Cumulonimbus calvus

Cumulonimbus calvus
Cumulonimbus calvus, with probable cumulonimbus incus in background over Milan Malpensa Airport, Milan, Italy
Abbreviation	Cb cal
Symbol
Genus	Cumulonimbus (heap, rain)
Species	Calvus (bald)
Variety	None
Classification	Family C (Low-level)
Appearance	Dark-based storm cloud capable of impressive vertical growth without cirriform, hairless.
Precipitation	Very common Rain, Snow, Snow pellets or Hail, heavy at times

Cumulonimbus calvus is a moderately tall cumulonimbus cloud that is capable of precipitation but has not yet reached the tropopause,^[1] which is the height of stratospheric stability at which cumulonimbus forms into cumulonimbus capillatus (fibrous-top) or cumulonimbus incus (anvil-top). Cumulonimbus calvus develops from cumulus congestus,^[2] and its further development, under auspicious conditions, will result in cumulonimbus incus.

This cloud consists mainly of water droplets. By definition of cumulonimbus cloud, at its top water droplets are transformed into ice crystals. But for cumulonimbus calvus, content of ice crystals are meager and polar are in early stage, so the cloud tops still look round and puffy.

Cumulonimbus calvus is categorized by size and development and lacks an anvil head found on Cumulonimbus capillatus or Cumulonimbus incus.^[3] While lightning is not required, it can be used to differenciate it from Cumulus congestus.^[4] Developing cumulonimbus calvus lose sharp outlines of the top as more water droplets transform into ice crystals. Strong updrafts may form pileus or thin vertical stripes may protrude upwards out of the cloud. When upper parts of the cloud freeze to a greater extent and clearly visible cirriforms appear, cumulonimbus calvus metamorphoses into another species of cumulonimbus.