Clouds Formation

These clouds can have strong vertical currents and rise far above bases which form anywhere in the low to lower-middle altitude range from near surface to about 10,000 ft (3,000 m) and consequently elude classification into the three conventional height ranges. Like smaller cumuliform clouds in family D1, these towering giants usually initially form in the low altitude range but the bases can rise into the middle range when the moisture content of the air is very low. Unlike families A through D1 that each include a cumuliform and stratiform-category genus, family D2 instead has one cumuliform-category genus and one cumulus species, but no stratiform-category genus or species except for the occasional inclusion of very thick nimbostratus.
Family D2 includes:
- Genus Cumulonimbus (Cb): Heavy towering masses of free convective cloud with dark grey to nearly black bases that are associated with thunderstorms and showers. Thunderstorms can produce a range of severe weather that includes hail, tornadoes, a variety of other localized strong wind events, several types of lightning, and local very heavy downpours of rain that can cause flash floods, although lightning is the only one of these that requires a thunderstorm to be taking place. Generally, cumulonimbus require moisture, an unstable air mass, and a lifting force (heat) in order to form. Cumulonimbus typically go through three stages: the developing stage, the mature stage, and the dissipation stage. The average thunderstorm has a 24 km (15 mi) diameter. Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through.
+ Species Cumulonimbus calvus (Cb cal): Cumulonimbus clouds with very high clear-cut domed tops similar to towering cumulus.