The Meath Weather Cam

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 (often the Earth's surface is heated by terrestrial radiation) above the typically cooler air above it, it starts to cool (as the heat rises it expands and cools, this is due to pressure at different altitudes), 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 down drafts 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 pop 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 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).

                                Calvus type Cumulonimbus cloud.

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 = heavy rain and thunderstorms.

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 praecipitatio
• Cumulonimbus virga
• Cumulonimbus pannus
• Cumulonimbus incus - subtype of Cumulonimbus capillatus, with flat anvil-like top.
• Cumulonimbus mammatus
• Cumulonimbus pileus
• Cumulonimbus velum
• Cumulonimbus arcus
• Cumulonimbus tuba

            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.

Effects

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. They also can snow because they are also in the higher part of the atmosphere.

The air convection can also form mesocyclones, which can cause hail and tornadoes.

Cumulonimbus Incus

A cumulonimbus incus (Latin incus, "anvil") is a cumulonimbus cloud which has reached the level of stratospheric stability and has formed the characteristic flat, anvil-top shape. It can cause a supercell and then a tornado.

A cumulonimbus incus is a sub-form of cumulonimbus capillatus.

Cumulonimbus Calvus

                      Cumulonimbus calvus with cloud-to-cloud lightning

Cumulonimbus calvus is a moderately tall cumulonimbus cloud which is capable of precipitation, but has not yet reached the height where it forms into a cumulonimbus capillatus (firbous-top) or cumulonimbus incus (anvil-top). Cumulonimbus calvus develops from cumulus congestus, and its further development under auspicious conditions will result in cumulonimbus capillatus.

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 is small and freezing is in early stage, so cloud top still looks round and puffy.

Cumulonimbus calvus is characterized by distinctive (between other types of cumulonimbus cloud) rounded shape and relatively sharp edges of its top area, unlike cumulonimbus incus or cumulonimbus capillatus, which have cirriform tops. Developing cumulonimbus calvus loses sharp outlines of the top as more water droplets transform into ice crystals. Strong updrafts may form pileus or thin vertical stripes protruding upwards out of the cloud. When upper part of the cloud freezes to greater extent and clearly visible cirriforms appears, cumulonimbus calvus turns into another species of cumulonimbus.

Cumulonimbus calvus arcus is a sub-type of cumulonimbus calvus, which has arcus cloud ahead of cloud's front.

Cumulonimbus with mammatus

Mammatus (also known as mammatocumulus, meaning "bumpy clouds") is a meteorological term applied to a cellular pattern of pouches hanging underneath the base of a cloud. The name "mammatus" is derived from the Latin mamma (breast), due to the resemblance between the shape of these clouds and human female breasts.

Mammatus Characteristics

Mammatus are most often associated with the anvil cloud that extends from a cumulonimbus, but may also be found under altocumulus, altostratus, stratocumulus, and cirrus clouds, as well as contrails and volcanic ash clouds. In the United States, sky gazers may be most familiar with the very distinct and more common cumulonimbus mammatus. When occurring in cumulonimbus, mammatus are often indicative of a particularly strong storm. These tend to form more often during warm months and over the midwest and eastern portions of the country, and more infrequently over the west and southwest. Due to the intensely sheared environment in which mammatus form, aviators are strongly cautioned to avoid cumulonimbus with mammatus.

Mammatus may appear as smooth, ragged or lumpy lobes and may be opaque or semitransparent. Because mammatus occur as a grouping of lobes, the way they clump together can vary from an isolated cluster to a field of mamma that spread over hundreds of kilometers to being organized along a line, and may be composed of unequal- or similarly-sized lobes. The individual mammatus lobe average diameters of 1-3 km and lengths on average of 0.5 km. A lobe can last an average of 10 minutes, but a whole cluster of mamma can range from 15 minutes to a few hours. Their composition is usually mostly ice, but can be a mixture of ice and liquid water or almost entirely liquid water.

Cumulus Congestus

Cumulus congestus clouds (also towering cumulus (TCu)) are characteristic of unstable areas of the atmosphere which are undergoing convection. They are often characterized by sharp outlines and great vertical development. Because cumulus congestus is produced by strong updrafts, it is typically taller than it is wide, and cloud tops can reach 5,000 metres (15,000 ft). Cumulus congestus clouds are formed by the development of cumulus mediocris generally, though they can also be formed from altocumulus castellanus or stratocumulus castellanus as well.

Cumulus congestus will mature into cumulonimbus calvus under conditions of sufficient instability. This transformation can be seen by the presence of smooth, fibrous, or striated aspects assumed by the cloud's upper part. This cloud type produces precipitation, often in abundance. The congestus species of cloud can only be found in the genus cumulus. cumulus congestus contains towers which contain many high working cells.

Pyrocumulus

                                  Pyrocumulus cloud generated by a forest fire                                                           smoke plume in California,                                                                                     off the Interstate 5 highway, looking west.

A pyrocumulus or fire cloud is a dense cumuliform cloud associated with fire or volcanic activity.

Formation

A pyrocumulus is produced by the intense heating of the air from the surface. The intense heat induces convection which causes the air mass to rise to a point of stability, usually in the presence of moisture. Phenomena such as volcanic eruptions, forest fires, and occasionally industrial activities can induce formation of this cloud. The detonation of a nuclear weapon in the atmosphere will also produce a pyrocumulus in the form of a mushroom cloud which is made by the same mechanism. The presence of a low level jet stream can enhance its formation. Condensation of ambient moisture (moisture already present in the atmosphere) as well as moisture evaporated from burnt vegetation or volcanic outgassing occurs readily on particles of ash.

Pyrocumuluses contain severe turbulence which also results in strong gusts at the surface which can exacerbate a large conflagration. A large pyrocumulus, particularly one associated with a volcanic eruption, may also produce lightning. This is a process not fully understood as yet, but is probably in some way associated with charge separation induced by severe turbulence, and perhaps, by the nature of the particles of ash in the cloud. Large pyrocumuluses can contain temperatures well below freezing, and the electrostatic properties of any ice that forms may also play a role. A pyrocumulus which produces lightning is actually a type of cumulonimbus, a thundercloud and is called pyrocumulonimbus.

A pyrocumulus is similar dynamically in some ways to a firestorm, and the two phenomena may occur in conjunction with each other. However, one may occur without the other.

Appearance

Pyrocumulus is often greyish to brown in color, because of the ash and smoke associated with the fire. It also tends to expand, because the smoke and the ash involved in the cloud's formation increase the amount of condensation nuclei. This poses a problem, as the cloud can trigger another thunderstorm whose lightning can set off new fires.

Information and photos on this page are courtesy of Wikipedia