Cumulus congestus over Long Island.JPG
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Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, generally less than in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters. Cumulus clouds are often precursors of other types of clouds, such as cumulonimbus, when influenced by weather factors such as instability, moisture, and temperature gradient. Normally, cumulus clouds produce little or no precipitation, but they can grow into the precipitation-bearing congests or cumulonimbus clouds. Cumulus clouds can be formed from water vapour, supercooled water droplets, or ice crystals, depending upon the ambient temperature. They come in many distinct subforms and generally cool the earth by reflecting the incoming solar radiation. Cumulus clouds are part of the larger category of free-convective cumuliform clouds, which include cumulonimbus clouds. The latter genus-type is sometimes categorized separately as cumulonimbiform due to its more complex structure that often includes a cirriform or anvil top. There are also cumuliform clouds of limited convection that comprise stratocumulus (low- étage), altocumulus (middle-étage) and cirrocumulus (high-étage). These last three genus-types are sometimes classified separately as stratocumuliform.


Formation

Cumulus clouds form via atmospheric convection as air warmed by the surface begins to rise. As the air rises, the temperature drops (following the
lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
), causing the relative humidity (RH) to rise. If convection reaches a certain level the RH reaches one hundred percent, and the "wet-adiabatic" phase begins. At this point a positive feedback ensues: since the RH is above 100%, water vapor condenses, releasing latent heat, warming the air and spurring further convection. In this phase, water vapor condenses on various nuclei present in the air, forming the cumulus cloud. This creates the characteristic flat-bottomed puffy shape associated with cumulus clouds. The height of the cloud (from its bottom to its top) depends on the temperature profile of the atmosphere and of the presence of any inversions. During the convection, surrounding air is entrained (mixed) with the thermal and the total mass of the ascending air increases. Rain forms in a cumulus cloud via a process involving two non-discrete stages. The first stage occurs after the droplets coalesce onto the various nuclei. Langmuir writes that surface tension in the water droplets provides a slightly higher pressure on the droplet, raising the vapor pressure by a small amount. The increased pressure results in those droplets evaporating and the resulting water vapor condensing on the larger droplets. Due to the extremely small size of the evaporating water droplets, this process becomes largely meaningless after the larger droplets have grown to around 20 to 30  micrometres, and the second stage takes over. In the accretion phase, the raindrop begins to fall, and other droplets collide and combine with it to increase the size of the raindrop. Langmuir was able to develop a formulaThe formula was t=, with t being the time to infinite radius, \eta being the viscosity of air, E being the fractional percentage of water droplets accreted per unit volume of air that the drop falls through, w being the concentration of water in the cloud in grams per cubic metre, and r_0 being the initial radius of the droplet. which predicted that the droplet radius would grow unboundedly within a discrete time period.


Description

The liquid water density within a cumulus cloud has been found to change with height above the cloud base rather than being approximately constant throughout the cloud. In one particular study, the concentration was found to be zero at cloud base. As altitude increased, the concentration rapidly increased to the maximum concentration near the middle of the cloud. The maximum concentration was found to be anything up to 1.25 grams of water per kilogram of air. The concentration slowly dropped off as altitude increased to the height of the top of the cloud, where it immediately dropped to zero again. Cumulus clouds can form in lines stretching over long called cloud streets. These cloud streets cover vast areas and may be broken or continuous. They form when wind shear causes horizontal circulation in the atmosphere, producing the long, tubular cloud streets. They generally form during high-pressure systems, such as after a cold front. The height at which the cloud forms depends on the amount of moisture in the thermal that forms the cloud. Humid air will generally result in a lower cloud base. In temperate areas, the base of the cumulus clouds is usually below above ground level, but it can range up to in altitude. In arid and mountainous areas, the cloud base can be in excess of . Cumulus clouds can be composed of
ice crystal Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth, na ...
s, water droplets, supercooled water droplets, or a mixture of them. The water droplets form when water vapor condenses on the nuclei, and they may then coalesce into larger and larger droplets. One study found that in temperate regions, the cloud bases studied ranged from above ground level. These clouds were normally above , and the concentration of droplets ranged from . This data was taken from growing isolated cumulus clouds that were not precipitating. The droplets were very small, ranging down to around 5  micrometres in diameter. Although smaller droplets may have been present, the measurements were not sensitive enough to detect them. The smallest droplets were found in the lower portions of the clouds, with the percentage of large droplets (around 20 to 30 micrometres) rising dramatically in the upper regions of the cloud. The droplet size distribution was slightly
bimodal In statistics, a multimodal distribution is a probability distribution with more than one mode (statistics), mode. These appear as distinct peaks (local maxima) in the probability density function, as shown in Figures 1 and 2. Categorical, ...
in nature, with peaks at the small and large droplet sizes and a slight trough in the intermediate size range. The
skew Skew may refer to: In mathematics * Skew lines, neither parallel nor intersecting. * Skew normal distribution, a probability distribution * Skew field or division ring * Skew-Hermitian matrix * Skew lattice * Skew polygon, whose vertices do not ...
was roughly neutral. Furthermore, large droplet size is roughly inversely proportional to the droplet concentration per unit volume of air. In places, cumulus clouds can have "holes" where there are no water droplets. These can occur when winds tear the cloud and incorporate the environmental air or when strong downdrafts evaporate the water.


