Cumulus congestus clouds are characteristic of unstable areas of the atmosphere which are undergoing convection. They are often characterized by sharp outlines and great vertical development. Because they are produced by strong updrafts, they are typically taller than they are wide, and cloud tops can reach, or higher in the tropics. Cumulus congestus clouds are formed by the development of cumulus mediocris generally, though they can also be formed from altocumulus castellanus or stratocumulus castellanus as well. The congestus species of cloud can only be found in the genuscumulus and is designated as towering cumulus by the International Civil Aviation Organization. Congestus clouds are capable of producing severe turbulence and showers of moderate to heavy intensity. This species is classified as vertical or multi-étage and is coded CL2 in the synop report. These clouds are usually too large and opaque to have any opacity or pattern-based varieties. Pyrocumulus is a free convective cloud associated with volcanic eruptions and large-scale fires. Pyrocumulus isn't recognized by the WMO as a distinct genus or species, but is, in essence, cumulus congestus formed under special circumstances that can also cause severe turbulence. Cumulus congestus will mature into cumulonimbus calvus under conditions of sufficient instability. This transformation can be seen by the presence of smooth, fibrous, or striated aspects assumed by the cloud's upper part. This cloud type produces precipitation, often in abundance. These clouds also producefair weatherwaterspouts forming from mesocyclones. These waterspouts tend to perish after precipitation or strong downdrafts form, by which time the congestus may have advanced to cumulonimbus. Cumulus congestus clouds can produce precipitation and even lightning. An approaching weather front often brings mid level clouds such as altostratus or altocumulus which usually stop any cumulus from reaching the congestus stage by reducing the Sun's heat or acting as a layer of stable air through which the cumulus cannot rise through. Occasionally, however, particularly if the air below the mid level cloud is very warm or unstable, some of the cumuli may become congestus and the tops of them may rise above the mid level cloud layer, sometimes resulting in showers ahead of the main rainband. This is often a sign the approaching front contains at least a few cumulonimbi amongst the nimbostratusrain clouds and therefore any rain may be accompanied by thunderstorms.
Turkey tower is a slang term for a narrow, individual towering cloud from a small cumulus cloud which develops and suddenly falls apart. Sudden development of turkey towers could signify the breaking or weakening of a capping inversion.
