Cumulonimbus cloud

Cumulonimbus cloud (calvus-type)
Abbreviation	Cb
Genus	Cumulonimbus (heap, cloud/severe rain)
Altitude	2,000–15,000 m (6,500–50,000 ft)
Classification	Family D (Vertically developed)
Appearance	Very tall and large clouds
Precipitation Cloud?	Yes, but may be virga

Cumulonimbus (Cb) is a type of cloud that is tall, dense, and involved in thunderstorms and other intense weather. The clouds can form alone, in clusters, or along a cold front in a squall line. Cumulonimbus clouds form from cumulus clouds (namely from cumulus congestus) and can further develop to a supercell, a severe thunderstorm with special features.

Formation

Three ingredients are needed for the formation of a perfect cumulonimbus cloud:

1. Plenty of moisture.
2. A mass of warm unstable air.
3. A source of energy to lift the warm, moist air mass rapidly upward.

Typically, the clouds form around front lines, near oceans where sea breezes provide storm energy, or over mountains which push the air upwards. When the warm air rises above the typically cooler air above it, it starts to cool and the water vapor condenses into water droplets. This condensation heats the surrounding air by releasing latent heat, thus continuing the rise of air. As the air mass continues to rise, the water droplets continue to cool and form ice crystals. Gravity causes these droplets and crystals to start to fall, causing a downward movement to compete with the upward lift. Instability between the updrafts and downdrafts causes static electrical charges to build up within the cumulonimbus cloud. The discharge of this electricity causes thunder and lightning.

Anvil tops of thunderclouds visible over the Pacific Ocean, as seen from the International Space Station

During the spring and summer, cumulonimbus clouds are more likely to form in the afternoon, due to the heating of the earth's surface. However, they can also form along a cold front when the warm buoyant air is forced upward by the heavier cold air mass that cuts under the warmer air like a wedge. This can happen at any time of the year, as demonstrated by thunderstorms that happen in conjunction with snowstorms in the winter. Cumulonimbus are sometimes mistaken for their parent, cumulus congestus. The distinctive feature of cumulonimbus is its frozen top, consisting of ice crystals, contrary to Cumulus cloud, which is formed entirely by water droplets. Frozen top of cumulonimbus loses puffy shape, resembles Cirrostratus and appears as homogeneous or fibrous-edged cirriform or as flat 'anvil'. The line between Cumulus cloud and cumulonimbus can be drawn at the moment, when cloud top reaches temperature of approximately minus 10 degrees Celsius and cirriforms are clearly seen. However, this does not fully apply to cumulonimbus calvus, which are closer by structure and look to cumulus clouds and are an intermediate form between cumulus congestus and cumulonimbus.

Appearance

Cumulonimbus cloud in central Oklahoma. The updraft is the large cloud mass at the center of the photo. The anvil is the flat layer at the top. The downdraft is the rainy area to the right.

Cumulonimbus clouds usually form from cumulus clouds at a much lower height, thus making them, like cumulus clouds, grow vertically instead of horizontally, thus giving the cumulonimbus its mushroom shape. The base of a cumulonimbus can be several miles across, and it can be tall enough to occupy middle as well as low altitudes; though formed at an altitude of about 3,000 to 4,000 meters (10,000 to 12,000 feet), its peak can reach up to 23,000 meters (75,000 feet) in extreme cases. Typically, it peaks at a much lower height (usually up to 5,000 meters / 16,500 feet). Well-developed cumulonimbus clouds are also characterized by a flat, anvil-like top (anvil dome), caused by straight line winds at the higher altitudes which shear off the top of the cloud, as well as by an inversion over the thunderstorm caused by rising temperatures above the tropopause. This anvil shape can precede the main cloud structure for many miles, causing anvil lightning. Cumulonimbus clouds can be subdivided into several species:

• Cumulonimbus calvus - cloud with puffy top, looking like cumulus congestus, but larger;
• Cumulonimbus capillatus - cloud with cirrus-like, fibrous-edged top;
• Cumulonimbus incus - subtype of Cumulonimbus capillatus, with flat anvil-like top.

Cumulonimbus capillatus incus

Effects

Exceptionally clearly developed single-cell Cumulonimbus incus, gusts will happen near and under it

Cumulonimbus storm cells can produce heavy rain (particularly of a convective nature) and flash flooding, as well as straight-line winds. Most storm cells die after about 20 minutes, when the precipitation causes more downdraft than updraft, causing the energy to dissipate. If there is enough solar energy in the atmosphere, however (on a hot summer's day, for example), the moisture from one storm cell can evaporate rapidly — resulting in a new cell forming just a few miles from the former one. This can cause thunderstorms to last for several hours. This multicell cloud structure exists until cold downdraft preceding cumulonimbus at ground level flows before cloud at distance sufficient to disrupt updraft (5-10 kilometers). From this moment on, cumulonimbus cloud quickly degrades and dissipates, forming cirrus spissatus, dense anvil-like cirrus, stratocumulus diurnalis or stratocumulus vesperalis. Cumulonimbus clouds sometimes form mammatus clouds. Cumulonimbus clouds contain severe convection currents, with very high, unpredictable winds, particularly in the vertical plane (updrafts and downdrafts). They are therefore extremely dangerous to aircraft. Smaller, propeller-driven planes cannot cope with the conditions and must fly around them; larger jet aircraft fly over the smaller ones and around larger examples. Larger planes are also equipped with weather radar and wind shear detectors to help guide them through, in the event that they need to pass through such clouds to land. The air convection can also form mesocyclones, which can cause hail and tornadoes.

See also

weather Portal

• Cumulus congestus cloud
• Cumulonimbus calvus
• Cumulonimbus incus
• Pyrocumulonimbus
• Supercell
• Mammatus cloud
• Funnel cloud
• Storm
• Tornado
• Pileus (meteorology)

External links

• Clouds-Online.com Cloud Atlas with many photos and description of the different cloud genus
• Cumulonimbus cloud at BBC Weather
• Weather Pictures and Storm Chasing – Cumulonimbus clouds