Subforms

Cumulus clouds come in four distinct species, ''cumulus humilis'', ''mediocris'', ''congestus'', and ''fractus''. These species may be arranged into the variety, ''cumulus radiatus''; and may be accompanied by up to seven supplementary features, ''cumulus pileus'', ''velum'', ''virga'', ''praecipitatio'', ''arcus'', ''pannus'', and ''tuba''. The species ''Cumulus fractus'' is ragged in appearance and can form in clear air as a precursor to cumulus humilis and larger cumulus species-types; or it can form in precipitation as the supplementary feature ''pannus'' (also called
scud A Scud missile is one of a series of tactical ballistic missiles developed by the Soviet Union during the Cold War. It was exported widely to both Second World, Second and Third World, Third World countries. The term comes from the NATO reporti ...
) which can also include stratus fractus of bad weather. ''Cumulus humilis'' clouds look like puffy, flattened shapes. ''Cumulus mediocris'' clouds look similar, except that they have some vertical development. ''Cumulus congestus'' clouds have a cauliflower-like structure and tower high into the atmosphere, hence their alternate name "towering cumulus". The variety ''Cumulus radiatus'' forms in radial bands called cloud streets and can comprise any of the four species of cumulus. Cumulus supplementary features are most commonly seen with the species congestus. ''Cumulus virga'' clouds are cumulus clouds producing virga (precipitation that evaporates while aloft), and ''cumulus praecipitatio'' produce precipitation that reaches the Earth's surface. ''Cumulus pannus'' comprise shredded clouds that normally appear beneath the parent cumulus cloud during precipitation. ''Cumulus arcus'' clouds have a gust front, and ''cumulus tuba'' clouds have
funnel clouds A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud (usually a cumulonimbus or towering cumulus cloud) but not reaching the ground or a water s ...
or tornadoes. ''Cumulus pileus'' clouds refer to cumulus clouds that have grown so rapidly as to force the formation of pileus over the top of the cloud. ''Cumulus velum'' clouds have an ice crystal veil over the growing top of the cloud. There are also cumulus cataractagenitus. These are formed by waterfalls.


Forecast

Cumulus humilis clouds usually indicate fair weather. Cumulus mediocris clouds are similar, except that they have some vertical development, which implies that they can grow into cumulus congestus or even cumulonimbus clouds, which can produce heavy rain, lightning, severe winds, hail, and even tornadoes. Cumulus congestus clouds, which appear as towers, will often grow into cumulonimbus storm clouds. They can produce precipitation.
Glider Glider may refer to: Aircraft and transport Aircraft * Glider (aircraft), heavier-than-air aircraft primarily intended for unpowered flight ** Glider (sailplane), a rigid-winged glider aircraft with an undercarriage, used in the sport of glidin ...
pilots often pay close attention to cumulus clouds, as they can be indicators of rising air drafts or thermals underneath that can suck the plane high into the sky—a phenomenon known as
cloud suck Cloud suck is a phenomenon commonly known in paragliding, hang gliding, and sailplane flying where pilots experience significant lift due to a thermal under the base of cumulus clouds, especially towering cumulus and cumulonimbus. The vertical ex ...
.


Effects on climate

Due to reflectivity, clouds cool the earth by around , an effect largely caused by stratocumulus clouds. However, at the same time, they heat the earth by around by reflecting emitted radiation, an effect largely caused by cirrus clouds. This averages out to a net loss of . Cumulus clouds, on the other hand, have a variable effect on heating the earth's surface. The more vertical ''cumulus congestus'' species and cumulonimbus genus of clouds grow high into the atmosphere, carrying moisture with them, which can lead to the formation of cirrus clouds. The researchers speculated that this might even produce a positive feedback, where the increasing upper atmospheric moisture further warms the earth, resulting in an increasing number of ''cumulus congestus'' clouds carrying more moisture into the upper atmosphere.


Relation to other clouds

Cumulus clouds are a genus of free-convective low-level cloud along with the related limited-convective cloud stratocumulus. These clouds form from ground level to at all latitudes. Stratus clouds are also low-level. In the middle level are the alto- clouds, which consist of the limited-convective stratocumuliform cloud altocumulus and the stratiform cloud altostratus. Mid-level clouds form from to in polar areas, in temperate areas, and in tropical areas. The high-level cloud, cirrocumulus, is a stratocumuliform cloud of limited convection. The other clouds in this level are cirrus and cirrostratus. High clouds form in high latitudes, in temperate latitudes, and in low, tropical latitudes. Cumulonimbus clouds, like cumulus congestus, extend vertically rather than remaining confined to one level.


