The word cumulonimbus is a combination of two Latin words “Cumulo” meaning puffy, and “nimbus” meaning dark. Cumulonimbus clouds (abbreviated “Cb”) is a tall and dense cloud, formed due to atmospheric instability, and which precipitates thunderstorms and other intense weather.
Cumulonimbus clouds do not fall under the classification as low, middle, or high altitude clouds. They are rather Vertical growth clouds that span all levels of the troposphere and can even shoot into the stratosphere. They usually start at 3000 meters (10000 feet) above sea level and fueled by vigorous convective updrafts, sometimes in excess 50 knots, the tops of cumulonimbus clouds can easily reach 39,000 feet (12,000 meters) or higher. In extreme cases, it can go up to a height of 23,000 meters.
APPEARANCE
A cumulonimbus cloud appears like a long funnel that stretches to the sky. The base can be several miles long. When these clouds reach a height of ten miles, high winds and the inversion of the thunderstorm caused by rising temperatures flatten the top of the cloud out into an anvil-like shape. This anvil shape can precede the main cloud structure for many miles, causing lightning.
Cumulonimbus clouds (abbreviated “Cb”) takes the form of several species and varieties, depending on the size, shape and form of the elements. They are:
Cb Arcus: An arcus cloud is a low, horizontal cloud formation associated with the leading edge of thunderstorm outflow.
Cb Calvus: Calvus clouds are moderately tall cumulonimbus clouds capable of precipitation, but which has not yet reached the height where it develops the characteristic anvil-top. They have a distinctive round shape and relatively sharp edges at the top. Cb calvus develops from cumulus congestus, and its further development under auspicious conditions result in Cb Capillatus.
Cb Capillatus are the embodiment of the fully developed cumulonimbus clouds, with a massive cirrus-like, fibrous-edged top. These are the only cloud that can extend from almost ground level up to the tropopause. The vertical movements inside and around this cloud can be a hazard for aviation.
Cb Incus is a subtype of Cumulonimbus capillatus, with flat anvil-like top.
Cb Tuba is a modification of the capillatus type, and is the type of cumulonimbus cloud that looks like a finger pointed towards the earth.
Cb Mammatus, also known as mammatocumulus, or “bumpy clouds” is a cellular pattern of pouches hanging underneath the base of a cloud, resembling the shape of human female breasts.
Cb Pannus is the formation of numerous cloud shreds below the main cloud. These shreds may constitute a layer, which separate from the main part of the cloud,
Cb Pileus is a small, horizontal cloud that can appear above a cumulus or cumulonimbus cloud, giving the parent cloud a characteristic “hood like” appearance. This cloud change shape regularly. Pileus is Latin for cap.
Cb Velum: When cumulus clouds grow from a sheet of thin layer, or when a pileus cap becomes part of the cumulus, these clouds row on to become cumulonimbus velum. Velum is Latin for “veil”.
Cb Praecipitatio is the type of cumulonimbus clouds that rains
Cb Virga: Virga is the type of cumulonimbus cloud that precipitates virga. Virga is an observable streak or shaft of precipitation that falls from a cloud but evaporates before reaching the ground.
HOW THEY FORM
As the sun heats the earth, bubbles of hot air accompanied by water vapors rise upward from the warm surface and undergo convention, or get diluted with the cold air on the top layers of the atmosphere. They continue to rise as long as the air within the parcel is warmer than the surrounding air. “Cold Front” is the transition zone where the cold air mass replaces a warmer air. The cool air, which now dominates the air parcel, condenses the moisture and produces clouds, and later induces precipitation.
Cumulonimbus clouds form when a front of cool air meets a body of hot, moist air that rises up from the surface. They develop from the cumulus congestus type of clouds that prevail at lower height, and grow vertically. They form alone, in clusters, or along a cold front in a squall line. A squall is a sudden, sharp increase in wind speed usually associated with active weather such as rain showers, thunderstorms, or heavy snow.
COMPOSITION
Lower levels of cumulonimbus clouds consist mostly of water droplets while at higher elevations, where temperatures are well below 0 degree Centigrade, these clouds are entirely composed of ice crystals.
EFFECTS ON THE ATMOSPHERE
Cumulonimbus clouds produce heavy rain showers, snow showers, hail, thunderstorms, lightning and tornadoes, and for this reason, they are also known as thunderstorm clouds. They are usually associated with flash flooding and straight-line winds.
Cumulonimbus clouds transform into super cells or large thunderstorms with deep rotating updrafts and can have a lifetime of several hours. Super cells can produce frequent meso-cyclones, lightning, large hail, damaging winds, and tornadoes.
Cumulonimbus clouds contain severe convection currents, with very high, unpredictable winds. They are therefore extremely dangerous to aircraft. Cumulonimbus incus type of cloud accompanies by gusts near and under it
HOW IT DISSIPATES
Cumulonimbus clouds precipitates down as violent and heavy rain, hail, or snow. They may also transform into a super cell, a type of severe thunderstorm.
The precipitation usually dissipates within 20 minutes. However, on hot sunny days when there is enough energy in the atmosphere, moisture from one cell evaporates rapidly, causing a new cell to form nearby, and so on. These cause thunderstorms to last for several hours.
Cumulonimbus clouds dissipate when the downdraft or the hot air flowing down is stronger than the updraft or cool air rising up. When that happens, these clouds degrade and form clouds like cirrus spissatus, dense anvil-like cirrus, stratocumulus diurnalis or stratocumulus vesperalis.
REFERENCES
http://www.windows.ucar.edu
http://ww2010.atmos.uiuc.edu
http://www.amsglossary.allenpress.com
http://www.weatheronline.co.uk
