Cumulonimbus () is a dense, towering, vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. Above the lower portions of the cumulonimbus the water vapor becomes ice crystals, such as snow and graupel, the interaction of which can lead to hail and to lightning formation, respectively.

When causing thunderstorms, these clouds may be called thunderheads. Cumulonimbus can form alone, in clusters, or along squall lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes, hazardous winds, and large hailstones. Cumulonimbus progress from overdeveloped cumulus congestus clouds and may further develop as part of a supercell. Cumulonimbus is abbreviated as Cb.

Description

upright=1.2|thumb|A cumulonimbus calvus over [[El Bajío, Guanajuato, Mexico.]]

Towering cumulonimbus clouds are typically accompanied by smaller cumulus clouds. The cumulonimbus base may extend several kilometres (miles) across, or be as small as several tens of metres (yards) across, and occupy low to upper altitudes within the troposphere - formed at altitude from approximately . Normal peaks usually reach to as much as , with unusually high ones typically topping out around and extreme instances claimed to be as high as or more. Well-developed cumulonimbus clouds are characterized by a flat, anvil shaped top (anvil dome), caused by wind shear or inversion at the equilibrium level near the tropopause. The shelf of the anvil may precede the main cloud's vertical component for many kilometres (miles), and be accompanied by lightning. Occasionally, rising air parcels surpass the equilibrium level (due to momentum) and form an overshooting top culminating at the maximum parcel level. When vertically developed, this largest of all clouds usually extends through all three cloud regions. Even the smallest cumulonimbus cloud dwarfs its neighbors in comparison.

Subtypes

Species

  • Cumulonimbus calvus: cloud with puffy top, similar to cumulus congestus which it develops from; under the correct conditions it can become a cumulonimbus capillatus.
  • Cumulonimbus capillatus: cloud with cirrus-like, fibrous-edged top.

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File:Wagga-Cumulonimbus.jpg|Cumulonimbus calvus

Storm cloud.jpg|A clearly developed cumulonimbus fibrous-edged top capillatus

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Types

  • Cumulonimbus flammagenitus (pyrocumulonimbus): rapidly growing cloud forming from non-atmospheric heat and condensation nuclei sources such as wildfires and volcanic eruptions.

thumb|center|Pyrocumulonimbus with pileus

Supplementary features

Accessory clouds
  • Arcus (including roll and shelf clouds): low, horizontal cloud formation associated with the leading edge of thunderstorm outflow.
  • Pannus: accompanied by a lower layer of fractus species cloud forming in precipitation.
  • Pileus (species calvus only): small cap-like cloud over parent cumulonimbus.
  • Velum: a thin horizontal sheet that forms around the middle of a cumulonimbus.
Supplementary features
  • Incus (species capillatus only): cumulonimbus with flat anvil-like cirriform top caused by wind shear where the rising air currents hit the inversion layer at the tropopause.
  • Mamma or mammatus: consisting of bubble-like protrusions on the underside.
  • Tuba: column hanging from the cloud base which can develop into a funnel cloud or tornado. They are known to drop very low, sometimes just above ground level.
  • Virga: precipitation that evaporates before reaching the ground. A dust storm caused by a cumulonimbus downburst is a haboob.

Aviation

thumb|Aerial view of a cumulonimbus with pileus, flying over Brazil

Cumulonimbus are a notable hazard to aviation mostly due to potent wind currents but also reduced visibility and lightning, as well as atmospheric icing and hail if flying inside the cloud. Within and in the vicinity of thunderstorms there is significant turbulence and clear-air turbulence (particularly downwind), respectively. Wind shear within and under a cumulonimbus is often intense with downbursts being responsible for many accidents in earlier decades before training and technological detection and nowcasting measures were implemented. A small form of downburst, the microburst, is the most often implicated in crashes because of their rapid onset and swift changes in wind and aerodynamic conditions over short distances. Most downbursts are associated with visible precipitation shafts, however, dry microbursts are generally invisible to the naked eye. At least one fatal commercial airline accident was associated with flying through a tornado.

Life cycle or stages

thumb|300px|Stages of a cumulonimbus cloud's life.

thumb|Transformation from a mature cumulus congestus cloud to a mature cumulonimbus incus

In general, cumulonimbus require moisture, an unstable air mass, and a lifting force in order to form. Cumulonimbus typically go through three stages: the developing stage, the mature stage (where the main cloud may reach supercell status in favorable conditions), and the dissipation stage. The average thunderstorm has a diameter and a height of approximately . Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through.

See also

  • Atmospheric convection
  • Atmospheric thermodynamics
  • Convective instability
  • Hot tower
  • Lifted condensation level (LCL), convective condensation level (CCL), level of free convection (LFC), and free convective layer (FCL)
  • William Rankin
  • Ewa Wiśnierska

References

  • Clouds-Online.com Cloud Atlas with many photos and description of the different cloud genera
  • MetOffice.gov.uk Learn about thunderstorms and how cumulonimbus clouds form