Cirrocumulus clouds

Cirrocumulus clouds form in patches and cannot cast shadows. They commonly appear in regular, rippling patterns or in rows of clouds with clear areas between. Cirrocumulus are, like other members of the cumuliform and stratocumuliform categories, formed via
convective Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convect ...
processes. Significant growth of these patches indicates high-altitude instability and can signal the approach of poorer weather. The ice crystals in the bottoms of cirrocumulus clouds tend to be in the form of hexagonal cylinders. They are not solid, but instead tend to have stepped funnels coming in from the ends. Towards the top of the cloud, these crystals have a tendency to clump together. These clouds do not last long, and they tend to change into cirrus because as the water vapor continues to deposit on the ice crystals, they eventually begin to fall, destroying the upward convection. The cloud then dissipates into cirrus. Cirrocumulus clouds come in four species which are common to all three genus-types that have limited-convective or stratocumuliform characteristics: ''stratiformis'', ''lenticularis'', ''castellanus'', and ''floccus''. They are
iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...
when the constituent supercooled water droplets are all about the same size.


Altocumulus clouds

Altocumulus clouds are a mid-level cloud that forms from high to in polar areas, in temperate areas, and in tropical areas. They can have precipitation and are commonly composed of a mixture of ice crystals, supercooled water droplets, and water droplets in temperate latitudes. However, the liquid water concentration was almost always significantly greater than the concentration of ice crystals, and the maximum concentration of liquid water tended to be at the top of the cloud while the ice concentrated itself at the bottom. The ice crystals in the base of the altocumulus clouds and in the virga were found to be dendrites or conglomerations of dendrites while needles and plates resided more towards the top. Altocumulus clouds can form via convection or via the forced uplift caused by a warm front.


Stratocumulus clouds

A stratocumulus cloud is another type of stratocumuliform cloud. Like cumulus clouds, they form at low levels and via convection. However, unlike cumulus clouds, their growth is almost completely retarded by a strong
inversion Inversion or inversions may refer to: Arts * , a French gay magazine (1924/1925) * ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas * Inversion (music), a term with various meanings in music theory and musical set theory * ...
. As a result, they flatten out like stratus clouds, giving them a layered appearance. These clouds are extremely common, covering on average around twenty-three percent of the earth's oceans and twelve percent of the earth's continents. They are less common in tropical areas and commonly form after cold fronts. Additionally, stratocumulus clouds reflect a large amount of the incoming sunlight, producing a net cooling effect. Stratocumulus clouds can produce drizzle, which stabilizes the cloud by warming it and reducing turbulent mixing.


Cumulonimbus clouds

Cumulonimbus clouds are the final form of growing cumulus clouds. They form when ''cumulus congestus'' clouds develop a strong updraft that propels their tops higher and higher into the atmosphere until they reach the tropopause at in altitude. Cumulonimbus clouds, commonly called thunderheads, can produce high winds, torrential rain, lightning, gust fronts,
waterspout A waterspout is an intense columnar vortex (usually appearing as a funnel cloud, funnel-shaped cloud) that occurs over a body of water. Some are connected to a cumulus congestus cloud, some to a cumuliform cloud and some to a cumulonimbus clou ...
s, funnel clouds, and tornadoes. They commonly have
anvil cloud A cumulonimbus incus (Latin ''incus'', "anvil"), also known as an anvil cloud, is a cumulonimbus cloud which has reached the level of tropopause, stratospheric stability and has formed the characteristic flat, anvil-top shape. It signifies the ...
s.


Horseshoe clouds

A short-lived
horseshoe cloud A horseshoe cloud is a relatively uncommon meteorological phenomenon which manifests as a cloud in the shape of a horseshoe or inverted letter "U". They occur when a horseshoe vortex deforms a cumulus cloud. The clouds are relatively short- ...
may occur when a
horseshoe vortex The horseshoe vortex model is a simplified representation of the vortex system present in the flow of air around a wing. This vortex system is modelled by the ''bound vortex'' (bound to the wing) and two '' trailing vortices'', therefore having ...
deforms a cumulus cloud.


Extraterrestrial

Some cumuliform and stratocumuliform clouds have been discovered on most other planets in the solar system. On Mars, the Viking Orbiter detected cirrocumulus and stratocumulus clouds forming via convection primarily near the polar icecaps. The
Galileo space probe ''Galileo'' was an American robotic space probe that studied the planet Jupiter and Moons of Jupiter, its moons, as well as the asteroids 951 Gaspra, Gaspra and 243 Ida, Ida. Named after the Italian astronomer Galileo Galilei, it consisted of ...
detected massive cumulonimbus clouds near the
Great Red Spot The Great Red Spot is a persistent high-pressure region in the atmosphere of Jupiter, producing an anticyclonic storm that is the largest in the Solar System. Located 22 degrees south of Jupiter's equator, it produces wind-speeds up to 432  ...
on Jupiter. Cumuliform clouds have also been detected on
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
. In 2008, the Cassini spacecraft determined that cumulus clouds near Saturn's south pole were part of a cyclone over in diameter. The
Keck Observatory The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and when comp ...
detected whitish cumulus clouds on Uranus. Like Uranus,
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
has methane cumulus clouds. Venus, however, does not appear to have cumulus clouds.


See also

* List of cloud types


Notes


References


Footnotes


Bibliography

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External links


AMS Glossary of Meteorology
{{DEFAULTSORT:Cumulus Cloud Cloud types Articles containing video clips