Clouds Formation

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.
Major precipitation clouds: Although they do not comprise a family as such, cloud genera with 'nimbo' or 'nimbus' in their names are the principal bearers of precipitation. Although nimbostratus initially forms in the middle height range, it can be classified as moderate vertical because it achieves considerable thickness despite not being a convective cloud like cumulonimbus, the other main precipitating cloud genus. Frontal lift can push the top of a nimbostratus deck into the high altitude range while precipitation drags the base down to low altitudes.