Cumulus clouds are that have flat cloud base and are often described as puffy, cotton-like, or fluffy in appearance. Their name derives from the Latin , meaning "heap" or "pile".
Cumulus clouds are often precursors of other types of clouds, such as cumulonimbus, when influenced by weather factors such as instability, humidity, and temperature gradient. Normally, cumulus clouds produce little or no precipitation, but they can grow into the precipitation-bearing cumulus congestus or cumulonimbus clouds. Cumulus clouds can be formed from water vapour, supercooling 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 irradiance.
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.
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), 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 formula was , with being the time to infinite radius, being the viscosity of air, being the fractional percentage of water droplets accreted per unit volume of air that the drop falls through, being the concentration of water in the cloud in grams per cubic metre, and 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.
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 Temperateness 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 , water droplets, supercooled water droplets, or a mixture of them.
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.
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) which can also include stratus fractus of bad weather.
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.
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
.
Cumulus congestus clouds, which appear as towers, will often grow into
cumulonimbus storm clouds. They can produce precipitation.
pilots often pay close attention to cumulus clouds, as they can be indicators of rising air drafts or
underneath that can suck the plane high into the sky—a phenomenon known as
cloud suck.
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
. 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
convection 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
iridescence 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. 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
. 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,
,
, and tornadoes. They commonly have
.
Horseshoe clouds
A short-lived
horseshoe cloud may occur when a
horseshoe vortex 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 detected massive cumulonimbus clouds near the Great Red Spot on
Jupiter.
Cumuliform clouds have also been detected on
Saturn. 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 detected whitish cumulus clouds on
Uranus.
Like Uranus,
Neptune has methane cumulus clouds.
, however, does not appear to have cumulus clouds.
See also
Notes
Footnotes
Bibliography
External links
