9th September 2005
Cumulonimbus Calvus
Source: Excerpt from The Book " Weather "
Distribution: Worldwide,
except Antarctica.Height: 3000 to 30,000 feet. Cause: Powerful
convection, enhanced
by atmospheric instability.Associated Weather:
Moderate to heavy showers, strong winds.Hazard Warning:
Significant turbulence at cloud level.Cumulonimbus calvus represents a transitional stage between cumulus congestus and a fully fledged cumulonimbus incus.
Calvus occurs when convection and atmospheric instability combine to push the cloud tops beyond the congestus stage to heights of up to 30,000 feet.
The mushrooming top of the calvus formation is a sign of the vigorous updrafts that may eventually force the cloud up into the highest levels of the troposphere. At this level of the troposphere, temperatures are normally well below freezing, and any condensation that takes place will produce ice crystals rather than water droplets. This gives the top of the cloud a brilliant, white appearance. However, the cloud will not yet have developed the anvil-like profile characteristic of cumulonimbus incus.
Calvus clouds always produce some form of precipitation, with rainshowers occurring in temperate zones and snowfalls in colder areas. Under certain conditions, these falls can be moderate to heavy. In dry areas, showers may fall from the cloud base but evaporate before reaching the ground, a phenomenon known as virga.
The powerful convective updrafts associated with calvus clouds can produce significant turbulence. However, the precipitation associated with the cloud can usually be located by onboard aircraft radar and evasive action taken.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
Check out Skyscapes for cloud photos taken from the aircraft.
