Cumulonimbus (from Latincumulus'swell' and nimbus'cloud') is a dense, towering, verticalcloud,[1] typically forming fromwater vapor condensing in the lowertroposphere that builds upward carried by powerfulbuoyant air currents. Above the lower portions of the cumulonimbus the water vapor becomesice crystals, such assnow andgraupel, the interaction of which can lead tohail and tolightning formation, respectively.
When causingthunderstorms, these clouds may be calledthunderheads. Cumulonimbus can form alone, in clusters, or alongsquall lines. These clouds are capable of producing lightning and other dangeroussevere weather, such astornadoes, hazardouswinds, and large hailstones. Cumulonimbus progress from overdevelopedcumulus congestus clouds and may further develop as part of asupercell. Cumulonimbus is abbreviated asCb.
A cumulonimbus calvus overEl Bajío, Guanajuato, Mexico.
Towering cumulonimbus clouds are typically accompanied by smallercumulus clouds. The cumulonimbusbase 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 200 to 4,000 m (700 to 10,000 ft). Normal peaks usually reach to as much as 12,000 m (39,000 ft), with unusually high ones typically topping out around 20,000 m (66,000 ft)[2] and extreme instances claimed to be as high as 21,000 m (69,000 ft) or more.[3] Well-developed cumulonimbus clouds are characterized by a flat,anvil shaped top (anvil dome), caused bywind shear orinversion at theequilibrium level near thetropopause. 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 anovershooting top culminating at themaximum 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.
Cumulonimbus calvus: cloud with puffy top, similar to cumulus congestus which it develops from; under the correct conditions it can become a cumulonimbus capillatus.
Incus (species capillatus only): cumulonimbus with flat anvil-like cirriform top caused by wind shear where the rising air currents hit theinversion layer at the tropopause.[8]
Mamma ormammatus: consisting of bubble-like protrusions on the underside.
Tuba: column hanging from the cloud base which can develop into afunnel cloud or tornado. They are known to drop very low, sometimes just 6 metres (20 ft) above ground level.[7]
Flanking line is a line of small cumulonimbus or cumulus generally associated with severe thunderstorms.
Anovershooting top is a dome that rises above the thunderstorm; it is associated with severe weather.
Cumulonimbus storm cells can produce torrentialrain of aconvective nature (often in the form of arain shaft) andflash flooding, as well asstraight-line winds. Most storm cells die after about 20 minutes, when theprecipitation causes moredowndraft thanupdraft, causing the energy to dissipate. If there is sufficient instability and moisture in theatmosphere, however (on a hot summer day, for example), theoutflowing moisture andgusts from one storm cell can lead to new cells forming just a few kilometres (miles) from the former one a few tens of minutes later or in some cases hundreds of kilometres (miles) away many hours later. This process cause thunderstorm formation (and decay) to last for several hours or even over multiple days. Cumulonimbus clouds can also occur as dangerous winter storms called "thundersnow" which are associated with particularly intense snowfall rates and withblizzard conditions when accompanied by strong winds that further reducevisibility. However, cumulonimbus clouds are most common intropical regions and are also frequent in moist environments during the warm season in themiddle latitudes.[10] Adust storm caused by a cumulonimbus downburst is ahaboob.
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 asatmospheric icing and hail if flying inside the cloud. Within and in the vicinity of thunderstorms there is significantturbulence andclear-air turbulence (particularlydownwind), respectively. Wind shear within and under a cumulonimbus is often intense withdownbursts being responsible for many accidents in earlier decades before training and technologicaldetection andnowcasting 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 airlineaccident was associated with flying through a tornado.
Stages of a cumulonimbus cloud's life.Transformation from a mature cumulus congestus cloud to a mature cumulonimbus incus
In general, cumulonimbus require moisture, anunstableair mass, and a lifting force in order to form. Cumulonimbus typically go through three stages: thedeveloping stage, themature stage (where the main cloud may reach supercell status in favorable conditions), and thedissipation stage.[11] The average thunderstorm has a 24 km (15 mi) diameter and a height of approximately 12.2 km (40,000 ft). Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through.[12]
