Clouds Formation

The individual genus types result from the physical categories being cross-classified by height range family within the troposphere. These include family A (high), family B (middle), family C (low), family D1 (moderate vertical with low to middle bases), and family D2 (towering vertical with low to middle bases). The family designation for a particular genus is determined by the base height of the cloud and its vertical extent. The base height range for each family varies depending on the latitudinal geographical zone.
Families A and B: All Cirriform-category clouds are classified as high range family A and thus constitute a single genus cirrus (Ci). Cumuliform and stratiform-category clouds in the high altitude family carry the prefix 'cirro', yielding the respective genus names cirrocumulus (Cc) and cirrostratus (Cs). Similar genera in the middle range family B are prefixed by 'alto', yielding the genus names altocumulus (Ac) and altostratus (As).
Families C and D1: Any cumuliform or stratiform genus in these two families either has no prefix or carries one that refers to a characteristic other than altitude. The two non-prefixed genera are non-convective low stratus (St: family C),and free-convective moderate vertical cumulus (Cu: family D1). One prefixed cloud in this group is stratocumulus (Sc), a limited convection genus of the low altitude family C that has some stratiform characteristics (as do the middle and high based genera altocumulus and cirrocumulus, the genus names of which exclude 'strato' to avoid double prefixing). The other prefixed cloud is nimbostratus (Ns), a non-convective genus of the moderate vertical family D1 that has some vertical extent and whose prefix refers to its ability to produce significant precipitation.
Family D2: This family comprises large towering free-convective clouds that typically occupy all altitude ranges and therefore also carry no height related prefixes. They comprise the genus cumulonimbus (Cb) and the cumulus species cumulus congestus (Cu con). Under conditions of very low humidity, free-convective clouds may form above the low altitude range and therefore be found only at middle and high tropospheric altitudes. In the modern system of cloud nomenclature, cumulonimbus is something of an anomaly. The cumuliform-category designation appears in the prefix rather than the root which refers instead to the cloud's ability to produce storms and heavy precipitation. This apparent reversal of prefix and root is a carry-over from the nineteenth century when nimbus was the root word for all precipitating clouds.