Clouds Formation

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.
• Species
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.