticularis, and castellanus, which are common to cumuliform genera of limited convection in the high, middle, and low height ranges (cirrocumulus, altocumulus, and stratocumulus respectively). Stratiformis species normally occur in extensive sheets or in smaller patches with only minimal convective activity. Lenticularis species tend to have lens-like shapes tapered at the ends. They are most commonly seen as orographic mountain-wave clouds, but can occur anywhere in the troposphere where there is strong wind shear. Castellanus structures, which resemble the turrets of a castle when viewed from the side, can also be found in convective patches of cirrus, as can the more detached tufted floccus species which are common to cirrus, cirrocumulus, and altocumulus. However floccus is not associated with stratocumulus in the lower levels where local airmass instability tends to produce clouds of the more freely convective cumulus and cumulonimbus genera whose species are mainly indicators of degrees of vertical development.
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.